FXUS63 KLOT 220914 AFDLOT AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL 314 AM CST SAT JAN 22 2011 .DISCUSSION... 853 PM CST LITTLE CHANGE MADE TO GOING FORECAST EXCEPT TO UP POPS TO LOWER END LIKELY NUMBERS MUCH OF THE FA AS FEEL WILL BE STILL BE A MINOR...NUISANCE EVENT BUT WITH ENOUGH LOWER AND MID LEVEL WARM AIR ADVECTION AND RESULTING LIFT OCCURRING AHEAD OF THE APPROACHING SHORT WAVE TROF TO LIKELY LEAVE BEHIND AT LEAST A TENTH OF AND INCH SF AND A HUNDREDTH LIQUID EQUIVALENT. NOT A WHOLE LOT MORE EXPECTED WITH GRIDDED SNOWFALL FORECAST TOTALS RANGING FROM 0.1-0.2 ACROSS THE IND COUNTIES OF THE FA UP TO 0.6-0.8 ACROSS THE WESTERN COUNTIES. INITIALLY CLEAR/MOSTLY CLEAR SKIES AND CALM OR NEAR CALM WINDS ALLOWED SOME LOCATIONS TO DIP BACK BELOW ZERO BUT WITH CLOUDS SPREADING W TO E ACROSS MOST ALL OF NORTHERN IL AS 9 PM CST AND SURFACE FLOW STARTING TO BECOME LIGHT SOUTH-SOUTHEASTERLY TEMPS ALREADY REBOUNDING. TRS PREVIOUS DISCUSSION... 310 PM CST THE WEATHER SITUATION WILL REMAIN QUITE ACTIVE AND CHANGEABLE FOR THE NEXT SEVERAL DAYS. HOWEVER...EVEN THOUGH THE LONG WAVE PATTERN IS SHAPING UP TO BE NEARLY STATIONARY OVER THE CONUS...WITH AN AMPLIFYING RIDGE OVER THE ERN PACIFIC AND WEST COAST...AND BROAD TROUGHING OVER THE CENTRAL AND ERN CONUS...AN NORTHERN STREAM SHORT WAVE PATTERN WILL BRING PERIOD CHANCES OF LIGHT SNOW WITH TEMPERATURES NEAR OR SLIGHTLY BELOW NORMAL. FOR THE NEAR TERM...SFC HIGH PRESSURE WILL MOVE THROUGH THE MIDWEST AS A SFC TROUGH DEVELOPS OVER THE NRN PLAINS AND APPROACHES THE MISSISSIPPI VALLEY. WITH THE RIDGE AXIS CROSSING IL/IN THIS AFTERNOON AND EVENING...WINDS WILL CONTINUE TO DROP OFF WHILE TEMPERATURES RANGE FROM THE LOWER TEENS IN DOWNTOWN CHICAGO TO THE MIDDLE SINGLE DIGITS OVER THE REST OF THE AREA. AS THE RIDGE AXIS MOVE TO THE EAST...LOW LEVEL FLOW WILL BECOME MORE SOUTHERLY SETTING UP A PATTERN OF WEAK WARM ADVECTION. WHILE SKIES REMAIN MOSTLY CLEAR...TEMPERATURES WILL BEGIN TO DROP AGAIN DURING THE LATE AFTERNOON AND EARLY EVENING HOURS. THE RESULTANT RADIATIONAL COOLING SHOULD ALLOW TEMPERATURES TO DROP RAPIDLY DURING THE FIRST FEW HOURS AFTER SUNSET. OVERNIGHT LOW TEMPERATURES SHOULD OCCUR AROUND 03Z AND HAVE TRENDED CLOSER TO THE LOWER END OF THE GUIDANCE ENVELOPE WITH TEMPERATURES DROPPING TO ARND -3F WEST OF THE FOX VALLEY AND NORTH OF THE INTERSTATE 80 CORRIDOR. WITH WINDS BECOMING VERY LIGHT...DO NOT EXPECT WIND CHILLS TO DROP MUCH LOWER THAN -10 TO -15F AT THE TIME MIN TEMP. SO...HAVE NO PLANS FOR ANOTHER WIND CHILL ADVISORY. TEMPERATURES SHOULD THEN BEGIN TO SLOWLY RISE THROUGH THE REMAINDER OF THE NIGHT AS FLOW BECOMES INCREASINGLY SOUTHERLY IN ADVANCE OF THE SFC TROUGH APPROACHING FROM THE WEST. THE NEXT WEATHER CONCERN WILL BE IN ASSOCIATION WITH THE AFOREMENTIONED SFC TROUGH AND AN ASSOCIATED MID LEVEL SHORT WAVE. THE LATEST MODEL RUNS HAVE BEEN LESS ORGANIZED WITH THIS SYSTEM...WHICH IS ACTUALLY 2 SEPARATE SHORTWAVES...ONE TRACKING THROUGH THE UPPER GREAT LAKES AND ANOTHER TRACKING INTO THE LOWER OHIO VALLEY. THESE 2 SHORTWAVES ARE LOOKING TO REMAIN UNPHASED...WITH THE NRN WAVE LOOKING TO BE THE STRONGER OF THE TWO. WITH THE LOCAL AREA REMAINING CUT OFF FROM ANY SIGNIFICANT LOW LEVEL MOISTURE BY A RIDGE OF HIGH PRESSURE EXTENDING FROM THE MID ATLANTIC TO WRN GULF COAST...ANY PCPN WILL BE THE RESULT OF WEAK UPPER LEVEL FORCING SQUEEZING OUT THE LIMITED MOISTURE IN PLACE. EXPECT SOME LIGHT SNOW TO REACH THE I-39 CORRIDOR BY LATE THIS EVENING AND THEN STEADILY SPREAD EWD OVERNIGHT. WHILE MOST OF THE AREA WILL LIKELY SEE SOME LIGHT SNOW OVERNIGHT AND EARLY TOMORROW MORNING...ACCUMULATIONS SHOULD RANGE FROM A DUSTING TO A COUPLE TENTHS OF AN INCH. THERE WILL BE ANOTHER CHANCE FOR SOME LIGHT SNOW AS ANOTHER NRN STREAM SHORTWAVE DROPS OUT OF THE NRN ROCKIES SATURDAY NIGHT. THE GUIDANCE IS TRENDING A LITTLE FARTHER SOUTH WITH THIS SYSTEM THAN PREVIOUS RUNS...INDUCING A WEAK SFC LOW OVER THE LOWER MISSISSIPPI VALLEY SUNDAY. WITH HIGH PRESSURE EXPECTED OVER THE ERN GREAT LAKES AND THE SFC LOW OVER THE LOWER MISSISSIPPI VALLEY...WEAK ELY FLOW WILL DEVELOP. HOWEVER...WITH THE MAIN FORCING FARTHER SOUTH THAN PREVIOUSLY ANTICIPATED...MUCH OF THE SNOW SHOULD BE CONFINED TO THE FAR SOUTHERN COUNTIES...WITH SOME LAKE EFFECT SNOW SHOWERS POSSIBLE FOR NERN ILLINOIS...PARTICULARLY LAKE COUNTY AND NRN COOK COUNTY. TEMPERATURES WILL BE ON A SLOW WARMING TREND...THOUGH REMAINING BELOW NORMAL THROUGH THE WEEKEND...WITH HIGHS TOPPING OUT IN THE UPPER TEENS TO ARND 20 AND OVERNIGHT LOWS IN THE UPPER SINGLE DIGITS TO LOW TEENS. BY MONDAY...HIGHS SHOULD REBOUND INTO THE LOW TO MIDDLE 20S AND TUESDAY SHOULD BE A NEAR BALMY UPPER 20S TO LOW 30S. ANOTHER WEAK CLIPPER SHOULD BRING ANOTHER CHANCE FOR SOME LIGHT SNOW EARLY NEXT WEAK...BUT AGAIN...ANY ASSOCIATED SNOW SHOULD BE LIGHT. KREIN && .AVIATION... //ORD AND MDW CONCERNS...UPDATED 09Z... * ONCL -SN THROUGH EARLY MORNING HOURS WITH MVFR/BRIEF IFR VSBYS * MVFR CIGS DEVELOP THIS AM AND CONTINUE INTO AFTERNOON IZZI //DISCUSSION...UPDATED 06Z... SFC HIGH PRES CONTINUES TO SLIDE SE AWAY FROM THE GREAT LAKES REGION WITH A WEAK ELEVATED LOW PRESSURE PUSHING EAST FROM THE DAKOTAS. AS THE HIGH PRES DEPARTS THE REGION...WINDS WILL CONTINUE TO BACK FROM THE SW TO S. CIGS WILL STEADILY LWR AS ADDTL MOISTURE POOLS EAST FROM APPROACHING UPPER LVL LOW PRES. LGT SN HAS CONTINUED TO FALL ACROSS MANY UPSTREAM LOCATIONS IN IA/SOUTHERN MN/FAR NW ILLINOIS ASSOCIATED WITH THIS WEAK LOW PRES APPROACHING THE REGION. EXPECT LGT SN TO OVERSPREAD NORTHERN IL AIRFIELDS...WITH PSBLY A 08-12Z WINDOW OF STEADIER SN. IT CURRENTLY DOESN/T APPEAR VSBYS WILL REDUCE TO 3SM...SO HAVE HELD VSBYS AT 4SM WITH THE STEADIER SN BTWN 10-12Z. WITH HOW DRY THE ATMOS CURRENTLY IS...SN IS EXPECTED TO REMAIN IN POWDER FORM OR FLUFFY SN. CIGS WITH THIS AREA OF SN SHUD REDUCE TO ARND 2KFT AGL. GUIDANCE HAS SLOWLY BACKED OFF ON A QUICK EROSION OR PROGRESSIVE DRY AIR ARRIVAL...WHICH HAS RESULTED IN A LOWER CONFIDENCE FOR A RETURN TO VFR CIGS. HAVE HELD ONTO REDUCED CIGS OR MVFR THRU 16Z FOR MOST AIRFIELDS. WINDS THEN TURN W/NW AFT DAYBREAK SAT. BEACHLER //ORD AND MDW CONFIDENCE...UPDATED 09Z... * HIGH CONFIDENCE IN MVFR/BRIEF IFR VSBY IN -SN THRU EARLY AM * HIGH CONFIDENCE IN MVFR CIGS THIS AM * MEDIUM CONFIDENCE IN MVFR CIGS LINGERING INTO THE AFTERNOON IZZI //OUTLOOK FOR ORD/MDW FOR 12Z SUNDAY-12Z THURSDAY...UPDATED 06Z... SUNDAY...VFR. PSBL MVFR WITH LGT SN. MONDAY...VFR. PSBL MVFR WITH LGT SN. TUESDAY...EARLY LGT SN AND MVFR TRENDING VFR. WEDNESDAY...VFR. THURSDAY...VFR. BEACHLER && .MARINE... 314 AM CST NO GALES CURRENTLY EXPECTED THROUGH 1ST HALF OF NEXT WEEK. A FRESH SW WIND THIS MORNING WILL VEER NW TODAY WITH WEAK FRONT MOVING ACROSS THE LAKE. WINDS WILL SLACKEN AND VEER AROUND THE COMPASS SUNDAY AS NEXT LOW APPROACHES FROM THE NORTHWEST. THIS LOW WILL CAUSE SOUTH WINDS TO FRESHEN UP SUNDAY NIGHT INTO MONDAY BEFORE WINDS SUBSIDE AND BECOME WESTERLY IN WAKE OF THIS SYSTEM BY TUESDAY AFTERNOON. BASED ON HIGH RESOLUTION VISIBLE MODIS IMAGERY FROM UNIV OF WISCONSIN FRIDAY AFTERNOON HAVE OPTED TO REMOVE WAVES FROM NSH FORECAST AS MAJORITY OF THE NEARSHORE WATERS ARE NOW ICE COVERED. IZZI && .LOT WATCHES/WARNINGS/ADVISORIES... IL...NONE. IN...NONE. LM...NONE. && $$  FXUS63 KLOT 310937 AFDLOT AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL 337 AM CST MON JAN 31 2011 ...MULTIFACETED...DANGEROUS AND POTENTIALLY LIFE THREATENING WINTER STORM TAKING AIM ON REGION... .DISCUSSION... SHORT TERM (MONDAY THROUGH WEDNESDAY).... THE HEADLINER OF WINTER EVENTS DOESNT ARRIVE UNTIL TUESDAY AFTERNOON...HOWEVER FIRST WE GET TO DEAL WITH AN OPENING ACT TONIGHT... LEAD SHORTWAVE IN THE SOUTHWESTERLY FLOW ALOFT IS SEEN ON WATER VAPOR IMAGERY EARLY THIS MORNING MOVING WESTERN KS/PANHANDLE REGION. AS THIS SHORTWAVE RIDES SW FLOW INTO THE REGION TONIGHT LOOK FOR SNOW TO BREAK OUT AS IT BEGINS TO TAP INTO BETTER MOISTURE SUPPLY AS A RESULT OF 30-40KT LOW LEVEL JET. BROAD REGION OF ISENTROPIC ASCENT AND LACK OF WELL DEFINED FOCUS MAKES QPF FORECAST TONIGHT TRICKY...HOWEVER MODELS DO SEEM TO BE KEYING IN ON ZONE OF STRENGTHENING LOW LEVEL FRONTOGENESIS OVER SOUTHERN CWA FOR SOMEWHAT HIGHER QPF AMOUNTS. SOUTHERN CWA WILL ALSO HAVE WARMER TEMP PROFILE AND WITH MODELS FREQUENTLY UNDERESTIMATING MAGNITUDE OF WARMING IN STRONG WAA REGIMES HAVE OPTED TO INTRODUCE CHC OF SLEET WITH THE SNOW OVER FAR SOUTHERN CWA TONIGHT. WARMER THERMAL PROFILE AND POTENTIAL FOR SLEET BOTH ARGUE FOR LOWER SNOW:LIQUID RATIOS SOUTH WHERE HIGHER QPF OCCURS. WHILE LOWER QPF AND HIGHER SLR'S FARTHER NORTH SUPPORT SIMILAR SNOWFALL TOTALS AND GENERALLY STICKING WITH A 1-3 INCH FORECAST TONIGHT...THOUGH LOCAL 4 INCH AMOUNTS WOULDNT BE AT ALL SURPRISING. LEAD SHORTWAVE MOVES EAST OF THE AREA BY EARLY TUESDAY MORNING WITH SNOW TAPERING OFF/ENDING AND POSSIBLY EVEN TRANSITIONING TO A PERIOD OF FREEZING DRIZZLE TUESDAY MORNING INTO THE EARLY AFTERNOON. HOPEFULLY ANY FRDZ WILL BE INSIGNIFICANT ENOUGH TO PROVIDE ROAD CREWS A CHANCE TO CATCH A FEW HOURS OF SLEEP EARLY TUESDAY BEFORE THE BIG HULLABALOO STARTS TUESDAY. OVERALL AGREEMENT IN MODELS ON SYNOPTIC DETAILS ASSOCIATED WITH THIS SYSTEM ARE EXCELLENT...WITH JUST THE 00Z NAM A LITTLE WEAKER/FASTER WITH THE SYSTEM AND LARGELY BEING DISCOUNTED. I CANNOT STRESS THIS POINT ENOUGH: MODEL ACCURACY AT THIS DISTANCE IS NOT SUCH THAT ITS WISE TO OFFER ANY DEGREE OF CERTAINTY WITH RESPECT TO SNOWFALL TOTALS. STRONG SYNOPTIC SYSTEMS...SUCH AS THIS ONE...MID-LEVEL DRY INTRUSIONS TYPICALLY RESULT IN VERY TIGHT GRADIENTS IN SNOWFALL TOTALS ALONG THE SOUTH EDGE OF THE HEAVY SNOW BAND. EVEN WITH VERY STRONG MODEL AGREEMENT...IT IS NOT UNCOMMON TO SEE FORECAST STORM TRACKS IN MODELS SHIFT BY 100 MILES OR MORE IN AS LITTLE AS 24 HOURS FROM THE ARRIVAL OF THE STORM WITH RAPIDLY DEEPENING/OCCLUDING LOWS. SUCH A SHIFT IN STORM TRACK COULD RESULT IN AREAS