Gusty winds and heavy rainfall may impact the Big Island of Hawaii as a nearby tropical system passes west of the islands. From the Southwest U.S., Rockies and across the Gulf Coast, heavy rainfall could lead to isolated flash flooding. Severe thunderstorms are expected for areas of the Plains, Gulf Coast and Southeast through Tuesday. Extreme heat lingers for the southern Plains and Southwest. Read More >
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147 FXUS66 KMTR 241827 AFDMTR Area Forecast Discussion National Weather Service San Francisco CA 1127 AM PDT Mon Aug 24 2026 ...New AVIATION, MARINE... .KEY MESSAGES... Updated at 1200 AM PDT Mon Aug 24 2026 - Temperatures near to slightly below seasonal averages persist within the marine layer through the rest of the week - Warm and dry conditions continue above 1000 feet - Pattern change Friday into the weekend brings cooler temperatures and possible tropical moisture && .SHORT TERM... Issued at 1200 AM PDT Mon Aug 24 2026 (Today and tonight) For the last couple of days we`ve written the tale of a battle between the eastern Pacific trough and the Desert Southwest ridge that has dominated our weather pattern. Today it`s the troughing that will take the upper hand as a cutoff low inches towards the Pacific Northwest, leading to slightly cooler temperatures and cloudier mornings across the region. Another interesting factor today is a weak shortwave trough coming into the region through the morning. The two main global weather models, the European ECMWF and the American GFS, are showing that there could be enough instability, moisture, and lift to maybe, just maybe, support a few high-based thunderstorms developing across the North Bay, particularly over Napa County. This is a very low probability forecast (around 5% chance through around 11 AM), along with a low confidence for storm development. The higher resolution NAM, HRRR, and RRFS models do not show any signs of developing storms, lapse rates aloft are marginal at best, and K-Index values below 20 degrees Celsius suggest no chance for developing thunderstorms. Any precipitation that does develop is also likely to end up as virga, evaporating within the vast dry low layer where relative humidity values as of the 0Z (5 PM PDT) OAK sounding fell into the single digits. Today`s highs range from the 80s into the lower 90s for the inland valleys, the middle 90s to the triple digits across the warmest interior spots, the upper 60s to the upper 70s near the Bays, and the upper 50s to the middle 60s along the Pacific coast. The higher elevations above the marine layer influence will remain rather day, especially during the afternoon and evening hours. Breezy onshore pattern winds will develop this afternoon and evening with wind gusts reaching 20 to 30 mph along coastal jet regions, through favored gaps and passes, and within the northern Salinas Valley. Smoke from the Timber Fire will continue impacting the immediate area around the actively burning fire through the day, with winds aloft lofting the smoke plume over Salinas and Hollister. For air quality information, refer to products issued by the Monterey Bay Air Resources District. && .LONG TERM... Issued at 1200 AM PDT Mon Aug 24 2026 (Tuesday through Sunday) Temperatures rebound Tuesday and Wednesday as the ridge reasserts itself, with the inland valleys reaching highs in the middle 80s to the middle 90s on Wednesday and Thursday while the Bays reach the lower 70s to the lower 80s. A pattern change will begin later on Thursday as a deep trough builds into the West Coast while the ridge begins to inch its way eastwards towards the Southern Plains. It will be interesting to see what the trough will do as the upcoming weekend comes closer and we are better able to refine the forecast. Also worth a mention is the possibility of tropical moisture coming into the region, with our colleagues at the National Hurricane Center tracking Tropical Storm Iselle and three other areas of interest off the coast of Mexico. While the circulations remain far to the south of our waters, the resultant moisture will be captured by the developing trough with a wide variation in just how far north the moisture goes. By way of example, the deterministic European ECMWF model brings a swath of tropical moisture into the San Francisco Bay Area by Friday morning, whereas the deterministic GFS keeps the moisture plume to the south of Monterey County. In addition, wave models are also showing an increase in southerly swells as the tropical eastern Pacific remains active into the early part of next week. && .AVIATION... (18Z TAFS) Issued at 1126 AM PDT Mon Aug 24 2026 A blanket of stratus has retreated to the coastline resulting in VFR conditions for most terminals, except for HAF. HAF may briefly scatter out this afternoon, but will mainly remain IFR through the TAF period. Moderate onshore flow persists during the day which will allow for a more prominent inland push of stratus tonight. The marine layer is expected to be around 1000 to 1200 feet overnight which will be sufficient enough to reach most terminals. High confidence for coastal sites (HAF, MRY, and SNS) to develop IFR to LIFR ceilings as early as sunset. Medium confidence on the exact arrival of IFR to MVFR ceilings elsewhere, though they may arrive as early as 04-06Z Tuesday. Vicinity of SFO...VFR through the afternoon as westerly winds increase to around 15 kt. Borderline MVFR-IFR ceilings are expected tonight with the shallow marine layer and persistent onshore push, it just becomes a matter of when. High confidence OAK will develop ceilings before SFO, though medium confidence on the arrival as they may roll in a few hours later than specified. SFO Bridge Approach...Similar to SFO. Monterey Bay Terminals...IFR to LIFR ceilings will continue to lift this morning with VFR conditions to prevail this afternoon. An early return of IFR conditions is likely with ceilings gradually lowering to LIFR by late this evening/overnight. Some potential for reductions in visibility early tomorrow morning but that will depend how much the marine layer compresses and if fog is able to develop. Breezy onshore winds continue this afternoon/evening before winds ease overnight. && .MARINE... (Today through Saturday) Issued at 1126 AM PDT Mon Aug 24 2026 Northwest light to moderate breezes continue into Tuesday and gradually increase to fresh by the middle of the week. Wind driven seas remain slight to moderate, though a long period swell is expected to approach our coastal waters by the end of the week. Smoke from the Timber Fire continues to impact visibility to as low as 2 miles along the Big Sur coast. && .MTR WATCHES/WARNINGS/ADVISORIES... CA...None. PZ...None. && $$ SHORT TERM...DialH LONG TERM....DialH AVIATION...Navarrete MARINE...Navarrete 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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