A multiple day heavy rainfall event will begin to set up from the Southern Plains into the Mississippi Valley Thursday, peaking in intensity over the holiday weekend. Widely scattered thunderstorms are expected to develop across eastern Colorado Thursday afternoon and evening. Large hail will be the primary hazard. Read More >
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434 FXUS66 KMTR 210702 AFDMTR Area Forecast Discussion National Weather Service San Francisco CA 1202 AM PDT Thu May 21 2026 ...New SHORT TERM, LONG TERM... .KEY MESSAGES... Updated at 1201 AM PDT Thu May 21 2026 - Diurnally driven winds, with onshore winds during the afternoon and early evening - Warm and dry weather today leads to elevated fire weather concerns for the interior Bay Area and Central Coast - Gradual cooling trend begins Friday with seasonal temperatures for the upcoming weekend && .SHORT TERM... Issued at 1201 AM PDT Thu May 21 2026 (Today and tonight) Coastal stratus lingers across the Central Coast and has filled in on the western coast of San Mateo County. A decaying Otter Eddy within Monterey Bay combined with a dry slot paralleling the Big Sur coast is making the stratus forecast somewhat uncertain with a chance that the immediate vicinity of Monterey remains clear overnight. Confidence in inland stratus development, particularly away from the immediate Bayshore or south of Salinas, is low. Stratus should generally clear back to the immediate coast in the hours after sunrise. The large scale pattern over central California is a balance between a ridge over the northeast Pacific and edging into the Pacific Northwest and a trough lingering over the Rocky Mountains with the axis extending into southern California and Baja California. This pattern keeps the upper level winds offshore, if light, but at the surface, a thermal trough over the Central Valley keeps the winds generally onshore with coastal breezes developing across the region during the afternoon and evening. Low temperature this morning range from the upper 40s to low 50s near the coast, lower to middle 50s in the inland valleys, and the middle 50s to lower 60s in the higher elevations. The NBM model output is slightly overshooting the high temperatures, especially in the immediate coastal areas under the marine layer influence, so I have tamped down today`s coastal highs to the upper 50s to the middle 60s. These high temperatures are liable to be an underestimate of as much as 5 degrees or so if stratus clears out earlier than the current forecast, or indeed never develops at all. Away from the immediate coast, temperatures remain near to well above the seasonal average, with highs in the 70s to the lower 80s near the bays, while across the inland valleys, high temperatures range from the middle 80s to the middle 90s. The continuing hot and dry conditions will contribute to another day of elevated fire weather concerns in the interior regions, and indeed a couple of grass fires were reported across the region, one in Contra Costa County and one in San Benito County. For those living or visiting those hot and dry regions, keep the following in mind: * Properly dispose of cigarettes and matches * Do not use a lawn mower on weeds or dry grass * Avoid using equipment that creates sparks * Make sure campfires are completely put out * Make sure no vehicle parts drag on the ground, including towing chains * Obey burn bans & only burn with a permit && .LONG TERM... Issued at 1201 AM PDT Thu May 21 2026 (Friday through Wednesday) A gradual cooling trend begins on Friday with the upper level ridge retreating westward, allowing for more zonal upper level flow across the West Coast and a gradually thickening marine layer. By Saturday, temperatures should be near the seasonal average with highs in the middle 70s to the middle 80s inland. By the early part of next week, a cold front associated with a deep upper level low is likely to sweep through the state, which would cause highs to drop into the 70s across the inland valleys. Close analysis of the ensemble model clusters reveals a small, but significant fraction of the ensemble members (around 10-20%) keeping some level of ridging over California. There is even a slight chance of light rain or drizzle associated with an incoming front, but it`s a low confidence forecast at this point with around a 20-30% probability of seeing any water falling out of the sky, and that is tilted towards the coastal regions where topography would tend to enhance the rainfall amounts. CPC extended guidance does show a slight lean towards precipitation totals above seasonal averages into the beginning of June, with the caveat that for the last weeks of May, just a couple hundredths of an inch will tie or exceed the daily average totals for downtown San Francisco. && .AVIATION... (06Z TAFS) Issued at 926 PM PDT Wed May 20 2026 Currently a mixed bag of IFR-VFR at the terminals. Moderate to high confidence on IFR conditions at HAF, MRY, and SNS with VFR elsewhere tonight. Relatively lower confidence for HAF than the Monterey Bay terminals as stratus has been wiped out north of the Otter Eddy in Monterey Bay. If stratus does redevelop, it`ll likely be around 12Z. Diurnal winds will prevail. Vicinity of SFO...Currently VFR with northwesterly flow. High confidence in VFR and diurnal winds prevailing through the TAF period. FEW/SCT IFR clouds will be possible on the north side of the terminal if stratus sneaks through the Golden Gate Gap. SFO Bridge Approach...The San Mateo Bridge Approach has a lower probability of IFR conditions. Monterey Bay Terminals...Currently VFR and calm at MRY and IFR with northwesterly flow at SNS. Moderate to high confidence on IFR conditions through the night; however, this Otter Eddy could be a case where SNS continues to get fed IFR stratus while MRY remains just on the periphery. VFR is expected by late morning with IFR stratus sticking close to the coast during the day. && .MARINE... (Tonight through next Monday) Issued at 441 PM PDT Wed May 20 2026 Hazardous conditions for small craft will continue through Thursday for the northern outers due to rough seas and strong northerly breezes. Elsewhere, moderate northwesterly breezes and moderate seas will prevail. Conditions improve into the weekend as seas and northwesterly breezes become moderate. && .MTR WATCHES/WARNINGS/ADVISORIES... CA...None. PZ...Small Craft Advisory until 3 AM PDT Friday for Pt Arena to Pt Reyes 10-60 NM. && $$ SHORT TERM...DialH LONG TERM....DialH AVIATION...Sarment MARINE...Sarment 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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