054 FXUS66 KMTR 101322 AFDMTR Area Forecast Discussion National Weather Service San Francisco CA 622 AM PDT Wed Sep 10 2025 ...New KEY MESSAGES... .KEY MESSAGES... Updated at 620 AM PDT Wed Sep 10 2025 - 15-25% slight chance of isolated to scattered thunderstorms Wednesday afternoon. - Marginal risk for flash flooding in northern Napa and Sonoma counties. - Drier and slightly warmer weather Friday and Saturday. - Unsettled weather may return to the Bay Area on Sunday. && .SHORT TERM... Issued at 125 AM PDT Wed Sep 10 2025 (Today and Thursday) Today's the day! The upper-level low that brought us rain the past two days will be overhead providing the best chance for thunderstorms with this system and the essential ingredients of lift, instability, and moisture in place. While the isolated to scattered thunderstorm threat is namely for northern Napa and Sonoma Counties, I cannot completely rule out the rest of the area although the ingredients become far less favorable southward. The September 10th 00Z sounding revealed a low-level temperature inversion/cap near 800 millibars - over 100 millibars below the freezing level. Thus, surface temperatures would have to be high enough to break that cap and/or terrain would have to do the heavy lifting (literally) to get a thunderstorm. This is important because no ice = no thunderstorm. The long, skinny CAPE profile on model point soundings suggests that it is highly dependent on the surface dew point temperature and surface temperature. MUCAPE will be surface based CAPE in this case; thus, thunderstorm activity will kickoff by the afternoon and diminish after sunset once daytime heating is lost. Flooding and lightning threats exist with all thunderstorms; remember: when thunder roars, go indoors! and see a flash, dash inside! Outside of potential thunderstorm development, rain showers will be possible this afternoon and to a lesser extent tomorrow afternoon - accumulation will be beneficial. && .LONG TERM... Issued at 125 AM PDT Wed Sep 10 2025 (Friday through Tuesday) Global ensemble clusters remain in agreement with the upper-level low exiting to the northeast on Friday and split flow developing across the area Saturday. A developing gale force low in the Gulf of Alaska will bring the next round of unsettled weather to the area on Sunday via cold front. The uncertainty right now lies in how deep that upper-level low will dig and what adverse impacts if any are brought to the region. This uncertainty gets propagated into Monday and Tuesday with global ensemble clusters split nearly 50/50 on if the trough will continue to dig or if ridging builds in from the Eastern Pacific Ocean. && .AVIATION... (12Z TAFS) Issued at 523 AM PDT Wed Sep 10 2025 Moderate confidence that MVFR CIGs will continue through the remainder of the morning but, given the upper level low progressing into the West Coast, fluctuations higher and lower may temporarily occur as well. Shower activity looks to pick up late morning into the afternoon with some potential for thunderstorms in the northernmost portions of Sonoma and Napa Counties. Current thinking is that thunderstorms will not impact any of the TAF sites and will be more terrain driven/confined to the north of STS and APC. That being said, if a thunderstorm were to impact any of the sites it would be STS with the highest potential for storms this afternoon. If a thunderstorm or stronger cell does impact an airport, gusts greater than in the TAF may develop and visibilities may temporarily reduce. General thinking is that CIGs will at least briefly rise above 3000 ft this afternoon before dropping back to MVFR conditions late this evening. Vicinity of SFO...CIGs will be right on the VFR-MVFR border with fluctuations expected through the remainder of the morning given the upper level low pushing into the West Coast. MVFR CIGs are likely to return later tonight and then persist through the end of the TAF period. Winds stay onshore through the TAF period with winds are 10- 12 knots this afternoon. While thunderstorms are not anticipated to impact SFO, if a stronger shower moves over the airport locally stronger gusts and temporarily reduced visibility may develop. SFO Bridge Approach...Similar to SFO. Monterey Bay Terminals...Highest confidence in MVFR CIGs for the majority of the TAF period. Expecting CIGs to briefly rise, becoming VFR, during the afternoon before MVFR CIGs return late afternoon/early evening. Adjusted the timing of MVFR CIGs returning earlier but this may need to be adjusted depending on what scattered showers do this afternoon. Low confidence that showers will impact MRY and SNS, but if they do develop it would most likely be this afternoon into tonight. If a shower moves directly over the airport, it may result in locally stronger gusts and temporary reductions in visibility. && .MARINE... (Today through Monday) Issued at 523 AM PDT Wed Sep 10 2025 Moderate NW winds and moderate seas continue through Thursday with localized fresh gusts across the northern outer waters. Winds increase Friday through the weekend with fresh NW winds spreading across the majority of the coastal waters. Wind waves will build Friday into the weekend as winds increase and a moderate period NW swell train will arrive Friday into Saturday, resulting in moderate to moderate rough seas. Isolated to scattered showers continue Wednesday and Thursday with a low (less than 5%) chance of thunderstorms this afternoon. && .MTR WATCHES/WARNINGS/ADVISORIES... CA...None. PZ...None. && $$ SHORT TERM...Sarment LONG TERM....Sarment AVIATION...Kennedy MARINE...Kennedy Visit us at www.weather.gov/sanfrancisco Follow us on Facebook, X, and YouTube at: www.facebook.com/nwsbayarea x.com/nwsbayarea www.youtube.com/nwsbayarea