FORECAST TO GET 1 TO 2 FEET OF SNOW ENDING UP DRY SLOTTED WITH DRASTICALLY LIGHTER ACCUMULATIONS. GIVEN THE LINGERING UNCERTAINTY...NO PLANS TO MAKE UPGRADE ANY OF THE WATCH AT THIS TIME. HAVING SAID ALL THIS...HERE ARE THE HIGHLIGHTS OF THE STORM AS THEY APPEAR NOW... SNOWFALL: INTENSE 60KT LOW LEVEL JET WILL BE VERY EFFICIENT IN TRANSPORTING GULF MOISTURE NORTHWARD...SUPPLYING THE CYCLONE WITH AMPLE MOISTURE BOTH WITHIN THE WARM CONVEYOR BELT AND WITHIN THE DEVELOPING TROWAL/DEFORMATION ZONE. QPF IN REASONABLY GOOD AGREEMENT AMONG THE VARIOUS MODELS AND USING A REASONABLY CONSERVATIVE 10-12:1 SLR RESULTS IN SNOWFALL TOTALS OF 8-14 INCHES TUESDAY NIGHT ALONE. PROBABLY WORTH NOTING THAT CALCULATING/MEASURING THE ACTUAL SNOW:LIQUID RATIO THAT OCCURS MAY WELL BE DIFFICULT OR NEARLY IMPOSSIBLE GIVEN THE EXTENT OF BLOWING/DRIFTING EXPECTED. IN ADDITION...VERY STRONG WINDS COULD RIP DENDRITES APART...FURTHER LOWERING THE EFFECTIVE SNOW:LIQUID RATIO. SOME ADDITIONAL ACCUMULATING SNOW IS LIKELY WEDNESDAY MORNING...WITH HEAVIEST TOTALS DOWNWIND OF LAKE MICHIGAN IN NE IL AS IMPROVING THERMAL PROFILES RESULT IN INCREASINGLY FAVORABLE LAKE EFFECT SET UP. LAKE EFFECT SHOULD SHIFT EAST INTO NW INDIANA DURING THE AFTERNOON WEDNESDAY WITH STRENGTHENING COLD AIR ADVECTION RESULTING IN INTENSIFYING LAKE EFFECT SNOW SHOWERS. LAKE EFFECT ALONE COULD RESULT IN SEVERAL MORE INCHES OF ACCUMULATION IN NE IL AND PROBABLY MORE THAN THAT IN NW INDIANA. AS MENTIONED EARLIER THERE WILL BE A SHARP NORTH-SOUTH SNOWFALL GRADIENT AND WITH GFS /WRF-NAM BOTH SUGGESTING DRY INTRUSION MAKING IT INTO SOUTHERN CWA LATE TUES NIGHT...CONFIDENCE IN TOTALS OVER A FOOT CWA-WIDE IS MODERATE. WIND: DEEPENING LOW SHOULD RESULT IN STRONG PRESSURE GRADIENT TUESDAY NIGHT...WITH STRONG WINDS CONTINUING INTO WEDNESDAY AS STRONG ISALLOBARIC COMPONENT KICKS IN WITH QUICK DEPARTURE AND FILLING OF THE LOW. FORECAST SOUNDINGS FROM 00Z GUIDANCE SUGGESTS WINDS WILL INCREASE TO 25-35 MPH WITH GUSTS OF 40 TO POSSIBLY 50 MPH COMMON. HOWEVER...NEAR THE LAKE WINDS WILL BE EVEN STRONGER...PROBABLY 30-40 MPH SUSTAINED WITH GUSTS OF 50-60 MPH POSSIBLE. BLIZZARD CONDITIONS: VERY HIGH QPF TOTALS RAISE DOUBTS ABOUT THE BLOW-ABILITY OF THE SNOW THAT FALLS. HOWEVER SNOWFALL RATES OF 2-3 INCHES PER HOUR APPEAR LIKELY FOR A TIME TUESDAY NIGHT WHICH WILL PROBABLY BE ENOUGH TO KNOCK VISIBILITIES DOWN TO LESS THAN 1/4SM SATISFYING THE BLIZZARD WARNING CRITERIA. FURTHER INCREASING THE THREAT OF NEAR ZERO VISIBILITIES IS THE POTENTIAL FOR THUNDERSNOW. WILL BE A VERY DYNAMIC STORM SYSTEM WITH INTENSE UPWARD VERTICAL VELOCITIES AS INTENSE FRONTOGENESIS WORKS IN CONCERT WITH TREMENDOUS UPWARD MOTION RESULTING FROM FAST MOVING...RAPIDLY DEEPENING/CLOSING OFF MID LEVEL CIRCULATION BECOMING NEGATIVELY TILTED AS IT APPROACHES FROM THE SOUTH. FORECAST SOUNDINGS SUGGEST THAT 700-500MB LAPSE RATES WITHIN THE DRY INTRUSION WILL INCREASE TO BETWEEN 7 AND 8C/KM...WITH SOME OF THAT INSTABILITY LIKELY GETTING TAPPED INTO AND PROBABLY RESULTING IN EMBEDDED THUNDERSTORMS. LAKESHORE FLOODING: NORTHEAST WINDS PROGGED TO INCREASE TO 30-40 MPH WITH GUSTS OF 50-60 MPH MAKES LAKESHORE FLOODING A BIG CONCERN. HIGH RESOLUTION MODIS VISIBLE SATELLITE IMAGERY FROM JAN 28TH SHOWED THAT ICE THAT HAD ACCUMULATED NEAR THE IL SHORE HAD BEEN BLOWN WELL OFFSHORE...AND APPEARED SOMEWHAT FRAGMENTED IN NATURE EAST OF THE ILLINOIS SHORE. THERE IS SOME UNCERTAINTY WITH RESPECT TO HOW MUCH THAT ICE WILL RETARD WAVE DEVELOPMENT NEAR THE SHORE...BUT WITH 14-18FT SIGNIFICANT WAVES AND OCCASIONAL WAVES OVER 25FT OFFSHORE POTENTIALLY TAKING A TOLL ON ICE HAVE OPTED TO ERR ON THE SIDE OF CAUTION AND HOIST A LAKESHORE FLOOD WATCH. ITS POSSIBLE THAT ICE COULD MITIGATE THE COASTAL FLOOD POTENTIAL...BUT HARD TO SAY FOR SURE AND IF ICE ISNT A BIG DETERRENT THEN THE COASTAL FLOODING COULD BE VERY SIGNIFICANT. IZZI TEMPERATURES AND LONG TERM... MONDAY... THE 850 MB PROFILER DATA AT 07 UTC SHOWS THE WIND BECOMING SOUTH OVER CENTRAL ILLINOIS. THE WARM AIR ADVECTION IS BEGINNING AND WE EXPECT IT TO CONTINUE THIS AFTERNOON. WE WILL FORECAST HIGH TEMPERATURES IN 20 TO 25 RANGE TODAY AND TUESDAY. A SMALL 500 MB RIDGE WILL MOVE OVER ILLINOIS AND NORTHWEST INDIANA TODAY. THE FORECAST FRONTOGENESIS VECTORS AT 850 MB SHOW A FRONT DEVELOPING ALOFT LATER TODAY. THIS IS SEEN USING OUR LOCAL WRF. THIS LOCAL WRF IS FORECASTING SNOW TONIGHT. TUESDAY THROUGH WEDNESDAY...THE LONGWAVE TROUGH WILL CONTINUE TO DIG IN ARIZONA AND NEW MEXICO. HEIGHT FALLS WERE LARGE OVER ARIZONA AT 00 UTC. A 70 KNOT JET STREAK WAS SEEN MOVING INTO THE NORTHERN CALIFORNIA COAST AT 00 UTC. SEEN ON THE WATER VAPOR LOOPS THIS JET IS MOVING INTO SOUTHERN CALIFORNIA ON THE WEST SIDE OF THE LONGWAVE. WARM AIR ADVECTION OVER TEXAS AND NEW MEXICO EXTENDS INTO KANSAS AND OKLAHOMA BY 07 UTC. THERE WILL BE AN INCREASE OF KINETIC ENERGY WITH THIS WAVE AND A CYCLONE IS FORECAST TO DEVELOP OVER TEXAS. THE 07 UTC SURFACE MAP SHOWS AN INCREASE IN THE SURFACE TEMPERATURE GRADIENT OVER ARKANSAS THROUGH WEST VIRGINIA. VERY COLD AIR IS SEEN MOVING INTO MONTANA AND THE DAKOTAS. THIS CYCLONE IS FORECAST TO BECOME INTENSE OVER SOUTHERN ILLINOIS OR SOUTH CENTRAL INDIANA TUESDAY NIGHT. THE 300 MB JET STREAK IS EXPECTED TO COUPLE WITH THE POLAR JET OVER THE GREAT LAKES. THE LOCATION OF THE TWO JET MAXIMUM WIND SPEEDS MAY PLACE THE MAXIMUM UPPER AIR DIVERGENCE IN A BAND OVER CENTRAL ILLINOIS TO NORTHERN INDIANA. SOME CONVEYOR BELT PRECIPITATION WILL OCCUR ALSO. THIS IS SEEN IN THE FORECAST FROM THE ECMWF AND GFS MODELS ISENTROPIC SURFACE FORECASTS AND WIND FORECAST AT 300 MB. THE 200 MB ANALYSIS SHOW COLD AIR OVER THE REGION FROM NORTHERN COLORADO TO NORTHERN OHIO AT 00 UTC. THERE IS WARM AIR ADVECTION IN ALL OF THE WESTERN UNITED STATES AND BRITISH COLUMBIA. THIS IS MORE EVIDENCE OF THE STRENGTH OF THE DEVELOPING 500 MB LONG WAVE TROUGH. THE LOW TEMPERATURE WILL BE IN THE TEENS. WITH THE SNOW OCCURRING WE EXPECT THE TEMPERATURES TO BE IN THE MIDDLE 20S WEDNESDAY. BUT LATE WEDNESDAY AND WEDNESDAY NIGHT THE TEMPERATUES MAY FALL TO BELOW ZERO IN NORTH CENTRAL ILLINOIS TO NORTHWEST INDIANA. SINGLE DIGITS ARE FORECAST FOR ALONG THE LAKE SHORE. THE NORTHWEST WIND WILL SET UP A LAKE EFFECT SNOW OVER NORTHWEST INDIANA WEDNESDAY NIGHT. THURSDAY THROUGH SUNDAY...AS THE CYCLONE MOVES INTO OHIO AND PENNSYLVANIA BY WEDNESDAY NIGHT. VERY COLD AIR WILL MOVE INTO NORTHERN ILLINOIS AND NORTHWEST INDIANA. STRONG NORTHWEST WINDS WILL SET UP A SIGNIFICANT LAKE SNOW FOR NORTHWEST INDIANA EARLY THURSDAY. A LARGE HIGH WILL MOVE INTO NEBRASKA TO NORTHERN TEXAS. THIS HIGH WAS A VERY COLD CONTINENTAL ARCTIC AIR MASS IN MANITOBA AND SASKATCHEWAN AT 07 UTC TODAY. AS THIS AIR MASS MOVES SOUTH IT WILL MODIFY A LITTLE...BUT THE COLD AIR WILL BE OVER A LARGE AND DEEP SNOW PACK. WE WILL FORECAST SINGLE DIGIT HIGH TEMPERATURES FOR THURSDAY. BY FRIDAY...A WEST WIND IS FORECAST OVER IOWA AND ILLINOIS. THE TEMPERATURES MAY BE IN THE 20S FRIDAY. THE 500 MB LONG WAVE TROUGH WILL BE MOVING THROUGH ILLINOIS AND INDIANA FRIDAY. BY SATURDAY...THE TROUGH IS EXPECTED TO BE OVER OHIO AND PENNSYLVANIA. WE USED THE ECMWF AND GFS MODELS FOR THIS FORECAST. A RIDGE IS EXPECTED TO BUILD OVER IOWA AND MINNESOTA WHICH IS A REFLECTION OF THE WARM AIR ADVECTION AT THE SURFACE TO 700 MB. WE WILL FORECAST A SLIGHT CHANCE OF SNOW SATURDAY WITH THE HIGH TEMPERATURES IN THE MIDDLE 20S. THE HIGH TEMPERATURES WILL BE IN THE UPPER 20S SUNDAY. && .AVIATION... //ORD AND MDW CONCERNS...UPDATED 08Z... * MVFR CIGS ABV015 THRU PREDAWN HRS...RETURN BY EARLY EVE. * GRADUALLY INCREASING EASTERLY WINDS BECOMING NORTHEAST TONIGHT. * LIGHT SNOW SPREADING W TO E ACROSS REGION...REACHING CHI AREA BY EARLY EVE. TRS //DISCUSSION...UPDATED 08Z... NO SIGNIFICANT CHANGES TO FORECAST EXCEPT TO TWEAK NEXT FEW HRS TO MESH WITH CURRENT CONDITIONS. PREVIOUS DISCUSSION...UPDATED 06Z... CURRENT SATELLITE IMAGERY SHOWS LAKE EFFECT CLOUDINESS CONTINUING TO STREAM WWD/SWWD INTO SERN WI...NERN IL. PERSISTENT ENELY LOW LEVEL WINDS ARE HELPING TO ERODE LOWER MVFR CLOUD DECK. AT 05Z...THE BACK EDGE OF LOWER CLOUDS EXTENDED FROM CENTRAL LAKE MICHIGAN...TO JUST E OF GYY...TO NEAR PNT. THIS BACK EDGE IS EXPECTED TO CONTINUE CLEARING WWD...MOVING THROUGH NERN IL BY 09Z. OVERLYING MID/HIGH LEVEL CLOUD DECK ASSD WITH A DISTURBANCE APPROACHING FROM THE PLAINS STATES WILL BE THICKENING AND LOWERING ACROSS AREA BY 12Z. PREVAILING VFR CONDITIONS ARE EXPECTED UNTIL LATE MONDAY. ON MONDAY...A WEAK WEATHER DISTURBANCE MAKING ITS WAY ACROSS THE NRN HIGH PLAINS IS FORECAST TO INTERACT WITH INCREASINGLY MOIST AIR ALOFT OVER THE MS VALLEY. THIS IS FORECAST TO ALLOW AN AREA OF LIGHT SNOW TO OVERSPREAD THE REGION FROM THE WEST LATE MONDAY AFTERNOON...OR MONDAY EVENING...WITH CIGS/VIS LOWERING TO MVFR BY 03Z TUE. WINDS WILL REMAIN ENELY THROUGH MONDAY AFTERNOON...WITH SPEEDS INCREASING AFTER 15Z MON...TO FQT GUSTS AOA 20 KT. MERZLOCK //ORD AND MDW CONFIDENCE...UPDATED 08Z... * MEDIUM ON TIMING OF IMPROVEMENT TO VFR AND ON RETURN OF MVFR AND IFR THIS EVE. * HIGH ON WINDS. * MEDIUM ON ONSET OF -SN LATER AFTERNOON-EARLY EVE. TRS //OUTLOOK FOR ORD/MDW FOR 12Z TUESDAY-12Z SUNDAY...UPDATED 00Z... TUESDAY...LIGHT SNOW IN THE MORNING. HEAVY SNOW DEVELOPING BY TUESDAY EVENING WITH POSSIBLE BLIZZARD CONDITIONS. WINDS GUSTING AS HIGH AS 35 KT TUESDAY EVENING/TUESDAY NIGHT. CONDITIONS EXPECTED TO DETERIORATE TO VLIFR BY 00Z WED. WEDNESDAY...SNOW ENDING MIDDAY. BLOWING SNOW. WINDS DIMINISHING IN THE AFTERNOON AND EVENING. VLIFR-IFR...IMPROVING TO MVFR LATE. THURSDAY...VFR. FRIDAY...SLIGHT CHANCE OF SNOW. MVFR POSSIBLE. SATURDAY...VFR. MERZLOCK && .MARINE... 