Dangerous heat will persist across the Southwest U.S. through Saturday with daytime highs of 110-115°F. Extreme Heat Warnings remain in effect. Dangerous heat will build across the southern Plains again beginning this weekend. Heavy to excessive rain over the Carolinas Friday may lead to isolated areas of flash flooding. Read More >
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703 FXUS66 KMTR 281146 AFDMTR Area Forecast Discussion National Weather Service San Francisco CA 446 AM PDT Fri Aug 28 2026 ...New AVIATION, MARINE... .KEY MESSAGES... Updated at 1041 PM PDT Thu Aug 27 2026 - Slight chance of rain showers and thunderstorms through Friday afternoon focused on the Central Coast. - Cooling trend begins Friday && .SHORT TERM... Issued at 1041 PM PDT Thu Aug 27 2026 (Tonight through Saturday) As the remnant moisture from Tropical Storm Iselle moves over the region we must analyze the risk for high-based thunderstorms. This type of thunderstorm forms high in the atmosphere above a dry layer of air. Most of the rain evaporates in the dry layer, but lightning has no trouble getting to the ground. This dry lightning is a well-known wildfire ignition source, so let`s take a close look. The 00Z sounding measured a PWAT of 1.24", with 307 J/kg of MUCAPE. The best lapse rate was 850-500 mb at 7.6 C/km, with the moisture all above 700 mb. As this moisture gradually moves lower in the atmosphere, the instability will increase. That, combined with subtle lift from the trough and terrain could trigger some thunderstorms today. Models are not doing a good job analyzing the (307 J/kg of MUCAPE) instability measured on the 00Z weather balloon sounding. For example the 00Z ECMWF model initialization at OAK has 0 MUCAPE. The lapse rates and precipitable water are in the ballpark, but not lined up as well as reality. It`s the same story with the HRRR and NAM. Specifically the models aren`t resolving the moisture at the base of the unstable layer. As such, we`ve increased the probability of thunderstorms to 15% across the Santa Cruz Mountains and most of Monterey County from 5AM to 2PM. The two big questions are will this instability be realized, and will the rain reach the ground. The instability piece looks better as the night goes on. Now that the balloon data is getting into the models, they are gradually getting more interesting. The 06Z HRRR is just starting to come in and the lightning flash density has trended up. It`s now showing a potential peak of 1 flash every 5 mins across parts of Monterey County, with more scattered activity across the rest of the Bay Area. The 00Z ECMWF has a similar trend across the Central Coast, but still doesn`t show much in the Bay Area. One factor working against convection is the timing. While the upper level support peaks in the mid to late morning, peak surface heating will lag behind by several hours. If these lined up, there would be some more help from surface based instability. On the other hand, if the storms preform well in the morning, the increasing surface temperatures may continue to support convection that keeps some activity going longer into the afternoon than currently expected. As the 500 mb trough axis approaches Saturday it will usher in a much drier air mass. This will end the threat of thunderstorms, while ushering in some gusty onshore winds and cooler temperatures this weekend. && .LONG TERM... Issued at 1041 PM PDT Thu Aug 27 2026 (Saturday night through next Thursday) The marine layer will gradually reform and deepen through the weekend. An active pattern will bring several short waves into the Pacific Northwest this week, keeping the Bay Area on the cool and breezy side. There is even a chance for some light rain midweek. && .AVIATION... (12Z TAFS) Issued at 432 AM PDT Fri Aug 28 2026 VFR will prevail through the day and evening hours. Stratus is expected to return late tonight into early Saturday morning. There is a slight chance of isolated thunderstorms (primarily for the South Bay and Monterey Bay terminals) with shower activity later this morning through the afternoon. Low confidence in how well the marine layer organizes in the wake of todays shower activity. Vicinity of SFO...VFR. There`s a slight chance of showers with isolated thunderstorms later this morning into the afternoon. Expect breezy onshore flow later today behind any shower activity. SFO Bridge Approach...Similar to SFO. Monterey Bay Terminals...VFR expected to prevail through the day and evening hours. There`s a slight chance of showers with isolated thunderstorms this morning and afternoon. Moderate confidence that MVFR/IFR ceilings will return around midnight and persist through the remainder of the forecast period. && .MARINE... (Today through Wednesday) Issued at 432 AM PDT Fri Aug 28 2026 Northwest breezes persist through the weekend then diminish to moderate for the beginning of next week. A long period southerly swell will build to 15 seconds by this evening, peak around 17-18 seconds on Saturday, and then gradually diminish Sunday into the beginning of next week. && .FIRE WEATHER... Issued at 1041 PM PDT Thu Aug 27 2026 A Red Flag Warning is in effect from 5 AM through 3 PM for dry lightning potential across the Santa Cruz Mountains and Central Coast. Refer to the Short Term section for more details. By Saturday a cooling trend will kick off as the marine layer gradually deepens. && .MTR WATCHES/WARNINGS/ADVISORIES... CA...Red Flag Warning until 3 PM PDT this afternoon for CAZ512-516- 517-528>530. PZ...Small Craft Advisory from 9 AM Saturday to 3 AM PDT Sunday for Mry Bay. Small Craft Advisory until 9 AM PDT Sunday for Pigeon Pt to Pt Pinos 10-60 NM-Pt Arena to Pt Reyes 0-10 nm-Pt Arena to Pt Reyes 10-60 NM-Pt Pinos to Pt Piedras Blancas 0-10 nm. Small Craft Advisory until 3 AM PDT Sunday for Pigeon Pt to Pt Pinos 0-10 nm. && $$ SHORT TERM...Flynn LONG TERM....Flynn AVIATION...JM MARINE...JM 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 |
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