A series of fronts will focus locally heavy showers and storms across portions of the central and eastern U.S. through the end of the workweek. Dangerous to extreme heat continues across the Southwest U.S., eventually spreading eastward over the weekend into early next week. Elevated fire weather conditions continue across the Great Basin, with degraded air quality lingering across the Northwest. Read More >
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079 FXUS66 KMTR 051209 AFDMTR Area Forecast Discussion National Weather Service San Francisco CA 509 AM PDT Wed Aug 5 2026 ...New AVIATION, MARINE... .KEY MESSAGES... Updated at 1148 PM PDT Tue Aug 4 2026 - Elevated fire weather threat across the interior North and East Bays, and the Diablo and Santa Lucia ranges through midweek. - Widespread Moderate HeatRisk returns for the end of the work week. - Critical fire weather conditions develop Friday and Saturday. && .SHORT TERM... Issued at 1148 PM PDT Tue Aug 4 2026 (Tonight through Thursday) The picture from space this evening shows stratus snaking through the the region. Earlier model guidance struggled with the onset of its arrival and to some extent the depth as well. It`s currently around 1000ft and has the potential to get to 1200ft, with coastal drizzle and patchy fog possible tonight and into Wednesday morning. The good news is that this will cool temperatures overnight and perhaps take a bit longer to retreat to the coast Wednesday morning. With ridging not as strong and those factors in place, the threat for Moderate HeatRisk has been pulled back with only a few pockets found across the interior Central Coast and interior E and S Bay regions. Some of those pockets include areas above the marine layer, where temperature recovery won`t be as great. Like the prior shift, will not be issuing a Heat Advisory for Wednesday. Wednesday night into Thursday morning, the forecast will be on repeat. Look for the marine layer and associated stratus to seep into the region leading to another cloudy night.&& .LONG TERM... Issued at 1148 PM PDT Tue Aug 4 2026 (Thursday night through next Tuesday) The ridge starts to build back into the region on Thursday, which will compress the marine layer to perhaps 500ft for the next few days. At the surface, winds will have shifted from the south to a more north/northwesterly flow; however, aloft, guidance shows the southerly flow holding. Deterministic models and ensembles show a slug on moisture pushing into our region Thurday-Saturday giving us PWATs ranging from 0.75-1.0, which could lead to things feeling humid. In terms of the heat portion of the forecast, it looks to be on track. Data continues to signal 850mb temps in the 28-30C range, leading to toastier daytime and overnight temperatures. Highs, especially for interior areas and those not affected by the marine layer, will be in the 90s to low 100s while low temperatures there fall into the upper 60s-70s. Widespread Moderate HeatRisk is expected, with some pockets of Major HeatRisk on Saturday. Sunday and Monday, the upper level pattern shows the ridge flattening some. The latest NBM seems to keep temperatures a tad warmer on Sunday, with the more notable interior cooling trend occuring on Monday and Tuesday. && .LONG TERM... Issued at 1148 PM PDT Tue Aug 4 2026 (Thursday night through next Tuesday) The ridge starts to build back into the region on Thursday, which will compress the marine layer to perhaps 500ft for the next few days. At the surface, winds will have shifted from the south to a more north/northwesterly flow; however, aloft, guidance shows the southerly flow holding. Deterministic models and ensembles show a slug on moisture pushing into our region Thurday-Saturday giving us PWATs ranging from 0.75-1.0, which could lead to things feeling humid. In terms of the heat portion of the forecast, it looks to be on track. Data continues to signal 850mb temps in the 28-30C range, leading to toastier daytime and overnight temperatures. Highs, especially for interior areas and those not affected by the marine layer, will be in the 90s to low 100s while low temperatures there fall into the upper 60s-70s. Widespread Moderate HeatRisk is expected, with some pockets of Major HeatRisk on Saturday. Sunday and Monday, the upper level pattern shows the ridge flattening some. The latest NBM seems to keep temperatures a tad warmer on Sunday, with the more notable interior cooling trend occuring on Monday and Tuesday. && .AVIATION... (12Z TAFS) Issued at 509 AM PDT Wed Aug 5 2026 Strong 500 mb high pressure remains over the area, however there is also a stationary and weak surface low pressure system over the CA coastal waters within the marine layer. Counter-clockwise winds with the surface low is entraining higher surface dewpoint air (water vapor) from the west, the water vapor upon closing in on the CA coastline is crossing a cooler