Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA

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331 FXUS66 KMTR 240030 AFDMTR Area Forecast Discussion National Weather Service San Francisco Bay Area 430 PM PST Tue Jan 23 2018 .SYNOPSIS...Dry and seasonable weather with increasing high clouds for the remainder of the day. Next front arrives Weds morning in the North Bay and then spreads rain southward later Weds afternoon into Weds night for the Bay Area into the Central Coast. Steady rain quickly turns to showers for Thursday. A round of more organized showers is possible Thursday afternoon as the upper low moves inland. Dry weather forecast Friday through the weekend and into early next week. && .DISCUSSION...as of 12:00 PM PST Tuesday... After starting the day 5 to 20 degrees cooler than yesterday and with areas of patchy dense fog-- dry conditions, seasonal temperatures, and increasing high level clouds are anticipated for the remainder of the day. These high clouds, generally at about 25000 ft, are arriving at the leading edge of an air mass boundary which is set to pass through the San Francisco and Monterey Bay areas in the form of a cold front through the day tomorrow. Forecast models are in fair agreement regarding the timing and strength of this storm system, with some minor discrepancies in timing and precipitation accumulations. Generally speaking, the forecast models anticipate the leading edge of precipitation to move into the northern periphery of the North Bay by or just after sunrise tomorrow. The main rain band will then transition southeastward and pass the Golden Gate, moving into the immediate San Francisco Bay area, around or just after lunchtime. Next, the storm system will arrive in the South Bay/Santa Cruz area by late afternoon tomorrow before shifting farther southward into the greater Monterey Bay region by tomorrow evening. Forecast models generally agree that this system will be stronger than the last storm system to move through the area Sunday night, but that this system will still be on the weaker side of mid- winter storm systems. Overall, sustained winds will be breezy with brief stronger gusts ahead of and with the frontal passage. Winds will peak at around 35mph over the highest peaks and some coastal areas, while most areas will generally experience breezy winds in the 10 to 20 mph range. Precipitation amounts range from 1.25-1.75", 0.66-1.00" in the North Bay valleys, 0.33-0.66" for the San Francisco Bay area, except lesser amounts in the South Bay due to rain shadowing (ie around .25-.33"), with .20-.40" in the Monterey Bay area, and less than .20" in the southern Salinas Valley/Pinnacles NP area. Post frontal instability will bring some light lingering rain showers and an isolated thunderstorm (mainly offshore of the Sonoma coast) on Thursday. We will continue to be running below normal seasonal precipitation with the forecast precipitation amounts. Here is where we stand as of midnight this morning: SITE SINCE OCT01 (SEASONAL) SINCE JAN01 (CALENDAR) OBSRV NORMAL %NORM OBSRV NORMAL %NORM Santa Rosa 12.49 19.25 65 6.18 5.50 112 Napa 6.04 10.69 57 3.01 3.37 89 Concord 5.68 8.09 70 2.99 2.54 118 DT San Francisco 7.99 12.02 66 4.70 3.18 148 SFO Airport 7.51 10.36 72 4.46 3.00 149 Oakland 7.52 10.60 71 4.30 3.29 131 Hayward 4.77 8.82 54 2.76 2.53 109 Livermore 4.80 7.92 61 2.81 2.34 120 San Jose 4.27 7.27 59 2.70 2.18 124 Moffett Field 4.44 6.85 65 2.49 2.01 124 Watsonville 5.57 10.42 53 3.32 3.20 104 Monterey 6.11 7.80 78 4.13 2.65 156 Santa Cruz 8.33 15.08 55 5.33 4.20 127 Weak ridging builds in behind tomorrow`s storm system for Friday through the weekend and into early next week, bringing dry conditions, mild afternoons, and cold nights. Forecast models bring a different low pressure into the area around Tuesday of next week, however, the trajectory of this system means it will most likely be dry and more of a wind producer, if anything, for our area. && .AVIATION...as of 4:30 PM PST Tuesday...A few stratocumulus clouds this afternoon from residual low-level moisture. Next system will bring increasing moisture Wednesday morning with lower cigs into the SFO Bay Area after 15Z. Timing of precip appears to be after 20Z for the SFO Bay Area. Southerly winds increase as the front approaches after 20Z with gusts to 25 kt. Vicinity of KSFO...VFR through tonight becoming MVFR after 15Z and light rain after 19-20Z. Light south to southeast wind overnight increasing to 15-18 kt gusting to 25 kt after 20Z. SFO Bridge Approach... Similar to SFO. Monterey Bay Terminals...VFR through Wednesday morning becoming MVFR after 20Z. && .MARINE...as of 4:30 PM PST Tuesday...An approaching storm system will approach tonight resulting in increasing southerly winds tonight and Wednesday. Increasing winds will result in hazardous conditions with steeper wind waves. The storm system will bring rain to the region and thunderstorms to the northern waters. && .MTR WATCHES/WARNINGS/ADVISORIES... .Tday...NONE. && $$ PUBLIC FORECAST: DRP AVIATION: W Pi MARINE: MM Visit us at www.weather.gov/sanfrancisco Follow us on Facebook, Twitter, and YouTube at: www.facebook.com/nwsbayarea www.twitter.com/nwsbayarea www.youtube.com/nwsbayarea

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