FXUS66 KMTR 292158 AFDMTR Area Forecast Discussion National Weather Service San Francisco Bay Area 258 PM PDT Sat Oct 29 2016 .SYNOPSIS...Lingering showers will remain possible today ahead of our next frontal system set to impact the region tonight into Sunday. Widespread rainfall will return to the North Bay tonight and will spread across the remainder of the region Sunday morning. Another weak system will move through Monday into Monday night. Mainly dry conditions return for the latter half of the upcoming work week. && .DISCUSSION...as of 03:00 PM PDT Saturday... Water vapor imagery shows a distinct 544DM low pressure trough situated offshore of the PacNW/NorCal coast this afternoon. This weather system will dominate the local weather through the coming 24 hours and has already been responsible for bringing increasing southerly winds especially along the coast and over ridgetops (max up to around 37mph at Mount Diablo so far) and light prefrontal rain showers to the the northern counties. Latest analysis shows a weak cold frontal boundary residing just north of the area from Mendocino to Mount Shasta early this afternoon. Precipitable water values with this frontal boundary are not overly impressive (.6-.9" TPW/100-150% of average) so not really expecting to see tremendous rainfall with the initial arrival of the frontal passage. 850-700MB moisture advection indicates rain rates will begin to increase by this evening, predominately over the North Bay, and then continuing to ramp up into the overnight hours. The higher POES derived rain rates are well upstream of the initial frontal passage, so this aligns with model data pushing for heavier rain rates later in the overnight hours. As this system moves across the area, it will gradually taper off and weaken, providing the highest rainfall accumulations for the North Bay where .75-1.5" could fall in the valleys where most populated areas, and 1.5-2.5" are possible for the higher terrain of Napa, Sonoma, and Marin counties. Further south, Oakland and San Francisco will see around half of an inch, and San Jose will be rain shadowed and see a lesser amount. Orographics will help bring locally higher amounts to those areas that favor southerly feeds, including both the Santa Cruz and Big Sur coastal mountain ranges. By the time the system pushes into the Salinas valley, amounts only ranging from a tenth to a quarter of an inch are expected. Most of this rain is expected to fall overnight tonight and into early tomorrow. Lingering rain showers and thunderstorms are possible on Sunday as the core of the upper shifts closer. There will be a brief hiatus from the wet weather Sunday night into early Monday before a different, weaker system descends into the region from the northwest. Days ago, model guidance suggested that the area would remain rain free on Monday, with maybe a hint of rain for the North Bay. Since then, model guidance has been trending towards a wetter/more progressive system that will bring precipitation over more expansive area. It now appears that most areas are now looking at seeing some rain during the prime trick or treating hours Monday evening. That said, precipitation accumulations are not anticipated to be nearly as impressive as previous systems. North Bay valleys will see a third to half an inch of rain, immediate Bay Area could see a tenth to a quarter of an inch, and areas to the south will see less than a quarter. Lingering showers into Tuesday are anticipated. Models have been trending increasingly dry from midweek and beyond as high pressure builds aloft. Further north, low pressure systems will continue to descend into the greater region, but should predominately be deflected to our north. The North Bay could see a few glancing blows from these systems on Wednesday and again on Friday. && .AVIATION...as of 11:11 AM PDT Saturday...Borderline MVFR/VFR at most terminals this late morning. Spotty showers possible through the afternoon. Cigs should lift to VFR in the next couple of hours, except for KSTS and areas of the North Bay, where MVFR/IFR should prevail. Light to moderate southwesterly winds this afternoon. Wind will switch out of the south tonight and become quite gusty early Sunday morning as a result of a cold front approaching the area. Along with the breezy winds, widespread rain will accompany the front. Cigs are expected to lower to MVFR and perhaps IFR ahead of the front, especially under heavier showers that occur. Low to moderate confidence. Vicinity of KSFO...Mainly VFR this afternoon, but intermediate MVFR cigs are possible. Very spotty showers will persist, but overall, mostly dry. Southwest winds around 12 kt this afternoon. Winds turn out of the south by 02z and will be very breezy by 09z. Gusts of 30 kt possible through Sun morning. Winds switch back out of the southwest around 19-20z Sun, but still gusty. MVFR cigs will be likely by 12z Sun, ahead of the front. Low confidence on exact timing. Moderate confidence on winds. SFO Bridge Approach...Similar to KSFO. Monterey Bay Terminals...Mainly VFR, but will have to monitor developing low clouds through the day, as forecast models have been struggling with cigs as of late. Westerly/southwesterly winds this afternoon, 10-12 kt, with occasional higher speeds through the Salinas Valley. Wind speeds increase Sunday morning, ahead of a cold front. For the Monterey Bay area, winds will be southeast by midnight tonight, sustained 10-15 kt. Gusts up to 25 kts are possible for KSNS. Expect MVFR cigs to return early tonight for the area, sometime around 02z, but will be dependent on how cigs develop this afternoon. Rain moves in around 15z. Low to moderate confidence. && .MARINE...as of 10:35 AM PDT Saturday...Generally light winds and seas with isolated showers possible today. More widespread rain moves in beginning Sunday morning as a low pressure system draws closer to the Northern California waters. Southerly winds increase tonight as a result. The winds will switch to west to southwest and decrease Sunday night. && .MTR WATCHES/WARNINGS/ADVISORIES... .Tday...NONE. && $$ PUBLIC FORECAST: DRP AVIATION: BAM MARINE: BAM Visit us at www.weather.gov/sanfrancisco Follow us on Facebook and twitter at: www.Facebook.com/nwsbayarea www.twitter.com/nwsbayarea