335 AM...HIGH END GALES EXPECTED TUESDAY NIGHT AND WEDNESDAY. MANY FORECAST CHALLENGES MAINLY DEALING WITH STRONG WINTER STORM THAT WILL IMPACT THE REGION THIS WEEK. MAIN VORT ASSOCD WITH THIS SYSTEM CURRENTLY OVER SOUTHWEST US. SFC LOW WILL EMERGE OVER TEXAS EARLY TUESDAY MORNING AND TRACK NORTHEAST...DEEPENING AS IT MOVES ACROSS THE OHIO RIVER VALLEY LATE TUESDAY NIGHT. MEANWHILE...STRONG AREA OF HIGH PRESSURE WILL BUILD ACROSS THE NORTHERN PLAINS SETTING UP TIGHT PRESSURE GRADIENT WHICH WILL LEAD TO GALES ACROSS LAKE MICHIGAN. HEAVY SNOW...AND HEAVY FREEZING SPRAY WILL ALSO BE PART OF THE FORECAST FOR THE FIRST HALF OF THE WEEK. FOR MONDAY THROUGH EARLY TUESDAY...RIDGE OF HIGH PRESSURE EXTENDS ACROSS THE GREAT LAKES WITH EAST TO NORTHEASTERLY FLOW ACROSS THE LAKE. WINDS WILL GRADUALLY INCREASE...ESPECIALLY AS WE HEAD THROUGH THE DAY TUESDAY AS AREA OF LOW PRESSURE DEVELOPS ACROSS THE SOUTH-CENTRAL CONUS. AREA OF WARM ADVECTION SNOWFALL WILL DEVELOP ACROSS THE SOUTHERN HALF OF LAKE MICHIGAN EARLY TUESDAY MORNING BEFORE A BRIEF LULL IN THE PRECIPITATION TUESDAY AFTERNOON. TUESDAY AFTERNOON THROUGH WEDNESDAY WILL SEE THE WORST CONDITIONS. STRONG FORCING WILL LEAD TO PERIODS OF MODERATE AND HEAVY SNOWFALL. TIGHT PRESSURE GRADIENT WILL LEAD TO GALES BEGINNING OVER SOUTHERN LAKE MICHIGAN TUESDAY AFTERNOON AND ACROSS THE ENTIRE LAKE BY TUESDAY EVENING. MODEL GUIDANCE CONTINUES TO SHOW NORTHERLY WINDS GUSTING TO AROUND 45 KTS ACROSS THE SOUTHERN HALF OF THE LAKE EARLY WEDNESDAY MORNING. GUSTS STILL LOOK LIKE THEY WILL APPROACH STORM FORCE CRITERIA. LATEST NAM GUIDANCE ACTUALLY BACKED OFF A COUPLE KNOTS WHILE GFS WENT STRONGER SHOWING STORM FORCE WINDS FOR ABOUT 6 HRS OR SO AFTER MIDNIGHT TUESDAY NIGHT. MODELS CONTINUE TO BOUNCE AROUND ON THE TRACK OF LOW...SO WILL HOLD OFF FOR NOW ON UPGRADE TO STORM WATCH. COLD AIR ADVECTING OVER THE LAKE DURING THIS TIME WILL LEAD TO HEAVY FREEZING SPRAY. ICE...WHICH MOVED FARTHER OFFSHORE THIS WEEKEND...SHOULD BE PUSHED BACK INTO THE NEARSHORE ZONES AS WINDSPEEDS INCREASE. THIS SHOULD PROHIBIT WAVE ACTION IN THE NEARSHORES DURING THE PERIOD OF STRONGEST WINDS. BMD && .LOT WATCHES/WARNINGS/ADVISORIES... IL...BLIZZARD WATCH...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010- ILZ011-ILZ012-ILZ013-ILZ014-ILZ019-ILZ020-ILZ021-ILZ022- ILZ023-ILZ032-ILZ033-ILZ039...NOON TUESDAY TO 6 PM WEDNESDAY. IN...BLIZZARD WATCH...INZ001-INZ002-INZ010-INZ011-INZ019...NOON TUESDAY TO 6 PM WEDNESDAY. LM...GALE WATCH...LMZ080-LMZ567-LMZ669-LMZ671-LMZ673-LMZ675-LMZ740- LMZ741-LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-LMZ779-LMZ868- LMZ870-LMZ872-LMZ874-LMZ876-LMZ878...NOON TUESDAY TO 6 PM WEDNESDAY. GALE WATCH...LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565...6 PM TUESDAY TO 6 PM WEDNESDAY. SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744- LMZ745...3 PM MONDAY TO NOON TUESDAY. && $$  FXUS63 KLOT 190928 AFDLOT AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL 328 AM CST SAT FEB 19 2011 .DISCUSSION... 310 PM CST MAIN CONCERNS FOR THE CURRENT FORECAST PACKAGE DEAL WITH PRECIP AMOUNT AND PRECIP TYPE OVER THE WEEKEND. MODELS CONTINUE TO VARY SOME ON TRACK OF LOW...BUT STILL SEE POTENTIAL FOR MIXED PRECIP EVENT OVER THE WEEKEND AS WELL AS SOME DECENT RAINFALL AMOUNTS THAT WILL ADD TO FLOODING ISSUES ACROSS THE UPPER MIDWEST. REST OF TODAY AND TONIGHT...RIDGE OF HIGH PRESSURE EXTENDS SOUTHEAST ACROSS THE NORTHERN PLAINS NOSING INTO WESTERN ILLINOIS THIS AFTERNOON PROVIDING SUNNY SKIES AND BRISK NORTHWESTERLY FLOW. TEMPERATURE FORECAST FOR TONIGHT WILL BE A BIT TRICKY. SOME HIGH CLOUDS RIDING OVER THE RIDGE WITH MID DECK ARRIVING LATER TONIGHT AS WELL AS THE STRONGER WINDS ACROSS THE REGION WILL ALL HELP TO DELAY/PREVENT DECOUPLING TONIGHT. QUITE A BIT OF RANGE IN THE GUIDANCE...BUT DECIDED TO GO SOMEWHAT ON THE WARM SIDE...A FEW DEGREES WARMER THAN MOS TEMPS. THIS WEEKEND...DIGGING TROF EVIDENT ON WATER VAPOR IMAGERY OFF THE CALIFORNIA COAST WILL QUICKLY LIFT NORTHEAST REACHING THE CENTRAL PLAINS BY SATURDAY NIGHT WHILE SURFACE LOW UNDERGOES CYCLOGENESIS. ISENTROPIC UPGLIDE GRADUALLY SATURATES ATMOSPHERE OVERNIGHT SATURDAY WITH PRECIP BEGINNING ACROSS THE FORECAST AREA AFTER MIDNIGHT. AGAIN...IMPORTANT DISCREPANCIES IN THE LOW TRACK CONTINUE BUT MODELS DID SHIFT BACK SOUTH FROM YESTERDAYS TRACK. GFS/NAM/ECWMF COMING INTO SOMEWHAT BETTER AGREEMENT WITH LOW TRACKING ACROSS THE NORTHERN THIRD OF ILLINOIS WHILE SREF TAKES A WORST CASE TRACK ACROSS CENTRAL ILLINOIS. BASED ON CURRENT NAM/GFS BUFR SOUNDINGS AND ANTICIPATED POSITION OF WARM FRONT...LOOKS LIKE PRECIP WILL MOSTLY BE IN THE FORM OF RAIN. THE EXCEPTION/MAIN CONCERN RIGHT NOW WILL BE THE NORTHERN COUPLE TIERS OF COUNTIES SUNDAY MORNING. SOUNDINGS SHOW A SUFFICIENTLY DEEP COLD LAYER BELOW ELEVATED WARM LAYER THAT WILL REFREEZE FALLING PRECIP. PER THIS SOLUTION...SLEET WOULD BE EXPECTED SUNDAY MORNING THROUGH EARLY SUNDAY AFTERNOON BEFORE BECOMING ALL RAIN. SHOULD SREF SOLUTION PAN OUT...MAIN THREAT WOULD BE FREEZING RAIN WHICH WOULD FALL ACROSS FAR NORTHERN ILLINOIS. RIGHT NOW...SREF SOLUTION IS AN OUTLIER. PRECIPITATION AMOUNTS ALSO A BIG CONCERN GOING INTO THE WEEKEND. RIGHT NOW IT LOOKS QUITE LIKELY THAT A SWATH OF HALF INCH COULD FALL ACROSS THE NORTHERN PORTIONS OF THE OUTLOOK AREA...WITH THE POTENTIAL FOR OVER AN INCH. EVEN IF THE HIGHER 1+ INCH VALUES FALL NORTH OF THE FORECAST AREA ACROSS SOUTHERN WISCONSIN...THIS COULD STILL TRIGGER PROBLEMS DOWNSTREAM ALONG THE FOX...DES PLAINES...AND ROCK RIVER VALLEYS. MANY LOCATIONS WILL REACH ACTION STAGE OR FLOOD STAGE FROM SNOWMELT ALONE THROUGH SATURDAY AND ANY ADDITIONAL PRECIPITATION WILL QUICKLY BECOME RUNOFF ATOP THE SATURATED/FROZEN GROUND. NEXT WEEK...WILL SEE SOME LINGERING LIGHT SNOW ON MONDAY BUT NOT EXPECTING MUCH IN THE WAY OF ACCUMULATION. SURFACE HIGH BUILDS ACROSS THE REGION TUESDAY AND WEDNESDAY PROVIDING FAIR WEATHER BUT SEASONABLY COOL TEMPERATURES. RIDGE WILL SLIDE EAST THROUGH MIDWEEK ALLOWING SOUTHERLY FLOW AND A GRADUAL WARMUP BEFORE ANOTHER LOW PRESSURE SYSTEM IMPACTS THE AREA BY THE END OF THE WEEK. BMD && .HYDROLOGY... 500 AM CST NO SIGNIFICANT CHANGES TO HYDRO CONCERNS AT THIS TIME. RUNOFF FROM SNOW MELT WILL CONTINUE TO RESULT IN INCREASING FLOWS ON AREA RIVERS. THE COMBINATION OF THE RELATIVELY GRADUAL MELT AND LOW INITIAL FLOW CONDITIONS ALONG WITH ONLY LIGHT RAINFALL AMOUNTS OVER THE PAST FEW DAYS SUGGEST THAT THERE SHOULD BE NO SIGNIFICANT IMMEDIATE PROBLEMS. HOWEVER...CONDITIONS ON AREA RIVERS WILL CONTINUE TO BE CLOSELY MONITORED FOR FLOOD POTENTIAL AS MILDER TEMPERATURES AND INCREASED STREAM FLOW WILL RESULT IN BREAK UP OF ANY REMAINING ICE ON AREA RIVERS. THE ICE BREAKUP CAN POSSIBLY RESULT IN LOCALIZED ICE JAM FLOODING. ALSO...THE LONGER RANGE MODELS ARE ADVERTISING AN ACTIVE WEATHER PATTERN THAT COULD RESULT IN A FEW EPISODES OF PRECIPITATION THROUGH NEXT WEEK. THERE ARE STILL QUESTIONS ON THE PRECIPITATION TYPE DUE TO UNCERTAINTY IN THE TRACK OF THE LOW. HOWEVER...IT DOES APPEAR THAT NORTHERN ILLINOIS WILL EXPERIENCE A SIGNIFICANT PRECIPITATION EVENT THAT COULD RESULT IN OVER AN INCH OF LIQUID FROM LATE SATURDAY THROUGH MONDAY. LATER NEXT WEEK YET ANOTHER SYSTEM COULD BRING MORE DECENT PRECIPITATION TO THE AREA. THESE EPISODES HAVE THE POTENTIAL TO LEAD TO MORE SIGNIFICANT RISES ON AREA RIVERS AND STREAMS. KREIN/KJB && .AVIATION... //ORD AND MDW CONCERNS...UPDATED 09Z... * DECEASING NORTHWEST WIND BECOMING NORTHEAST THIS MORNING. //DISCUSSION...UPDATED 06Z... A LARGE HIGH OVER SASKATCHEWAN EXTENDS SOUTHEAST TO FLORIDA TONIGHT THIS LARGE AREA OF HIGH PRESSURE WILL EXTEND FROM SASKATCHEWAN AND MANITOBA TO SOUTH CAROLINA BY SATURDAY AFTERNOON. THE WIND WILL DECREASE OVERNIGHT 9 TO 12 KNOTS AND WILL BECOME NORTH BY THURSDAY MORNING. THE WIND IS FORECAST TO BECOME NORTHEAST AT 7 TO 9 KNOTS LATE THURSDAY MORNING AND AFTERNOON. THE WIND WILL BECOME SOUTHEAST THURSDAY NIGHT. A LOW WILL INTENSIFY OVER COLORADO THURSDAY NIGHT. CLOUDS AND VISIBILITY WILL REMAIN VFR THROUGH THE ENTIRE FORECAST PERIOD. THE OUTLOOK FOR LATE SATURDAY NIGHT SHOWS IFR CLOUDS AND A CHANCE OF RAIN...SLEET AND FREEZING RAIN AFTER MIDNIGHT. //ORD AND MDW CONFIDENCE...UPDATED 09Z... * HIGH CONFIDENCE IN THE WIND FORECASTS. * HIGH CONFIDENCE IN THE VFR FORECASTS. //OUTLOOK FOR ORD/MDW FOR 12Z SUNDAY-12Z FRIDAY...UPDATED 00Z... SUNDAY...IFR/LIFR. SLEET AND RAIN IN THE MORNING. RAIN IN THE AFTERNOON. RAIN AND SNOW SUNDAY NIGHT. MONDAY...IFR. CHANCE OF SNOW. TUESDAY...VFR. WEDNESDAY...VFR. CHANCE OF RAIN OR SNOW WEDNESDAY NIGHT. THURSDAY...MVFR. CHANCE OF RAIN OR SNOW. && .MARINE... 317 AM CST MAIN MARINE FORECAST ISSUES ARE DEALING WITH ENDING OF CURRENT GALE WARNING AND SMALL CRAFT ADVISORY HEADLINES THIS MORNING THROUGH MIDDAY TODAY...THEN POTENTIAL FOR NORTHEASTERLY GALES MAINLY ON NORTHERN LAKE MICHIGAN AGAIN FROM SUNDAY AFTERNOON THROUGH MIDDAY MONDAY. WINDS ARE GRADUALLY DIMINISHING ACROSS THE LAKE EARLY THIS MORNING AS LOW PRESSURE CONTINUES TO PULL AWAY ACROSS QUEBEC AND THE MARITIME PROVINCES...AND HIGH PRESSURE RIDGE SPREADS EAST FROM THE UPPER MIDWEST AND MISSISSIPPI RIVER VALLEY. STILL A FEW OBS INDICATING GUSTS OF GALE FORCE ACROSS FAR NORTHERN LAKE MICHIGAN AT 09Z/3 AM CST...SO HAVE EXTENDED GALE WARNING FOR NORTHERN LAKE MICHIGAN THROUGH 15Z/9 AM CST DURING WHICH TIME WINDS SHOULD CONTINUE TO SLOWLY DIMINISH. FARTHER SOUTH...WINDS DIMINISHING BELOW 20 KTS ACROSS FAR SOUTHERN LAKE MICHIGAN WHICH WILL ALLOW SMALL CRAFT ADVISORY TO EXPIRE FOR IL NEARSHORE WATERS AT 4 AM CST. WILL BE EXTENDING SMALL CRAFT ADVISORY THROUGH MIDDAY HOWEVER FOR NEARSHORE WATERS EAST OF GARY INDIANA HOWEVER...AS WAVES OF 4 FT OR HIGHER EXPECTED TO ONLY GRADUALLY SUBSIDE THERE AFTER LONG PERIOD OF STRONGER NORTHWEST FETCH. HIGH RESOLUTION MODIS POLAR ORBITING SATELLITE IMAGERY FROM YESTERDAY INDICATES LITTLE SIGNIFICANT ICE COVER IN IL/IND NEARSHORE WATERS. SFC HIGH PRESSURE RIDGE SLIDES ACROSS THE LAKE LATER TODAY...BEFORE LOW PRESSURE DEVELOPS TONIGHT ACROSS EASTERN COLORADO AND THEN MOVES EAST INTO IL/IND AND PASSES JUST SOUTH OF THE LAKE ON SUNDAY. AS WINDS SHIFT TO THE NORTHEAST AS THE LOW PASSES... MODEL GUIDANCE INDICATES A PERIOD OF NORTHEAST GALES UP TO 40 KTS OR SO ARE LIKELY FROM SUNDAY AFTERNOON THROUGH ABOUT MIDDAY MONDAY BEFORE THE GRADIENT SLACKENS. WILL MOST LIKELY SEE A PERIOD OF STRONGER WINDS AND HIGHER WAVES ESPECIALLY ALONG IL NEARSHORE AREAS AS EASTERLY FLOW STRENGTHENS SUNDAY AHEAD OF LOW...AND AGAIN MONDAY AS NORTHEAST/NORTH FLOW STRENGTHENS BEHIND THE DEPARTING SYSTEM. SMALL CRAFT ADVISORIES WILL LIKELY BE HOISTED DURING THIS PERIOD...WHICH WILL LIKELY STRETCH INTO TUESDAY BEFORE CONDITIONS IMPROVE. RATZER && .LOT WATCHES/WARNINGS/ADVISORIES... IL...WINTER STORM WATCH...ILZ003-ILZ004-ILZ005-ILZ006...3 AM SUNDAY TO 6 PM SUNDAY. IN...NONE. LM...GALE WARNING...LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-LMZ567- LMZ868 UNTIL 9 AM SATURDAY. SMALL CRAFT ADVISORY...NEARSHORE WINTHROP HARBOR TO GARY IN UNTIL 4 AM SATURDAY. SMALL CRAFT ADVISORY...NEARSHORE GARY TO MICHIGAN CITY IN UNTIL NOON SATURDAY. && $$  FXUS63 KLOT 050227 AAA AFDLOT AREA FORECAST DISCUSSION...UPDATED NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL 927 PM CDT WED JUL 4 2012 .DISCUSSION... 