433 FXUS66 KMTR 100828 AFDMTR Area Forecast Discussion National Weather Service San Francisco CA 128 AM PDT Wed Sep 10 2025 ...New SHORT TERM, LONG TERM... .KEY MESSAGES... Updated at 125 AM PDT Wed Sep 10 2025 - Isolated to scattered thunderstorm chances this afternoon, particularly for northern Napa and Sonoma Counties - Isolated to scattered rain showers this afternoon && .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... (06Z TAFS) Issued at 938 PM PDT Tue Sep 9 2025 Post frontal cumulus clouds combined with some possible stratus in a disturbed marine layer is creating a complex symphony of cloud coverage tonight. Most model guidance is very aggressive with IFR ceilings returning to coastal terminals, but due to the unstable airmass and absence of any organized marine layer, the TAFs lean optimistic with more MVFR impacts tonight. That said, the confidence is very low due to the transient nature of cumulus clouds and the fact that most model guidance disagrees with me. Wednesday will bring renewed shower chances in the afternoon, with an outside chance for a quick thunderstorm in the North Bay. Although a lack of shear will limit any prolonged organization. Vicinity of SFO...VFR-MVFR ceilings are likely through the TAF period from cumulus cloud bases (LCL 2500 ft). Coverage will increase through the day Wednesday, though the ceiling will likely lift back to VFR by midday. A few light showers are possible Wednesday afternoon, but still look too light and brief to include in the SFO TAF. SFO Bridge Approach...Similar to SFO Monterey Bay Terminals...MRY and SNS are holding out with clear skies for much longer than the guidance suggested, which is giving me more confidence in the optimistic MVFR/VFR TAF. If I'm wrong and the ceilings do drop to IFR tonight, they will certainly lift by mid morning as the atmosphere become more mixed with convective currents. && .MARINE... (Tonight through next Monday) Issued at 938 PM PDT Tue Sep 9 2025 Gentle to moderate NW winds and moderate seas will continue through Thursday. By Friday, winds will increase a fresh NW breeze. Meanwhile, a moderate period NW swell will arrive by Friday. This swell, combined with the rougher wind waves, will build moderate to rough seas Friday through Saturday. Otherwise isolated showers with a slight chance for thunderstorms could bring locally hazardous conditions this afternoon through Wednesday night. && .MTR WATCHES/WARNINGS/ADVISORIES... CA...None. PZ...None. && $$ SHORT TERM...Sarment LONG TERM....Sarment AVIATION...Flynn MARINE...Flynn 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