FXUS65 KTWC 281653

AFDTWC



Area Forecast Discussion

National Weather Service Tucson AZ

953 AM MST Wed Dec 28 2016



.SYNOPSIS...Expect dry weather and mild temperatures again

today with the potential for showers and mountain snow showers

starting late Thursday and continuing through the New Year's Day

holiday weekend.



&&



.DISCUSSION...A fairly thick veil of high clouds stretches across

much of SE AZ and stretches upwind into the Pacific. The clouds

are not as thick in Cochise Co. as per satellite imagery, but

made appropriate changes to sky cover today and tonight. Also

lowered high temperatures a few degrees due to cloud thickness.



First look at 12z models shows NAM still most emphatic developing

rain Thursday, mainly east of Tucson. This is in fair agreement

with 00z ECMWF. 12z GFS backed off on all precip in AZ and NM

through Thursday night. Based on how much better the NAM performed

with the initial moisture push with the system during the middle

of last week, will hold firm with precip chances Thursday and may

even raise chances east of Tucson.



Will discuss the rest of the forecast period later after more data

becomes available.



&&



.AVIATION...Valid through 29/18Z. 

BKN to occasionally OVC clouds AOA 20k ft AGL thru the forecast

period, except less sky cover vcnty KDUG today. SFC wind will

generally remain less than 12 kts. Aviation discussion not updated

for TAF amendments.



&&



.FIRE WEATHER...Dry conditions are expected through tonight

with above normal temperatures persisting through Friday. Unsettled

weather returns late this week into this weekend with a chance of

rain and snow showers Thursday through Sunday. Some gusty 20-ft

winds may occur this weekend, with speeds mainly less than 15 mph

otherwise.



&&



.PREV DISCUSSION...Water vapor imagery continues to show a large

ridge centered over the southwest Gulf of Mexico, with weak

ridging to zonal flow across most of the southern tier of the

nation. Meanwhile, the closed low continues to spin well offshore,

west of the Baja Spur. Ahead of this low, plenty of mid and high

level moisture can be seen streaming northward across much of the

eastern Pacific waters, the Baja peninsula, northwest Mexico and

into portions of the Desert Southwest. IR imagery shows this swath

of extensive cloudiness spreading north and east and generally

covering the southern third of Arizona. These clouds are mostly

high cirrus clouds, so will see a mix of sun and clouds from time

to time today as cirrus at times will be thick enough to obscure

the sun, but mostly sunny to partly cloudy to the north and partly

to mostly cloudy to the south.



Models continue to indicate that the eastern Pacific upper low will

continue to retrograde through early Thursday before beginning to

move northeast toward the southern California coast by early Friday.

Models eventually show the low evolving into just an open wave as it

moves across southern California, Arizona and into New Mexico late

Friday through late Saturday. Meanwhile, a secondary system right on

the heels of the first one, dives southeast from California and

sweeps through the Desert Southwest late Saturday through Sunday.

The GFS solution has the system farther north than the ECMWF, which

has it move over northern Sonora. As a result of this difference in

timing and position, the GFS is keeping the bulk of the associated

moisture to our north, while the ECMWF has it generally over much of

the state. 



At this time yesterday I thought the models were finally converging

on a similar outcome for the later part of the week and into the

weekend. However, when I was looking at the 27/00Z MOS PoP numbers

from the GFS it was advertising PoPs for Tucson in the 50 to 60%

range for Saturday night into Sunday. The 27/12Z run trimmed those

values down to the 10 to 30 percent range and the latest 28/00Z run

has only single digit PoPs for Tucson through the next seven days.

On the other hand the ECMWF has maintained a wet scenario, with PoPs

still in the likely to categorical range from Friday night through

Sunday. Given this disagreement between the models, as well as a

lack of run-to-run consistency, at least from the GFS, I will just

'stand pat' with the PoP forecast that I inherited, which looks

eerily similar to what I used yesterday at this time. The bottom

line is that this means with very low confidence in this situation

will show slight chances creeping in from the southeast late

Thursday and then spreading farther west toward the Tucson area by

the overnight and early morning hours Thursday into Friday, with

just slight chances for central zones and chance category toward New

Mexico and including the mountain tops. I still show scattered type

PoPs across the board for Saturday into Sunday. With models all

showing some flavor of trough and thus disturbed weather during this

period, think having at least slight chance to chance PoPs in the

forecast is the way to go. Subsequent shifts can fine tune the

forecast as the system gets closer in both space and time.  



For Tucson, high temperatures will range from 8 to 10 degs above

normal today through Friday, then near normal Saturday. Highs 4 to 6

degs below normal Sunday, then near normal for early next week. Low

temps will range from 9 to 12 degs above normal Thursday through

Saturday mornings, then about 3 to 5 degs above normal Sunday and

only slightly above normal for early next week. 



&&



.TWC WATCHES/WARNINGS/ADVISORIES...None.



&&



$$



DROZD



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