925 PM CDT ONLY MINOR CHANGES MADE TO THE GOING FORECAST WITH CONDITIONS GENERALLY ON TRACK WITH THE FORECAST. THE HOT TEMPERATURES OF TODAY ARE SLOWLY FALLING ACROSS THE AREA WITH DEW POINTS INCHING UP A FEW DEGREES AS THE MIXING OF THE DAY SUBSIDED. HEAT INDEX READINGS IN CHICAGO REMAIN RIGHT AROUND 100 AT 9 PM...WITH MINIMUM VALUES ONLY EXPECTED TO BE IN THE MID 80S. LOWS IN THE LOWER 80S IN CHICAGO WITH LOW TO MID 70S IN OUTLYING AREAS ARE STILL FAVORED. IT IS POSSIBLE FOR SOME SHALLOW LIGHT FOG TRAPPED UNDERNEATH THE THERMAL INVERSION IN OUTLYING AREAS...AS 9 PM OBSERVATIONS AT RPJ OF 80 OVER 76 AND PNT OF 82 OVER 77 HINT AT. LOOKING AHEAD TO THURSDAY...A LAKE BREEZE REMAINS FAVORED IN LATEST GUIDANCE ASSESSMENT WITH WIND FLOW IN THE MIXED LAYER AROUND 5-8 KT LESS THAN TODAY. SIMILAR TO SE WI TODAY WHERE THE SYNOPTIC WINDS WERE SLIGHTLY LESS...THE LAKE BREEZE IS FAVORED TO INCH VERY SLOWLY INLAND AGAINST THE WEST SOUTHWEST WIND. MUCH OF THE LAKE COUNTIES STILL WILL BE SEEING HEAT INDEX READINGS NEAR 110 DUE TO BOTH THE LIMITED INLAND IMPINGEMENT AND THE FACT THE WIND SHIFT MAY BE LATE ENOUGH AFTER THE HIGHS ARE REACHED /HIGH AT ORD TODAY WAS EARLY AT 133 PM/. CONVECTIVE CHANCES ALONG THE LAKE BREEZE ARE SMALL...BUT ARE NON-ZERO. THERE IS SLIGHT COOLING IN THE MID-LEVELS FORECAST FROM THIS EVES RAOB READINGS. FORECAST MLCIN IS NEAR ZERO IN NORTHEAST IL. THE 04.18 NAM HAD THE CENTRAL MN SHORT WAVE IMPULSE TONIGHT DRIFTING OVER THAT AREA DURING THE LATE AFTERNOON...WHILE THE MOST RECENT NAM HAS TRENDED SLIGHTLY FASTER. IF THAT SLIGHT IMPULSE CAN INTERACT OVER THE LAKE BREEZE CONVERGENCE...IT MAY NEGATE ANY CIN FOR WIDELY SCATTERED BUT PROBABLY STRONG THUNDERSTORM DEVELOPMENT. ALSO OF NOTE...ON THE MODIS AND GOES VISIBLE IMAGES TODAY AND THIS EVENING WAS QUITE A BIT OF SMOKE IN THE DAKOTAS...NE...MN...AND IA. NAM PARCEL TRAJECTORIES FROM 20000 FT AND HIGHER INDICATE THIS AREA SHOULD DRIFT OVER THE AREA THURSDAY...ALTHOUGH IT MAY BE MORE DISPERSED. THERE WERE NO INDICATIONS OF VISIBILITY REDUCTIONS IN THOSE UPSTREAM AREAS TODAY AND THIS EVE...BUT IF THAT WERE TO SETTLE CLOSER TO THE TOP OF THE MIXED LAYER THURSDAY...SLIGHTLY MORE HAZE AND POTENTIALLY WORSE AIR QUALITY COULD DEVELOP. MTF && .PREV DISCUSSION... 445 PM CDT FORECAST FOCUS SQUARELY ON CONTINUATION OF INTENSE HEAT THROUGH FRIDAY. AN EXCESSIVE HEAT WARNING IS IN EFFECT FOR THE ENTIRE FORECAST AREA THROUGH 10 PM CDT FRIDAY. AIR TEMPERATURES CLIMBED TO 95 TO 102 ACROSS THE FA THIS AFTERNOON WITH HEAT INDEX NUMBERS UP TO 103 TO 108. NORTHEASTWARD EXTENSION OF THE 850KPA THERMAL RIDGE THAT STRETCHES ACROSS THE MID MO VALLEY TO THE UPPER MIDWEST TODAY IS TO BE SUPPRESSED A BIT TO THE S ACROSS SOUTHERN WI AND NORTHERN IL FOR THU AND FRI AS THE UPPER RIDGE OVER THE UPPER MIDWEST AND WESTERN ONTARIO GETS FLATTENED BY THE UPPER LOW TRACKING E OVER THE CANADIAN PRAIRIE. EXPECT MAX TEMPS ON THU THAT ARE A DEG OR TWO ABOVE TODAY/S...AND THEN FOR FRI AN ADDITIONAL DEG HIGHER OVER THU AT MOST LOCATIONS. WHILE A WEAK LAKE BREEZE IS EXPECTED TO DEVELOP TOMORROW AFTERNOON IT IS NOT EXPECTED TO MOVE MORE THAN MARGINALLY INLAND UNTIL LATE IN THE AFTERNOON AND EARLY EVENING SO ANY RELIEF WILL BE VERY LIMITED AND SHORT LIVED AS LIGHT SW LOW LEVEL FLOW PUSHES THE HOT AIR RIGHT BACK OUT OVER THE SHORE LINE BY LATER THU EVENING. TIMING OF COOL FRONT AND LAKE BREEZE DURING FRI COMPLICATES THINGS A BIT BUT FEEL THAT THE ACTUAL FRONT WILL REACH NORTHERN IL UNTIL VERY LATE FRI OR MORE LIKELY FRI NIGHT. WEAK W TO SW LOW LEVEL FLOW DURING FRI WILL LIKELY ALLOW A LAKE BREEZE TO PUSH INLAND BUT NOT ANTICIPATED UNTIL DURING THE LATER AFTERNOON-EARLY EVENING SO FRI WILL BE ANOTHER VERY HOT DAY ACROSS THE VAST MAJORITY OF THE FA WITH THE FAR NE CORNER OF IL LIKELY THE ONLY PART OF THE FA TO GET ANY REAL RELIEF UNTIL THE ACTUAL COOL FRONT DROPS ACROSS THE AREA. WITH MAX TEMPERATURES GENERALLY IN THE 100 TO 105 RANGE THU AND FRI THE LOWERING OF SURFACE DEW POINTS BACK UPPER 60S AND LOWER 70S PER GUIDANCE WILL KEEP HI NUMBERS UP IN THE 105-115 ACROSS MUCH OF THE AREA THU AND FRI AFTERNOONS. WITH OVERNIGHT HEAT INDEX NUMBERS ONLY FALLING TO AROUND 75 TONIGHT AND THU NIGHT EXCESSIVE HEAT WARNING CRITERIA FOR ALL OTHER COUNTIES SURROUNDING COOK WHICH HAS IT/S OWN SET FOR THE CITY WILL BE REACHED SO ADVISORY EXPANDED ACROSS THE ENTIRE AREA. HEAT AND HUMIDITY RESULT IN LARGE VALUES OF POTENTIAL INSTABILITY BUT LOW LEVEL FOCUS AND MID-UPPER DISTURBANCES LACKING THUS ONLY SLIGHT CHANCES OF ISOLATED TS IF ALL INGREDIENTS COME TOGETHER AT THE RIGHT TIME AT A PARTICULAR SPOT. THUS...POP NUMBERS TOO LOW FOR ANY MENTION OF EVEN SLIGHT CHANCE UNTIL FRI NIGHT WITH THE COOL FRONT MOVING INTO NORTHERN IL. MODELS NOT BULLISH WITH PUSHING THIS BOUNDARY FURTHER S SAT OR SUN SO WILL HAVE SLIGHT CHANCE TO LOW END CHANCE POPS GOING ALL OR PARTS OF THE FA FOR SAT THROUGH MON UNTIL BOUNDARY FINALLY GETS SHOVED ACROSS CENTRAL TO SOUTHERN IL TAKING LOW LEVEL MOISTURE POOLED ON EITHER SIDE OF IT OUT OF THE LOCAL FA. TRS && .CLIMATE... 925 PM CDT RECORDS... CHICAGO THU JUL 5 MAX 102 1911 MIN 82 1911 FRI JUL 6 MAX 99 1988 MIN 80 1977 ROCKFORD THU JUL 5 MAX 100 1911 MIN 77 1911 FRI JUL 6 MAX 102 1936 MIN 73 1948 THE ALL TIME RECORD HIGH FOR CHICAGO IS 105 ON JULY 24 1934... AND FOR ROCKFORD IS 112 ON JULY 14 1936. FOR CHICAGO THE MOST CONSECUTIVE 100 DEGREE OR HIGHER DAYS IS THREE...JULY 3RD THROUGH 5TH OF 1911...AND ALSO AUGUST 4TH THROUGH 6TH OF 1947. FOR ROCKFORD THE MOST CONSECUTIVE 100 DEGREE OR HIGHER DAYS IS NINE...JULY 6TH THROUGH 14TH OF 1936. IZZI/TRS && .AVIATION... //ORD AND MDW CONCERNS...UPDATED 00Z... * LAKE BREEZE POTENTIAL THURSDAY AFTERNOON/EVENING. BMD //DISCUSSION...UPDATED 00Z... NO SIGNIFICANT CONCERNS FOR THE REST OF THE EVENING AND OVERNIGHT HOURS. EXPECT THE AREA TO CONTINUE TO SIT BETWEEN AREA OF HIGH PRESSURE TO THE SOUTHEAST WITH SLOW MOVING AREA OF LOW PRESSURE OVER THE PLAINS. THIS LEAVES NORTHERN ILLINOIS AND NORTHWEST INDIANA UNDER WESTERLY FLOW. GRADIENT SHOULD BE A LITTLE WEAKER THURSDAY LEADING TO SOMEWHAT LIGHTER WIND SPEEDS. THIS SHOULD ALLOW A LAKE BREEZE TO DEVELOP...THOUGH THERE IS UNCERTAINTY AS TO HOW FAR INLAND IT WILL PUSH GIVEN THE OPPOSING WINDS. SHOULD IT MAKE IT ACROSS THE CHICAGO TERMINALS...WINDS WOULD SWITCH AROUND TO AN EASTERLY DIRECTION. ALSO WILL HAVE TO MONITOR CONVECTIVE CHANCES ALONG THE LAKE BREEZE. THERE SHOULD BE DECENT CONVERGENCE ALONG THE BOUNDARY AND SOUNDINGS INDICATE UNCAPPED ATMOSPHERE POINTING TO THE POTENTIAL FOR DEVELOPMENT ALONG THE BOUNDARY AND THEN MOVING SOUTHEAST. BMD //ORD AND MDW CONFIDENCE...UPDATED 00Z... * LOW CONFIDENCE IN LAKE BREEZE REACHING TERMINALS THURSDAY AFTERNOON/EVENING. BMD //OUTLOOK FOR ORD/MDW FOR 12Z FRIDAY-12Z WEDNESDAY...UPDATED 00Z... FRIDAY...VFR. SATURDAY AND SUNDAY...VFR. CHANCE TSRA SATURDAY NIGHT INTO SUNDAY. MONDAY AND TUESDAY...VFR. BMD && .MARINE... 250 PM CDT A WEAK COLD FRONT HAS BECOME NEARLY STATIONARY OVER CENTRAL LAKE MICHIGAN THIS AFTERNOON...BUT WILL LIFT BACK NORTH TONIGHT AS LOW PRESSURE OVER THE EASTERN DAKOTAS AND CANADIAN PRAIRIES LIFTS TOWARD HUDSON BAY. THE LOW WILL CONTINUE EAST ACROSS HUDSON BAY THURSDAY AND THURSDAY NIGHT...WITH A COLD FRONT TRAILING ACROSS THE WESTERN LAKES REGION AND SLIDING SOUTH DOWN LAKE MICHIGAN AND SETTLING ACROSS THE SOUTH END BY EARLY FRIDAY. THE FRONT WILL SLOW BRIEFLY FRIDAY NIGHT AS A WEAK SURFACE WAVE RIPPLES EAST ALONG THE FRONT... WITH NORTHERLY FLOW THEN STRENGTHENING SATURDAY AS HIGH PRESSURE BUILDS ACROSS THE UPPER MIDWEST AND NORTHERN GREAT LAKES. WINDS WILL GENERALLY BE ON THE LIGHT SIDE OUTSIDE OF ANY THUNDERSTORM GUSTS UNTIL SATURDAY...WHEN THE TIGHTENING NORTHERLY GRADIENT AND PUSH OF SOMEWHAT COOLER AIR RESULTS IN NORTH-NORTHEAST WINDS INCREASING INTO THE 15-20 KT RANGE PERHAPS WITH A FEW GUSTS IN THE 20-25 KT RANGE SATURDAY INTO SATURDAY NIGHT. RATZER && .LOT WATCHES/WARNINGS/ADVISORIES... IL...EXCESSIVE HEAT WARNING...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008- ILZ010-ILZ011-ILZ012-ILZ013-ILZ014-ILZ019-ILZ020-ILZ021- ILZ022-ILZ023-ILZ032-ILZ033-ILZ039 UNTIL 10 PM FRIDAY. IN...EXCESSIVE HEAT WARNING...INZ001-INZ002-INZ010-INZ011-INZ019 UNTIL 10 PM FRIDAY. LM...NONE. && $$  FXUS63 KLOT 080840 AFDLOT AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL 240 AM CST SAT FEB 8 2014 .DISCUSSION... 