gradient in sea surface temperatures. This is resulting in stratus and mist/fog /MVFR-IFR/ reaching the coastline and inland on onshore winds. Even under strong high pressure, the marine layer has deepened to ~ 1800 feet. It`s sharply drier and warmer aloft at and above the marine layer temperature inversion, supporting nocturnal radiative cooling to space. Onshore winds are supported by a 2.4 mb SFO-SAC pressure gradient. It`ll take the morning for stratus, mist/fog to mix out to VFR. Wildfire smoke from the Pacific Northwest may also get entrained into the aforementioned low over the coastal waters today based on recent high resolution near surface smoke forecasts. If smoke is drawn into the stratus, this may lower surface visibilities in a mix of smoke, haze and mist today. Coastal stratus, mist/fog /MVFR-IFR/ return inland on onshore winds tonight and Thursday morning. Vicinity of SFO...Stratus /MVFR/ until 20z today then mixing out to VFR. Stratus returns late tonight-early Thursday by 10z. Light and variable wind becoming northwest 10 to 15 knots in the afternoon and evening. SFO Bridge Approach...Similar to SFO. Monterey Bay Terminals...Stratus /IFR/ until late morning then mixing out to MVFR-VFR. Stratus /IFR/ returns this evening and overnight. Mainly northwest winds 5 to 15 knots. && .MARINE... (Today through Monday) Issued at 425 AM PDT Wed Aug 5 2026 Stationary and weak surface low pressure will remain over the coastal waters today through Thursday morning before dissipating. This low will combine with generally onshore breezes including more locally gusty winds across the San Francisco Bay and the Delta. After the low dissipates Thursday, onshore breezes will continue late in the week and over the weekend. Northwest winds will begin to increase out over the northern coastal waters during the weekend resulting in building seas and hazardous conditions for small craft. && .FIRE WEATHER... Issued at 155 PM PDT Tue Aug 4 2026 After coordinating with neighboring offices and fire partners fire weather concerns over the next 5 to 7 days remain with elevated to near critical conditions. Temperatures remain around 10-15 degrees above normal across the interior with the exception of Wednesday and Thursday where temperatures are more seasonal. High temperatures largely remain in the upper 80s to 90s with the warmest locations reaching the low 100s. Daytime humidity retention remains poor across the interior (15-30%) with humidity values dropping into the 10s across the interior Central Coast, East Bay Hills/Valleys, and the higher elevations. Portions of the interior Central Coast may see daytime humidity values drop into the single digits Tuesday, then again towards the end of the week into the weekend. Overnight RH recovery remains poor at locations above 2,000 feet with values between 25 to 40% expected with some favored locations struggling to reach even 25%. This will be particularly true across the Mayacamas, highest peaks of the East Bay Hills, the Diablo Range, the Santa Cruz Mountains, the Santa Lucia and Gabilan Ranges. Nocturnal drying (rapid decrease in RH through the pre- dawn hours) may also transpire which will increase the burn period or time in which fire behavior may remain active. Energy Release Component (ERC) values have climbed above the 97th percentile within the Interior Central Coast and Diablo-Santa Cruz (Diablo Mountains and Santa Cruz Mountains) zones with the Mid Coast to Mendocino Zone (North Bay) to reach the 97th percentile this week. Large fires become more frequent after ERC values cross the 90th percentile with extreme burning conditions expected with ERC values above the 97th percentile. 100 hr fuels are also responding to the hot and dry conditions with values reaching critical levels across the region. Latest intel also shows Live Fuel samplings are also heading toward critical levels. The one saving grace is that we`re not predicting any large wind events or offshore flow wind events during this time period. That being said, diurnally breezy onshore winds are expected along the coast, mountain gaps and passes, and along the ridgelines each afternoon/evening. Given the elevated fire weather concerns and very dry fuels, individual across the highest risk areas (interior North and East Bays, the Diablo Range, the Santa Lucia Range, and interior Central Coast) are advised to avoid activities that may spark a fire. This includes campfires/barbecues and making sure chains are secured to avoid sparks if towing anything. Fire crews are urged to exercise extra caution when executing any prescribed burns, especially where fuels, topography, and weather align. MM/KR && .MTR WATCHES/WARNINGS/ADVISORIES... CA...None. PZ...None. && $$ SHORT TERM...KR LONG TERM....KR AVIATION...Canepa MARINE...Canepa 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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