856 PM CST NO BIG CHANGES THIS EVENING...MAINLY SLIGHT ADJUSTMENTS TO SKY COVER TRENDS OVERNIGHT. LATEST SATELLITE IMAGERY DEPICTING MID LEVEL CLOUDS AND LOW LEVEL STRATUS CONTINUING TO EXPAND NORTHWARD ACROSS THE REGION...WITH THIS TREND EXPECTED TO CONTINUE OVERNIGHT AS SEVERAL LARGE SCALE FEATURES APPROACH FROM THE WEST. IT DOES APPEAR AS A SMALLER IMPULSE CURRENTLY MOVING THROUGH THE CENTRAL PLAINS WILL CONTINUE EAST THROUGH MISSOURI AND WEST CENTRAL ILLINOIS TONIGHT. WAA OUT AHEAD OF IT WILL LIKELY CONTINUE WITH MOST GUIDANCE DEVELOPING SNOW OUT AHEAD OF THESE FEATURES...WHICH APPEARS REASONABLE. ALTHOUGH THIS AREA OF SNOW SHOULD STAY TO THE SOUTH OF THE CWA...DO THINK IT HAS A CHANCE TO CLIP THE SOUTHERN CWA MORE TOWARDS SATURDAY MORNING. SO HAVE MADE SOME MINOR ADJUSTMENTS TO SATURDAY MORNINGS POP GRID...BRINGING IN SLIGHT CHANCE POPS FOR LIGHT SNOW. ONLY OTHER ADJUSTMENTS THIS EVENING WERE SOME MINOR CHANGES TO MIN TEMPS TONIGHT AS IT APPEARS AS IF LOW TEMPS MAY HAVE ALREADY BEEN OBSERVED WITH EITHER A STEADY OR SLIGHT WARMING TREND EXPECTED. RODRIGUEZ //PREV DISCUSSION... 313 PM CST THE MAIN FORECAST CONCERNS WILL THE ACCUMULATING SNOW EXPECTED ACROSS THE AREA ON SATURDAY AFTERNOON AND NIGHT. SURFACE HIGH PRESSURE CONTINUES TO DOMINATE THE REGION THIS AFTERNOON WITH CLEAR SKIES ACROSS THE AREA. THIS HIGH WILL SLOWLY SHIFT EASTWARD ACROSS OHIO VALLEY THIS EVENING...THEN CLOUDS WILL INCREASE FROM THE WEST. THIS SHOULD ALLOW TEMPERATURES TO DROP RAPIDLY THIS EVENING AFTER SUNSET...WITH STEADY OR SLOWLY RISING TEMPERATURES OVERNIGHT AS THE CLOUDS INCREASE ACROSS THE AREA. THEREFORE...THE SUB ZERO LOWS IN MOST AREAS WILL BE THIS EVENING...WITH TEMPERATURES LIKELY RISING BACK ABOVE ZERO OVERNIGHT. WATER VAPOR IMAGERY THIS AFTERNOON SHOWS THAT THE FAST QUASI-ZONAL FLOW ACROSS THE EASTERN PACIFIC AND PACIFIC NORTHWEST HAS EVOLVED. A RATHER STOUT PV ANOMALY...CURRENTLY BEING EJECTED EASTWARD ACROSS IDAHO...IS EXPECTED TO TRACK RIGHT ACROSS NORTHERN ILLINOIS ON SATURDAY AFTERNOON. ISENTROPIC ASCENT SHOULD INCREASE DURING THE MORNING...WITH SNOW LIKELY ONSETTING ACROSS NORTH CENTRAL ILLINOIS BY MID TO LATE SATURDAY MORNING...AND ACROSS THE REST OF NORTHEASTERN ILLINOIS AND NORTHWESTERN INDIANA AROUND MIDDAY. MODEL GUIDANCE CONTINUES TO ADVERTISE QUIET IMPRESSIVE MID LEVEL LAPSE RATES...700-500 MB LAPSE RATES OF 6.5-7.0 DEGREES C PER KM. THIS COMBINED WITH GOOD SYNOPTIC SCALE ASCENT WITH THE APPROACHING PV ANOMALY WILL LIKELY LEAD TO SOME PERIODS OF MODERATE TO PERHAPS EVEN A PERIOD OF HEAVIER SNOW SATURDAY AFTERNOON. MODEL BUFFER SOUNDINGS ALSO INDICATE THAT THERE WILL BE A VERY DEEP DGZ...POSSIBLY 200 MB DEEP AT TIMES...DURING THE PERIOD OF BEST SNOWFALL. WITH THIS IN MIND...I WENT WITH A 16 TO 18 TO 1 RATIO FOR THE SNOW TOMORROW AFTERNOON...AND THIS SHOULD YIELD 2 TO 3 INCHES IN MOST AREAS ALONG AND NORTH OF INTERSTATE 80...WITH THE POTENTIAL FOR SOME ISOLATED AMOUNTS UP AROUND 4 INCHES GIVEN THE STEEP MID LEVEL LAPSE RATES AND THE POTENTIAL FOR SOME PERIODS OF HEAVIER SNOW. AREAS ACROSS MY SOUTHERN CWA...SOUTH OF THE KANKAKEE AND ILLINOIS RIVERS...LOOK TO SEE SOME SLIGHTLY LIGHTER SNOW AMOUNTS DURING THE DAY SATURDAY...POSSIBLY ONLY AN INCH OR TWO. HOWEVER...IT APPEARS THAT AN ADDITIONAL ROUND OF SNOW MAY DEVELOP ACROSS THE AREA SATURDAY NIGHT AS ANOTHER MID LEVEL DISTURBANCE IMPACTS THE AREA. IT APPEARS THAT MY SOUTHERN CWA WILL BE BETTER FAVORED FOR THE SECOND ROUND OF SNOW SATURDAY NIGHT INTO SUNDAY. THIS AREA LOOKS TO REMAIN IN THE VICINITY OF THE BAROCLINIC ZONE...AND HENCE THE BETTER AREAS OF FGEN. IN ADDITION...MODEL GUIDANCE SUGGESTS THAT THESE AREAS WILL ALSO BE WITHIN THE LEFT EXIT REGION OF A 120 KT UPPER LEVEL JET INTO SUNDAY MORNING. GIVEN THIS...I HAVE INCREASED POPS INTO THE HIGH CHANCE CATEGORY...MAINLY FOR AREAS SOUTH OF INTERSTATE 80...THOUGH AREAS NORTH COULD ALSO SEE SOME LIGHT SNOW OR FLURRIES DURING THIS PERIOD. ANOTHER INCH OR TWO COULD OCCUR WITH THIS ACTIVITY SATURDAY NIGHT INTO SUNDAY. MOST IF NOT ALL OF THE SNOW SHOULD SHIFT SOUTH OF MY AREA SUNDAY AFTERNOON AS THE BAROCLINIC ZONE SAGS SOUTHWARD. HOWEVER...WITH THE END OF THE SNOW...WILL COME MORE COLD. A COLDER AIR MASS WILL OVERSPREAD THE AREA AGAIN LATER SUNDAY AS A CANADIAN SURFACE HIGH DROPS SOUTHWARD ACROSS THE MISSOURI RIVER VALLEY. THIS SHOULD SETUP SUB ZERO LOWS SUNDAY NIGHT INTO MONDAY AND AGAIN MONDAY NIGHT INTO TUESDAY. TUESDAY MORNING LOWS COULD EVEN BOTTOM OUT IN THE TEENS BELOW ZERO AGAIN IN MOST AREAS...WITH THE SURFACE HIGH POSSIBLY MOVING RIGHT OVERHEAD. BELIEVE IT OR NOT...THERE IS SOME LIGHT AT THE END OF THE TUNNEL. IT APPEARS THAT WE MAY SEE AT LEAST A FEW DAYS WITH NO NEW SNOWFALL. MODEL GUIDANCE ALSO SUGGESTS THAT TEMPERATURES WILL MODERATE BY THE MIDDLE TO END OF NEXT WEEK AS SOUTHWESTERLY SURFACE FLOW SETS UP ACROSS THE CENTRAL PART OF THE COUNTRY. DEPENDING ON THE TRACK AND SPEED OF ANOTHER SYSTEM LATER IN THE WEEK...WE COULD GET TEMPERATURES TO POSSIBLY CRACK THE FREEZING MARK IN SOME AREAS BY THURSDAY. HOWEVER...UNCERTAINTY IN THE EXTENDED PERIOD REMAINS SOMEWHAT LOW AS THE MODELS ARE STRUGGLING WITH THE TIMING AND TRACK OF THE CONTINUED SMALLER SCALE MID/UPPER LEVEL DISTURBANCES LIKELY TO EJECT OUT OF THE ACTIVE QUASI-ZONAL FLOW ACROSS THE EASTERN PACIFIC AND PACIFIC NORTHWEST. KJB && .AVIATION... //ORD AND MDW CONCERNS...UPDATED 06Z... * A PERIOD OF SNOW/IFR CONDITIONS LATER TODAY...MAINLY DURING THE AFTERNOON HOURS. FLURRIES/LIGHT SNOW SHOWERS AND MVFR CONDITIONS MAY LINGER OVERNIGHT THROUGH EARLY SUNDAY MORNING. RATZER //DISCUSSION...UPDATED 06Z... VFR OVERCAST WILL GRADUALLY LOWER FROM MID-LEVEL TO 4000-5000 FT EARLY THIS MORNING...WITH SNOW EXPECTED TO DEVELOP LATE MORNING/MIDDAY. PERIOD OF LIGHT-MODERATE ACCUMULATING SNOW THEN ANTICIPATED DURING THE AFTERNOON WITH VISIBILITIES QUICKLY DETERIORATING TO IFR. SNOW SHOULD TAPER OFF TO LIGHT SNOW SHOWERS/FLURRIES BY EARLY EVENING AS INITIAL DISTURBANCE MOVES OFF TO THE EAST...THOUGH LIGHT SNOW/FLURRIES IN SOME FASHION WILL LIKELY PERSIST AT LEAST INTERMITTENTLY OVERNIGHT AS A SECOND MID- LEVEL WAVE AND SURFACE LOW PRESSURE TROUGH APPROACH. SOME INDICATION IN MODEL GUIDANCE THAT A PERIOD OF IFR CIGS AND SOME FOG MAY LINGER/DEVELOP DURING THE EVENING...THOUGH LOW CONFIDENCE IN THIS AT THIS TIME. WEAK LOW/SURFACE TROUGH EXPECTED TO BRING A SHIFT FROM LIGHT SOUTH WINDS TO NORTHWEST DURING THE PRE-DAWN HOURS OF SUNDAY MORNING...WITH MORE SOLID IMPROVEMENT IN CIGS TO MVFR LIKELY ALONG WITH RAPIDLY DIMINISHING SNOW POTENTIAL. RATZER //ORD AND MDW CONFIDENCE...UPDATED 06Z... * MEDIUM CONFIDENCE IN SNOW TIMING/INTENSITY...AND MVFR CIG/VIS DETAILS SATURDAY NIGHT. * HIGH CONFIDENCE ALL OTHER ELEMENTS. RATZER //OUTLOOK FOR ORD/MDW FOR 12Z SUNDAY-12Z FRIDAY...UPDATED 00Z... SUNDAY THROUGH THURSDAY...PREDOMINANTLY VFR. IZZI && .MARINE... 240 AM CST WEST WINDS WILL DIMINISH THIS MORNING ACROSS LAKE MICHIGAN...AS AN AREA OF HIGH PRESSURE CONTINUES TO DRIFT EAST ACROSS THE OHIO VALLEY AND A TROUGH OF LOW PRESSURE OVER EASTERN CANADA MOVES NORTHEAST. THE GRADIENT WILL WEAKEN BY THIS AFTERNOON...AS A WEAK AREA OF LOW PRESSURE MOVES EAST FROM THE PLAINS AND REACHES THE WESTERN LAKES AS AN ELONGATED LOW/TROUGH THIS EVENING. THE TROUGH WILL PUSH EAST OF THE LAKE EARLY SUNDAY MORNING...ALLOWING WINDS WHICH HAD DIMINISHED AND BACKED SOUTH-SOUTHWEST THIS AFTERNOON TO SHIFT TO THE NORTHWEST. VARIOUS MODEL SOLUTIONS DIFFER A BIT WITH HOW QUICKLY COLDER AIR SPREADS IN BEHIND THE DEPARTING TROUGH SUNDAY...AND THE MAGNITUDE OF THE PRESSURE GRADIENT ON THE EASTERN/SOUTHEASTERN PERIPHERY OF A SPRAWLING AREA OF CANADIAN HIGH PRESSURE WHICH WILL SPREAD SOUTHEAST INTO THE MID-MISSISSIPPI VALLEY THROUGH MONDAY. WITH SIGNIFICANT DIFFERENCES MODEL WIND SPEEDS...HAVE GENERALLY USED A BLEND OF AVAILABLE GUIDANCE FOR TIMING...WHICH BRINGS NORTHWEST WINDS UP INTO THE 15-25 KT RANGE MAINLY ACROSS WESTERN/SOUTHERN PARTS OF THE LAKE SUNDAY AFTERNOON. BLUSTERY NORTHWEST WINDS CONTINUE TO GUST NEAR 25 KT EAST OF THE HIGH PRESSURE RIDGE INTO MONDAY...BEFORE SLOWLY DIMINISHING AS THE RIDGE DRAWS NEAR LATER MONDAY NIGHT AND TUESDAY. FOR OUR ILLINOIS/INDIANA NEARSHORE WATERS...BEAUTIFUL MODIS POLAR ORBITER SATELLITE IMAGERY FROM THE LAST TWO SUNNY DAYS INDICATES MUCH OF THE INDIANA WATERS ARE SOLIDLY ICE-COVERED...WHILE THICKER ICE HAD PUSHED EAST OFF THE ILLINOIS SHORE WITH THE STRONGER WEST WINDS OF THE LAST SEVERAL DAYS. HOWEVER...WEB CAMS SUGGEST THAT OUR SINGLE DIGIT TEMPS HAVE ALLOWED AT LEAST SOME ICE TO CONTINUE TO DEVELOP ALONG THE ILLINOIS SHORE AND THUS WILL OMIT WAVE HEIGHTS IN THE NEARSHORE FORECAST AS LIKELY LITTLE WAVE DEVELOPMENT GIVEN LARGE AMOUNT OF ICE. RATZER && .LOT WATCHES/WARNINGS/ADVISORIES... IL...NONE. IN...NONE. LM...NONE. && $$ VISIT US AT HTTP://WEATHER.GOV/CHICAGO (ALL LOWERCASE) FOLLOW US ON FACEBOOK...TWITTER...AND YOUTUBE AT: WWW.FACEBOOK.COM/US.NATIONALWEATHERSERVICE.CHICAGO.GOV WWW.TWITTER.COM/NWSCHICAGO WWW.YOUTUBE.COM/NWSCHICAGO  FXUS63 KLOT 041955 AFDLOT AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL 155 PM CST WED FEB 4 2015 .UPDATE... 1032 AM CST GOING FORECAST APPEARS TO BE IN VERY GOOD SHAPE...WITH BANDED SNOW AFFECTING AREAS MAINLY ALONG/SOUTH OF THE I-80 CORRIDOR THIS MORNING. SURFACE OBS INDICATE MODERATE SNOW WITHIN THE BANDS OF MORE REFLECTIVE RADAR RETURNS...WITH VISIBILITY 1/2-3/4 MILE NOTED IN SEVERAL SFC OBS FROM THE IA/MO BORDER REGION THROUGH THE PONTIAC AREA RECENTLY. ANALYSIS OF MORNING RAOB AND MODEL DATA INDICATES FORCING IS LIKELY BEING MAXIMIZED THIS MORNING...WITH LARGE SCALE LIFT PROVIDED BY MINOR AMPLITUDE SHORT WAVE APPROACHING FROM THE PLAINS AS WELL AS ANOTHER DIGGING ACROSS THE UPPER MIDWEST. SOME ENHANCEMENT FROM UPPER DIVERGENCE IN LEFT EXIT REGION OF UPPER JET STREAK TO OUR WEST...AND MORE IMPORTANTLY STRONG FRONTOGENETIC FORCING IN 600-700 MB LAYER AND THE RESULTING VERTICAL AGEOSTROPHIC CIRCULATION...WAS HELPING TO PRODUCE THE STRONGER BANDING SEEN IN RADAR RETURNS INTO THE SOUTHERN HALF OF OUR CWA. GUIDANCE TRENDS CONTINUE TO SUPPORT GRADUAL WEAKENING OF F-GEN SUPPORT INTO THIS AFTERNOON AS WELL AS SHIFTING IT MORE TO THE SOUTHEAST AS THE STRONGER UPPER MIDWEST SHORT WAVE DIGS SOUTHEAST AND AMPLIFIES THE LARGER SCALE UPPER TROUGH ACROSS THE REGION. THIS IS IN LINE WITH EARLIER FORECAST THINKING...SO OTHER THAN EXPANDING HIGHER POPS INTO AREAS WHERE LIGHTER SNOW IS FALLING... HAVE MADE NO CHANGES TO GOING FORECAST OR SNOW AMOUNTS WHICH APPEAR REASONABLE GIVEN RADAR TRENDS AND AFOREMENTIONED VISIBILITY OBSERVATIONS. RATZER && .SHORT TERM... 336 AM CST THROUGH THURSDAY... FAST MOVING BUT POTENTIALLY HARD HITTING SNOW EXPECTED TODAY FOLLOWED BY BITTERLY COLD TEMPERATURES TONIGHT...PARTICULARLY AWAY FROM THE NORTHWEST INDIANA AND THE CHICAGO URBAN CORRIDOR. FIRST ON THE DOCKET IS SNOW. SHORTWAVE TROUGH AND ASSOCIATED 120KT+ 300MB JET STEAK OVER THE DAKOTAS IS FORECAST TO ROUND THE BASE OF THE SLIGHTLY AMPLIFYING TROUGH OVER THE WESTERN LAKES TODAY WITH JET PROGGED TO INCREASE TO NEARLY 140KT AS IT EXITS THE BASE OF THE TROUGH. THE JETLET IS PROGGED TO EXTENDS FROM SOUTHERN WI TO UPSTATE NY BY 18Z PLACING OUR CWA IN THE SWEET SPOT RIGHT ENTRANCE REGION. DEEP TROPOSPHERIC THERMALLY DIRECT CIRCULATION ASSOCIATED WITH THIS JET STEAK AND DEVELOPING DEEP F-GEN FIELD SHOULD SUPPORT RAPID EASTWARD DEVELOP OF THE SNOW OVER NEBRASKA INTO SOUTHERN PORTIONS OF NORTHERN ILLINOIS THIS MORNING. COMPARED TO YESTERDAYS SYSTEM...GUIDANCE IS MUCH STRONGER AND DEEPER WITH THE F-GEN. MID-UPPER LEVEL LAPSE RATES AND RESULTANT INSTABILITY ARE JUST A TICK WEAKER THAN YESTERDAY...BUT STILL SEE POCKETS OF NEGATIVE SATURATED EPV WITH THE WARM SIDE OF THE F-GEN CIRCULATION...WHICH SHOULD ALLOW A FAIRLY STRONG AND SOMEWHAT NARROW BAND OF SNOW TO DEVELOP. OMEGA IS ALSO FORECAST TO BE STRONGER THAN YESTERDAY AND PEAK OMEGA VALUES ARE FORECAST TO BE FAIRLY CLOSELY COLLOCATED WITH THE PRIME DGZ TEMP ZONE...WHICH RAISES THE THREAT OF MORE EFFICIENT HIGHER RATIO SNOWFALL WITHIN THE BAND. LEANED HEAVILY ON HIGHER RES CONVECTIVE ALLOWING GUIDANCE FOR QPF AMOUNTS GIVEN THE EXPECTATION OF MESOSCALE BANDING TO DEVELOP...WHICH THESE MODELS SHOULD TEND TO RESOLVE BETTER. GIVEN WE SAW BRIEF SNOWFALL RATES >1IN/HOUR YESTERDAY WITH 1/4SM SNOWS AND THE SETUP LOOKS EVEN BETTER TODAY...SEE NO REASON THAT WE WONT HAVE A REPEAT OF HEAVY SNOWFALL RATES DEVELOP WITH THE BANDING AGAIN TODAY WITH RATES OF 1-2 INCHES PER HOUR POSSIBLE. UNLIKE YESTERDAY...THE MESOSCALE BANDING LOOKS TO ALIGN A BIT BETTER WITH THE MOVEMENT OF THE PRECIP WHICH RAISES THE POTENTIAL FOR SOME LONGER RESIDENCE TIME UNDER THE HEAVY SNOW BAND FOR SOME LOCATIONS. HAVE BUMPED UP SNOW ACCUMS IN THE GRIDS TO 2-5" RANGE SOUTH OF I-80 AND HAVE GONE WITH A WINTER WX ADV. WHILE AMOUNTS ARE ON THE LOWER THRESHOLD OR JUST BELOW ADVISORY THRESHOLD...FELT THE POTENTIAL FOR +SN AND RATES OF 1-2" PER HOUR MADE IT EASY TO JUSTIFY THE HEADLINE. NOT EXPLICITLY FORECASTING THIS TO OCCUR...BUT IF THE HIGHER SNOWFALL RATES MATERIALIZE AS FORECAST THEN IT WOULD NOT TAKE TERRIBLY LONG FOR THE BAND TO STALL OUT TO HAVE SNOWFALL AMOUNTS PILE UP TO LOCALLY AROUND 6 INCHES WHICH IS REASONING FOR INCLUSION OF LOCALLY HEAVIER WORDING IN WSW. THE SNOWS WILL QUICKLY SHIFT SOUTH OF THE CWA BY 23-00Z WITH FORECAST SOUNDINGS AND T/H X-SECTIONS SHOWING STRONG SUBSIDENCE AND RAPID CLEARING TAKING PLACE EARLY THIS EVENING. STRONG 1038MB ARCTIC HIGH WILL BUILD INTO THE CORN BELT THIS EVENING WITH MODEST NW WINDS USHERING IN BITTERLY COLD ARCTIC AIR MASS...AIDED BY THE DEEP AND IN MANY CASES REFRESHED SNOW PACK. AFTER MIDNIGHT THE RIDGE AXIS IS FORECAST TO BUILD EAST TOWARD N IL AND AM CONCERNED THAT WINDS COULD LARGELY DECOUPLE...WHICH WOULD SUPPORT TEMPS TANKING...ESPECIALLY IN THE TYPICALLY COLD DRAINAGE AREAS LIKE THE FOX AND ROCK RIVER VALLEYS. HAVE CONTINUED TREND FROM PREVIOUS FORECAST OF UNDERCUTTING LOW TEMP GUIDANCE TONIGHT WITH DOUBLE DIGIT SUB ZERO LOWS FORECAST. IT WOULD TAKE VERY LITTLE WIND TO RESULT IN ADVISORY WORTHY WIND CHILLS...BUT CONFIDENCE IN THE TEMPS/WIND SPEEDS IS NOT HIGH ENOUGH TO JUMP ON A WIND CHILL ADVISORY NOW...WOULD RATHER LET SUBSEQUENT SHIFTS MAKE FINAL CALL ON. IZZI && .LONG TERM... 336 AM CST THURSDAY NIGHT THROUGH TUESDAY... UPPER TROUGH AND ARCTIC AIR ARE FORECAST TO QUICKLY MOVE OUT OF THE AREA LATER THURSDAY AS UPPER FLOW TRANSITIONS TO A MILDER ZONAL FLOW PATTERN THROUGH THE WEEKEND. DEEP SNOWPACK WILL UNDOUBTEDLY RETARD OUR WARM UP LOCALLY...THOUGH TEMPS DO LOOK TO EVENTUALLY CLIMB TO AROUND OR JUST ABOVE FREEZING BEGINNING THE VERY SLOW MELTING PROCESS OF THE FOOT TO FOOT AND A HALF OF SNOW THAT IS ON THE GROUND OVER A LARGE PORTION OF THE CWA. OTHER THAN SMALL CHANCE OF SOME FZG DRIZZLE/FOG FRI NIGHT INTO SAT WITH THE WARMER AIR MOVING INTO LOOK FOR DRY WX THROUGH SATURDAY. WEAK SHORTWAVE COULD SIDESWIPE US LATER IN THE WEEKEND WITH SOME LIGHTER PRECIP WHICH COULD TAKE ABOUT ANY FORM GIVEN THE SPREAD IN FORECAST THERMAL PROFILES THAT FAR OUT. AFTER TONIGHT/THURSDAY'S COLD SNAP LOOK FOR A RETURN TO NEAR/SLIGHTLY ABOVE AVG TEMPS THROUGH EARLY NEXT WEEK (DRIVEN ALMOST ENTIRELY BY THE MUCH ABOVE AVG NIGHTTIME LOWS). IZZI && .AVIATION... //ORD AND MDW CONCERNS...UPDATED 18Z... * MODERATE SNOW AT TIMES REDUCING VSBY TO IFR. * NNW/NW WINDS GUSTING TO AROUND 20 KT AT TIMES THIS AFTERNOON/EVENING. BMD //DISCUSSION...UPDATED 18Z... BAND OF HEAVY SNOW HAS SET UP SOUTH OF THE TERMINALS THIS AFTERNOON...STRETCHING FROM ACROSS EAST CENTRAL ILLINOIS INTO NORTHWEST INDIANA. MODERATE SNOW CONTINUES ACROSS THE CHICAGO TERMINALS REDUCING VISIBILITY GENERALLY INTO THE 1-2SM FRAME...THOUGH A FEW BANDS HAVE RESULTED IN VSBY BRIEFLY 1/2 TO 3/4 SM. THE BETTER FORCING IS PROGGED TO SLIDE SOUTHEAST THROUGH THE AFTERNOON AND EXPECT THE BETTER SNOW TO AS WELL...WITH CONDITIONS STARTING TO IMPROVE AFTER THE NEXT COUPLE HOURS. SNOW SHOULD END ALTOGETHER AROUND MID AFTERNOON WITH CONDITIONS IMPROVING TO HIGHER END MVFR IF NOT VFR. WINDS SHOULD GENERALLY STAY IN THE 320-340 RANGE REST OF TODAY WITH GUSTS PUSHING 20KT AT TIMES THIS AFTERNOON. HIGH PRESSURE WILL BUILD FROM THE MID MISSOURI VALLEY THIS EVENING TO THE MID MISSISSIPPI VALLEY TONIGHT. WINDS SHOULD GRADUALLY BACK AND WEAKEN OVERNIGHT COMING AROUND TO THE SOUTHWEST DURING THE DAY TOMORROW. GUIDANCE SUGGESTS THAT SKIES WILL BEGIN TO CLEAR OUT AS THE HIGH BUILDS IN THIS EVENING. BMD //ORD AND MDW CONFIDENCE...UPDATED 18Z... * MEDIUM-HIGH CONFIDENCE IN SNOW TIMING/VSBY. * LOW CONFIDENCE IN FREQUENT GUSTS ABOVE 20 KT THIS AFTERNOON. MEDIUM CONFIDENCE IN GUSTS STAYING AT OR BELOW 20 KT. * MEDIUM-HIGH CONFIDENCE IN CIG TRENDS. BMD //OUTLOOK FOR ORD/MDW FOR 00Z FRIDAY-12Z WEDNESDAY...UPDATED 12Z... THURSDAY NIGHT...VFR/WX NIL. SW WINDS. FRIDAY...VFR/WX NIL. SW WINDS. SATURDAY...VFR/WX NIL DURING THE DAY. CHANCE RAIN OR FREEZING RAIN OVERNIGHT. MVFR CIGS/VIS LIKELY. IFR CONDITIONS POSSIBLE. E WINDS. SUNDAY...CHANCE OF A WINTRY MIX THROUGH THE DAY. CHANCE OF SNOW SHOWERS OVERNIGHT. MVFR CIGS/VIS LIKELY. IFR CONDITIONS POSSIBLE. NE WINDS. MONDAY...VFR/WX NIL. NE WINDS. TUESDAY...VFR/WX NIL. S WINDS. KREIN && .MARINE... 154 PM CST MAIN MARINE CONCERNS THIS AFTERNOON ARE INCREASING NORTHWEST WINDS THIS EVENING AND TONIGHT...THEN INCREASING SOUTHWEST WINDS THURSDAY AFTERNOON WITH THE POTENTIAL FOR SOUTHWEST/WEST GALES THURSDAY AFTERNOON AND THURSDAY NIGHT. A PERIOD OF MORE MODEST NORTHEAST-EAST FLOW ALSO APPEARS ON THE HORIZON THIS WEEKEND. STRONG HIGH PRESSURE WAS BUILDING SOUTH ACROSS THE PLAINS THIS AFTERNOON...WITH COLD ARCTIC AIR AND A TIGHTENING PRESSURE GRADIENT BETWEEN THE HIGH AND LOW PRESSURE TO THE EAST OVER NEW ENGLAND HELPING TO SLOWLY INCREASE NORTHWEST WINDS ACROSS LAKE MICHIGAN. A PERIOD OF NORTHWEST WINDS 25-30 KT THROUGH LATE TONIGHT WILL AFFECT MAINLY THE SOUTHERN 2/3 OR SO OF THE LAKE...BEFORE HIGH PRESSURE RIDGING NORTHEAST OF THE MAIN HIGH CENTER OVER THE PLAINS MOVES ACROSS THE WESTERN LAKES THURSDAY MORNING. THE GRADIENT WILL TIGHTEN UP AGAIN QUICKLY THURSDAY MORNING/MIDDAY...AS THE HIGH SPREADS SOUTH ACROSS THE OHIO VALLEY AND ANOTHER LOW PASSES NORTH OF THE LAKES ACROSS ONTARIO. THE RAPID PRESSURE FALLS WILL LEAD TO A QUICK INCREASE IN SOUTHWEST-WEST WINDS DURING THE DAY...WITH THE POTENTIAL FOR LOW-END GALES ACROSS MUCH OF THE LAKE EXCEPT THE FAR NORTH AND FAR SOUTH ENDS THURSDAY AFTERNOON THROUGH AFTER MIDNIGHT THURSDAY NIGHT. FORECAST GUIDANCE HAS SHOWN THIS TREND FOR THE PAST SEVERAL RUNS...THUS HAVE ELECTED TO HOIST A GALE WATCH FOR THE OPEN WATERS FROM 3 PM CST THURSDAY THROUGH 3 AM CST FRIDAY MORNING. WINDS THEN DECREASE FAIRLY QUICKLY FRIDAY MORNING AS WEAK HIGH PRESSURE QUICKLY CROSSES THE LAKE. LATER IN THE FORECAST PERIOD...WEAK LOW PRESSURE IS EXPECTED TO CROSS THE NORTHERN PART OF THE LAKE EARLY SATURDAY...WITH A TRAILING COLD FRONT SAGGING TO THE SOUTH END OF THE LAKE BY SATURDAY EVENING. ANOTHER LOW THEN LOOKS TO TRACK ALONG THIS FRONT NEAR OR JUST SOUTH OF THE SOUTH END OF THE LAKE SATURDAY NIGHT/SUNDAY. THIS SHOULD SET UP A PERIOD OF MODESTLY STRONG EAST-NORTHEAST FLOW...DEPENDING UPON THE ULTIMATE STRENGTH OF THE LOW AND THE SOUTHEAST PORTION OF A HIGH PRESSURE RIDGE OVER ONTARIO. FOR THE ILLINOIS/INDIANA NEARSHORE WATERS...CURRENT SMALL CRAFT ADVISORY CONTINUES THROUGH THURSDAY MORNING FOR IL...EARLY THURSDAY AFTERNOON FOR IND. GIVEN THE AMOUNT OF ICE SEEN IN MODIS POLAR ORBITER SATELLITE IMAGERY FROM YESTERDAY IT MAY NOT MATTER MUCH... BUT WILL LIKELY NEED ANOTHER SCA FOR WINDS BY THURSDAY EVENING. WILL LIKELY HOLD OFF FOR NOW AND ALLOW CURRENT ONE TO PLAY OUT AND ALLOW MIDNIGHT SHIFT TO ASSESS THE DURATION OF LESS THAN SCA CRITERIA THURSDAY. RATZER && .LOT WATCHES/WARNINGS/ADVISORIES... IL...WINTER WEATHER ADVISORY...ILZ019-ILZ021-ILZ022-ILZ023 UNTIL 3 PM WEDNESDAY. WINTER WEATHER ADVISORY...ILZ032-ILZ033-ILZ039 UNTIL 5 PM WEDNESDAY. IN...WINTER WEATHER ADVISORY...INZ001-INZ002 UNTIL 3 PM WEDNESDAY. WINTER WEATHER ADVISORY...INZ010-INZ011-INZ019 UNTIL 5 PM WEDNESDAY. LM...SMALL CRAFT ADVISORY...ILLINOIS NEARSHORE WATERS UNTIL NOON THURSDAY. SMALL CRAFT ADVISORY...INDIANA NEARSHORE WATERS UNTIL 6 PM THURSDAY. && $$ VISIT US AT HTTP://WEATHER.GOV/CHICAGO (ALL LOWERCASE) FOLLOW US ON FACEBOOK...TWITTER...AND YOUTUBE AT: WWW.FACEBOOK.COM/NWSCHICAGO WWW.TWITTER.COM/NWSCHICAGO WWW.YOUTUBE.COM/NWSCHICAGO  FXUS63 KLOT 060809 AFDLOT AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL 309 AM CDT MON JUL 6 2015 .SHORT TERM... 303 AM CDT THROUGH TUESDAY... NEAR TERM FOCUS WILL BE ON TEMPERATURES TODAY..THEN ON CONVECTIVE EVOLUTION LATER TODAY AND OVERNIGHT. ANOTHER QUIET MORNING IN OUR NECK OF THE WINDS. TEMPERATURES ARE HOLDING HIGHER TONIGHT GIVEN A LIGHT SSE WINDS AND DEWPOINTS CREEPING UP INTO THE LOW TO MID 60S. PATCHY FOG WILL CONTINUE THROUGH DAYBREAK BUT GENERALLY NOT EXPECTING ANYTHING DENSE EXCEPT LOCALLY WHERE WINDS DECOUPLE OR IN RIVER VALLEYS. ATTENTION WILL BE ON THE PROGRESSION OF A COLD FRONT CURRENTLY STRETCHED FROM NORTHERN LAKE SUPERIOR SOUTHWESTWARD INTO THE CENTRAL PLAINS. ACTIVE CONVECTION ALONG THE FRONT CONTINUES TO SLOWLY MARCH SOUTH AND EASTWARD...WITH AN EXTENSIVE DEBRIS SHIELD HEADING SOUTHEASTWARD. 925 MB THERMAL RIDGE TO OUR WEST WILL SHIFT OVERHEAD THIS AFTERNOON. MIXING TO AT LEAST 925 MB AND LIKELY HIGHER TO 850 WHERE TEMPS ARE FORECAST TO REACH 25/19C RESPECTIVELY WOULD SUPPORT HIGHS AROUND 90. QUESTION HOLDS AS TO ANY LINGERING SMOKE...WHICH PER MODIS IMAGERY IS LARGELY ALONG AND EAST OF THE AREA...AND ANY HIGH CLOUDINESS FROM CONVECTION TO THE WEST PLAYS OUT. HAVE LARGELY HELD ONTO THE MID TO UPPER 80S AND AM RELUCTANT TO GO HIGHER GIVEN THESE UNCERTAINTIES AND THINK THE CLOUD COVER MAY BE MORE EXTENSIVE TO SUGGEST THE HIGHER TEMPS WOULD BE HARDER TO REACH...WHICH COULD EVEN SUGGEST SOME COOLER TEMPS IN OUR NORTH/WEST. STILL...HEAT INDEX VALUES WILL GET CLOSE TO OR INTO THE 90S IN MANY AREAS WITH THE INCREASED HUMIDITY. MODEL CONSENSUS IS NOT EXACTLY CLEAR JUST YET AS TO HOW THINGS PLAY OUT THROUGH TONIGHT BUT ARE COMING INTO BETTER GENERAL AGREEMENT IN FRONTAL TIMING. MID LEVEL LAPSE RATES...WHICH ARE MORE IMPRESSIVE CURRENTLY...WILL WEAKEN AS THE FRONT NEARS THE AREA TONIGHT. GFS/ECMWF/SREF STILL SUGGEST FRONT CLEARS THE CHICAGO AREA BY DAYBREAK TUESDAY...WHILE WRF-NAM AND NAM 4KM SUGGEST IT WILL STILL BE NEAR THE MISSISSIPPI RIVER BY THIS TIME. THERE ARE SEVERAL RIPPLES IN THE FLOW OUT AHEAD OF THE MAIN UPPER TROUGH CURRENTLY MOVING ALONG THE CANADIAN BORDER MAY SLOW TINGS A TOUCH AS THE UPPER JET DOES REMAIN FOCUSED TO THE NORTH...A SURFACE LOW DOES LOOK TO TRACK ALONG FRONT FROM THE PLAINS AND NORTH INTO WISCONSIN TONIGHT. GLOBAL MODELS HAVE BEEN CONSISTENT IN THEIR TIMING...WHICH IS NOT A GREAT DIURNAL TIME TO MOVE THROUGH...AND THERE IS NOTHING IN THE RAP RUNS TO SUPPORT THE WRF-NAM MODEL SLOWER LOW PROGRESSION AND THEREFORE HOLD WITH A GFS/ECMWF BLEND FOR FRONTAL TIMING. THEREFORE CURRENT FEELING IS THAT THE CONVECTION CURRENTLY ONGOING DUE TO LOW LEVEL WARM/MOIST ADVECTION IN IOWA WILL CONTINUE TO SHIFT EAST AND RIDE ALONG THE MOISTURE TRANSPORT ACCESS NORTHEASTWARD. SOME CONTINUED DEVELOPMENT IS EXPECTED OUT AHEAD OF THE COLD FRONT AS THE BEST MOISTURE TRANSPORT AHEAD OF THE FRONT CONTINUES TO PROGRESS ACROSS WISCONSIN. THE IMPLICATIONS OF THIS ARE THAT SOME CONVECTION MAY CLIP FAR NORTH CENTRAL ILLINOIS LATE THIS MORNING INTO THE EARLY AFTERNOON TIME FRAME AS SUGGESTED BY THE HRRR AND SEVERAL HOPWRF-ARW ENSEMBLE MEMBERS. SEVERAL MEMBERS DO SUGGEST A WEAKENING OF THE SOUTHERN PORTIONS OF THE CONVECTION WHICH WOULD KEEP A QUIET MORNING PERIOD...AND THIS MAKES SENSE GIVEN THE MOISTURE TRANSPORT WHICH IS NORTHEASTWARD DIRECTED AT THE MOMENT WILL FOCUS NORTH THROUGH THE MORNING AND EARLY AFTERNOON. THEN BOUNDARY LAYER HEATING/MOISTENING WOULD SUPPORT SOME REGENERATION IN WESTERN ILLINOIS AND INTO NORTH CENTRAL ILLINOIS IN LATER IN THE AFTERNOON...POTENTIALLY ENHANCED BY ANY OUTFLOW FROM THE OVERNIGHT CONVECTION. DEWPOINT POOLING AHEAD OF THE FRONT WILL ALLOW PW VALUES TO NEAR 2 INCHES...MEANING LOCALLY HEAVY DOWNPOURS WOULD BE A POTENTIAL HAZARD. 30 KT OF DEEP LAYER SHEAR DOES STILL SUPPORT SOME STORM ORGANIZATION AS WELL...WHICH IF THE STORMS FORM INTO A LINE COULD ALSO POSE A WIND/MARGINAL HAIL THREAT AS WELL. CURRENT SLIGHT/MARGINAL RISK AREAS FROM SPC HAVE CHANGED VERY LITTLE FROM PREVIOUS FORECAST WITH THE BEST CHANCE OF TAPPING INTO BETTER INSTABILITY COMBINED WITH THE FRONTAL FORCING IN NORTH CENTRAL ILLINOIS AND POINTS WEST. THEN TONIGHT...STILL SHOULD BE SOME SHOWERS AND THUNDERSTORMS OVERNIGHT THOUGH AS DEWPOINTS HOLD STRONG IN THE SOUTHERLY FLOW AHEAD OF THE FRONT IN THE LOWER 70S AND MAINTAIN SOME LEVEL OF INSTABILITY...THOUGH THE BETTER ACTIVITY LOOKS TO BE WEST OF THE AREA AND EXTENDING INTO NORTH CENTRAL ILLINOIS TIED TO THE COLD FRONT...AND NUMEROUS SOLUTIONS SUGGEST A WEAKENING OF THE ACTIVITY AS THE FRONT PROGRESSES THROUGH NORTHEAST ILLINOIS AND NORTHWEST INDIANA...ESPECIALLY AFTER MIDNIGHT. HIGH PRESSURE WILL MOVE FROM THE UPPER MIDWEST TO THE MISSISSIPPI VALLEY ON TUESDAY WHICH WILL CONTINUE THE COLD FRONTS SOUTH AND EASTWARD MOVEMENT. LOOKS LIKE THERE WILL BE SOME SHOWER ACTIVITY...POSSIBLY A FEW THUNDERSTORMS TO START THE DAY TUESDAY. WINDS TURN OFF THE LAKE AND ONSHORE...USHERING A MUCH COOLER AND DRIER AIRMASS FOR AREAS ALONG AND NORTH OF I-80. THE FRONT WILL BE ON THE EDGE OF OUR AREA TUESDAY AFTERNOON...MEANING A CHANCE FOR PRECIP LINGERS FOR SOUTH AND EAST AREAS AS THINGS FIRE UP AGAIN AHEAD OF THE FRONT...BUT THINGS COULD LARGELY BE EAST OF OUR AREA BY THAT TIME. KMD && .LONG TERM... 303 AM CDT TUESDAY NIGHT THROUGH SUNDAY... LOOKS LIKE A BRIEF RESPITE TUESDAY NIGHT BEFORE THE FRONT GETS SHIFTED BACK NORTHWARD AND BECOMES THE FOCUS FOR ADDITIONAL ROUNDS OF PRECIPITATION THROUGH WEEKS END. THE UPPER AIR PATTERN FEATURES HIGH PRESSURE OVER THE SE UNITED STATES EXTENDING OFFSHORE TO THE ATLANTIC...WITH A TROUGH JUST OFF THE CALIFORNIA COAST. THE HIGH IN THE EAST WILL SLOWLY RETROGRADE AND SHIFT INLAND TO THE WEST WHILE THE TROUGH IN THE WEST SLOWLY MOVES EASTWARD ONSHORE INTO THE GREAT BASIN. THIS KEEPS OUR AREA IN A SOMEWHAT MOIST WSW FLOW WITH MULTIPLE WAVES PROPAGATING NE IN THIS FLOW PATTERN AND INTERSECTING WITH THE STALLED OUT FRONTAL BOUNDARY. THIS PATTERN SUGGEST OCCASIONAL PERIODS OF SHOWERS AND THUNDERSTORMS. FUTURE FORECASTS WILL LIKELY NEED TO REFINE THE TIMING OF THESE CHANCES...WHICH CURRENTLY LOOK TO OCCUR WEDNESDAY PM...FRIDAY...AND POSSIBLY INTO SATURDAY. UPPER LEVEL HEIGHTS CONTINUE TO BUILD FROM THE EAST AS THE HIGH EXPANDS OVER THE ENTIRE SOUTHERN UNITED STATES WHICH SHOULD EVENTUALLY LIFT THE FRONTAL BOUNDARY NORTH OF THE AREA AT SOME POINT BUT NOT UNTIL THE WEEKEND. TEMPERATURES WILL BE MUCH COOLER THROUGH MID WEEK...WITH READINGS BACK INTO THE 70S...WITH LAKESHORE AREAS STUCK IN THE 60S/NEAR 70 BEFORE HEIGHTS BUILD AND THE FRONT GETS LIFTED NORTHWARD. TEMPERATURES AT THIS POINT CONTINUE TO RECOVER BACK WELL INTO THE 80S FOR THE WEEKEND. KMD && .AVIATION... //ORD AND MDW CONCERNS...UPDATED 06Z... * SHOWERS WITH POSSIBLE THUNDER LATER THIS EVENING. RODRIGUEZ //DISCUSSION...UPDATED 06Z... QUIET AND DRY CONDITIONS EXPECTED THIS MORNING WITH ANY PRECIP ACTIVITY STAYING WELL WEST OF THE TERMINALS AT THIS TIME. PATCHY FOG WITH INCREASING HIGH LEVEL CLOUD COVER IS LIKELY THIS MORNING...WITH SOUTHERLY WINDS OBSERVING A STEADY INCREASE THROUGH MIDDAY. MAIN FORECAST CHALLENGE FOR THE TERMINALS IS PRECIP TRENDS AND TIMING. AM MONITORING SHOWERS/THUNDERSTORMS ACROSS MINNESOTA AND FAR WESTERN IOWA...WHICH COULD FIRST IMPACT RFD BY LATE MORNING. THIS DEVELOPMENT WILL LIKELY DRIFT EASTWARD THROUGH MIDDAY AND POSSIBLY REACH RFD...BUT SHOULD ALSO OBSERVE A WEAKENING TREND AS IT APPROACHES. BEST FOCUS THIS FORECAST PERIOD FOR ALL TERMINALS WILL ARRIVE LATER THIS AFTERNOON AND EVENING ALONG AN APPROACHING SURFACE TROUGH AND FRONTAL BOUNDARY. HOWEVER...GUIDANCE VARIES TO THE EXTENT OF COVERAGE AND INTENSITY OF ANY PRECIP AS WELL AS TIMING AND DURATION. BASED ON LATEST TRENDS HAVE INCLUDED PRECIP IN THE TAFS WITH RFD HAVING THE BETTER CHANCE TO OBSERVE IT EARLIER IN THE AFTERNOON...AND THEN THE REMAINING TERMINALS NOT UNTIL LATER IN THE EVENING. CONFIDENCE IS LOWER AT THIS TIME...AS SOME GUIDANCE IS BRINGING THIS PRECIP IN EARLIER. OTHER CHALLENGES WILL ARRIVE VERY LATE IN THE FORECAST PERIOD...WITH LOW CEILINGS AND WIND SPEED/DIRECTION LIKELY BECOMING PROBLEMATIC. RODRIGUEZ //ORD AND MDW CONFIDENCE...UPDATED 06Z... * HIGH CONFIDENCE WITH VFR CONDITIONS THIS MORNING/TODAY. * HIGH CONFIDENCE WITH WIND SPEED/DIRECTION TODAY. * LOW MEDIUM CONFIDENCE WITH SHOWERS/THUNDER AND TIMING/DURATION LATER THIS EVENING. RODRIGUEZ //OUTLOOK FOR ORD/MDW FOR 12Z TUESDAY-12Z SUNDAY...UPDATED 00Z... TUESDAY...CHC MORNING TSRA AND POSSIBLE MVFR CIGS...TRENDING TOWARDS VFR CONDS BY AFTN. WEDNESDAY...SLIGHT CHANCE OF SHRA LATE. THURSDAY THROUGH SATURDAY...CHANCE OF TSRA. BEACHLER && .MARINE... 220 PM CDT WITH HIGH PRESSURE DEPARTING THE LAKES REGION AND A LOW PRESSURE TAKING SHAPE IN THE HIGH PLAINS...SOUTHERLY WINDS WILL CONTINUE TO STRENGTHEN ACROSS THE LAKE. ON THESE WINDS WILL COME HIGHER DEW POINT AIR. WEBCAM IMAGERY FROM AROUND THE LAKE INDICATES HAZE THAT MAY BE A LITTLE MORE THICKER ALREADY OVER THE WATER. CONCERNS WITH DENSE FOG WILL INCREASE DURING THE DAY AND EVENING ON MONDAY WHEN DEW POINTS OF 10-20 DEGREES HIGHER THAN THE LAKE WATER TEMPERATURES MOVE OVER THE LAKE. THE OFFSHORE SOUTHWESTERLY WINDS ON MONDAY AFTERNOON AND EVENING LOOK TO REACH SMALL CRAFT ADVISORY CRITERIA ALONG THE ILLINOIS SHORE AND IT IS POSSIBLE ALONG THE INDIANA SHORE. WILL GO AHEAD AND ISSUE A SMALL CRAFT ADVISORY ON THIS SHIFT FOR THE ILLINOIS SHORE. AS FOR THE OPEN WATERS...THE STABILITY WILL KEEP GUSTS FAR BELOW WHAT THEY COULD BE MONDAY AFTERNOON...AS WINDS JUST OFF THE DECK /WITHIN 700 FT/ ACROSS THE NORTHERN PART OF THE LAKE ARE FORECAST TO BE 40-50 KT. WILL MENTION A FEW GALE FORCE GUSTS POSSIBLE ACROSS THE NORTH AND THAT WILL BE PARTICULARLY TRUE FROM THE AREA OF THE NORTH BUOY TO THE U.P. SHORE. THE SYSTEM COLD FRONT WILL MOVE SOUTHWARD DOWN THE LAKE OVERNIGHT MONDAY NIGHT. THE ONSHORE FLOW FROM THIS ON TUESDAY...IF A LITTLE STRONGER THAN FORECAST...COULD BRING WAVES TO NEAR SMALL CRAFT ADVISORY CRITERIA ALONG MAINLY THE INDIANA SHORE. MTF && .LOT WATCHES/WARNINGS/ADVISORIES... IL...NONE. IN...NONE. LM...SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742...NOON MONDAY TO 10 PM MONDAY. && $$ VISIT US AT HTTP://WEATHER.GOV/CHICAGO (ALL LOWERCASE) FOLLOW US ON FACEBOOK...TWITTER...AND YOUTUBE AT: WWW.FACEBOOK.COM/NWSCHICAGO WWW.TWITTER.COM/NWSCHICAGO WWW.YOUTUBE.COM/NWSCHICAGO  FXUS63 KLOT 270948 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 348 AM CST Tue Dec 27 2016 .SHORT TERM... 347 AM CST Today through Wednesday night... Relatively tranquil period of late December weather is expected into mid-week, as surface high pressure spreads across the Ohio Valley into Wednesday. Main weather features today will be a gradually diminishing but continued blustery west wind gusting 20-25 mph, closer to, but still slightly above average temperatures, and a period or two of cloud cover brushing the northern tier or two of Illinois counties through mid-day or early afternoon. Despite modest but persistent low level cold advection, model 925-950 mb thermal fields support temperatures generally in the 35-40 degree range this afternoon, especially across the southern 2/3 of the forecast area where MODIS-Terra satellite imagery from yesterday afternoon shows no remaining snow cover. Coolest temps, at or a little above freezing, are expected across the northern tier of IL counties where some snow cover still exists. Guidance in fairly decent agreement with temps, though favored slightly cooler MET numbers over snow cover in north central IL, and slightly warmer MAV farther south. While high pressure passes largely south of the area tonight, weaker ridging will extend north across the cwa beneath a region of 70-80 meter 500 mb height rises which develop in the wake of a mid-level short wave trough which passes across the region late in the day. Winds will finally weaken substantially this evening as the gradient weakens with the approach of the surface ridge. Temps should fall off fairly quickly through the 20's this evening with light winds and clear skies especially over those north/northwesternsnow covered locations. Looking to the west, a strong upper jet streak propagates across the Pacific Northwest and central Rockies tonight into Wednesday, leading to amplification of an upper trough across the upper Midwest and Great Lakes region through Wednesday night. Strong H5 height falls, on the order of 150+ meters, develops across the region by Wednesday night in association with the deepening trough and a 140 knot upper level jet streak which translates through the base of the amplifying trough into the Ohio Valley overnight. Low level winds back to the south and increase late tonight and Wednesday, with warm advection and resulting isentropic ascent eventually leading to development of a mid-level overcast by late Wednesday afternoon. Low levels remain dry however, with large temperature/dew point spreads persisting through Wednesday night despite fairly vigorous forced ascent. Thus no precipitation is expected prior to the associated surface trough/cold front which moves through Wednesday night. South flow and warm advection would support temperatures in the 40's Wednesday per low-level model thermal fields. MAV/MET numbers pretty close up north, but prefer the slightly warmer MET guidance across much of the central and southern cwa where mid-upper 40's appear reasonable given lack of snow cover. Quick tightening of the surface pressure gradient should make for a fairly windy day however, with gusts around 30 mph from the south keeping wind chills in the 30's. The warm-up is fairly brief, as the surface trough/cold front pushes through Wednesday night with a return to colder west flow. Ratzer && .LONG TERM... 204 PM CST Tuesday night through Monday... Not looking for a very active week weather-wise. Robust clipper tracking well to our north should supply a quick shot of robust warm air advection Wednesday. NAM continues to think there's snow on the ground and is holding temps back as a result. Have continued to trend highs warmer than the NAM tainted blended model guidance and closer in line with the MAV which has highs well into the 40s. Cold front moves through with little fan fare Wednesday night with breezy and much cooler conditions Thursday. Forecast soundings suggest the potency of the CAA will be greater aloft resulting in steepening lapse rates which still look like they could result in flurries and perhaps some scattered snow showers Thursday afternoon. Not looking like a measurable precip event at this time. Another clipper fast on the heels of the first will attempt to moderate temps late Friday into early Saturday before another cold front moves through likely cooling temps off a bit. Later in the weekend into early next week forecast confidence is MUCH below average. ECMWF and GFS both dig a vigorous shortwave south down the California coast eventually closing off this mid/upper level low, before ejecting the system eastward. This is where the spread in solutions grow exponentially, with some models (12z operational GFS) phasing this system with northern stream and spinning up a powerful cyclone, while other guidance tracks a weakening southern stream wave across the southern states missing us altogether. Individual ensemble members run the gamut from unseasonable warmth to potential snow storm by early next week, in fact the ECMWF ensembles have high temps ranging from mid 50s (warmest member) to barely 10 above (coldest member) by next Tuesday. Needless to say, no big changes made to the blended guidance in that time range except to lower pops from likelys Monday given this uncertainty. Izzi && .AVIATION... For the 06Z TAFs... Easing but still gusty westerly winds along with a period of MVFR clouds into early Tuesday morning are the two items of interest in the aviation forecast. Gusty westerly winds have started to slowly ease, though still cannot rule out sporadic gusts to 30 kt through 09Z. The overall cyclonic, cold advection weather pattern will keep regular gusts through Tuesday morning, with direction between 250 and 280 degrees. The gust magnitude will ease gradually through Tuesday afternoon and then more quickly at sundown. Scattered to broken cold advection stratocumulus clouds will move over the TAF sites through Tuesday morning. Confidence in cloud bases remaining between 2000 and 3500 ft is high. MTF && .MARINE... 205 AM CST Low pressure continues to shift eastward into western Quebec early this morning. Westerly winds continue to be in the 30 to 35 KT over the open waters of Lake Michigan as of this writing, with slightly lighter flow of 25 to 30 KT in the Illinois and Indiana near shore waters. The current gale headline is set to end at 3 AM CST this morning. While the winds are expected to ease below gale force this morning, a couple of lingering Gales may extend an hour or two beyond this time, especially over Lake Michigan. I don't anticipate needing to extend the warning end time, but I will make a last minute decision on this prior to the 3 AM GLF issuance. Otherwise, the gale warning will be transitioned to a small craft advisory for today, especially for the Indiana waters where high waves will linger through the day. A period of lighter winds are expected tonight as a surface ridge of high pressure shifts over the region. The winds will back to a southerly direction by early Wednesday morning, then increase out of the south Wednesday. These strengthening southerly winds on Wednesday will be driven by a clipper system shifting eastward into western Ontario by late Wednesday afternoon. Pressure falls ahead of this low over the Upper Great Lakes will likely help drive a period of 30 to perhaps 35 KT southerly winds over the lake. While some low end gales are possible during the day, it appears they will be somewhat short lived. Because of this, no gale headline is planned at this time, but I will mention the potential of gale force gusts in the GLF. This same clipper system will push a cold front over Lake Michigan Wednesday night, and this will set up another period of strong west-northwesterly winds over the lake for Thursday and Thursday night. It appears probable that 35 to 40 KT west-northwesterly gales will develop during the day Thursday before abating Thursday night. The strongest winds are expected over the southern half of the lake (including the Illinois and Indiana near shore waters). A gale headline will likely need to be issued for this period as we get closer. An active weather pattern is expected to continue this weekend and into next week, with another clipper system forecast to shift towards the western Lakes region by Saturday. This could result in another period of southerly winds followed by a quick return to northwesterly winds Saturday night. Following this, yet another storm system may develop and shift northward into the lower Great Lakes region by early next week. Depending on the strengthen, and the track of this potential storm system, this could yet again produce strong winds over the lake. KJB && .LOT WATCHES/WARNINGS/ADVISORIES... IL...NONE. IN...NONE. LM...Gale Warning...LMZ777-LMZ779 UNTIL 6 AM Tuesday. Small Craft Advisory...NEARSHORE WINTHROP HARBOR TO GARY IN UNTIL 4 PM Tuesday. Small Craft Advisory...NEARSHORE GARY TO MICHIGAN CITY IN UNTIL 10 PM Tuesday. && $$ VISIT US AT HTTP://WEATHER.GOV/CHICAGO (ALL LOWERCASE) FOLLOW US ON FACEBOOK...TWITTER...AND YOUTUBE AT: WWW.FACEBOOK.COM/NWSCHICAGO WWW.TWITTER.COM/NWSCHICAGO WWW.YOUTUBE.COM/NWSCHICAGO