textproduct: San Francisco Bay Area

KEY MESSAGES

Updated at 1206 AM PDT Sun Aug 9 2026

- Widespread moderate with areas of major HeatRisk for the interior Sunday

- Near-critical fire weather conditions in the higher terrain Sunday

- Minor coastal flooding for bayshore locations along the San Francisco Bay, San Pablo Bay, and Monterey Bay through Thursday morning

UPDATE

Issued at 907 AM PDT Sun Aug 9 2026

The marine layer has compressed to around 800 feet in depth this morning per the OAK Upper Air Sounding. As such, will maintain current Heat Advisory across the interior through 10 PM this evening. With strong high pressure over the Desert Southwest, still expecting today to be the warmest of the next 7 days. Will need to evaluate late this morning and/or early afternoon whether or not to cancel the Heat Advisory for portions of the region.

RGass

SHORT TERM

Issued at 1206 AM PDT Sun Aug 9 2026 (Today and tonight)

The 00Z balloon observed a 925 mb temperature of 30.35 degrees Celsius (third hottest for the date and time), 850 mb temperature of 28.75 degrees Celsius (second hottest for the date and time), and 500 mb height of 5970 meters (record for the date and time). Satellite shows stratus clearing off the coast due to a strengthening ACV-SFO surface pressure gradient. Northerly/offshore flow down the Northern Sacramento Valley Sunday morning will help push the marine layer back to the coast. Both of these factors should support Sunday being the hottest day of this event; however, if the marine layer prevails and northerly/offshore winds don't develop we could have a repeat of Saturday with our forecast being too hot for the lower elevations. Nonetheless, temperatures will be 10 to 15 degrees above normal outside of the marine layer influence with widespread moderate HeatRisk and areas of major HeatRisk in the higher terrain. Minor coastal flooding is expected during the highest high tide (evening/overnight) for bayshore locations along the San Francisco Bay, San Pablo Bay, and Monterey Bay through Thursday morning. Have opted to leave the Pacific Coast out as it would be marginal and overnight if it occurred; however, high tide may cutoff access to some beaches.

LONG TERM

Issued at 1206 AM PDT Sun Aug 9 2026 (Monday through Saturday)

Cooling trend kicks off Monday as upper-level shortwave troughing develops off the West Coast. This will moderate temperatures to near normal, deepen the marine layer bringing a chance for drizzle, and increase onshore flow. A reinforcing trough Wednesday will bring more prominent troughing off the West Coast Thursday and into the weekend. This along with a resurgence in monsoonal moisture may be enough to bring elevated rain showers/thunderstorms to the forecast. While there's no mention in the official forecast now, it may be warranted as we get closer.

AVIATION

(12Z TAFS) Issued at 523 AM PDT Sun Aug 9 2026

A narrow channel of coastal stratus, fog and mist /MVFR-IFR-LIFR/ have held together and remains trapped beneath a strong lower level temperature inversion associated with a strong 500 mb high pressure system. Part of this stratus, fog and mist has reached into the Bay Area and the north Central Coast. The marine layer depth is approx 1000 feet.

Satellite shows an area of clear sky out over the outer waters and offshore waters which corresponds to an area of warmer air, primarily drifting southward. Satellite also shows stratus and fog redeveloping to our north. The leading edge of this stratus may join the nearly stationary stratus south of Point Arena during and just after daybreak. This pattern is characterized by large temperature differences over short distances, mainly in the vertical and also from the coastline to inland valleys (sharply warmer aloft, at the August maximum otherwise not atypical of either July or August). It's noteworthy that air temperatures in the lower level temperature inversion are in the 80s and lower 90s. With the wind flow weighted to onshore, we're essentially in a persistence forecast. Diurnal surface warming will tap the lower level temperature inversion warmth, vertical mixing and warming will mix out the stratus, fog and mist to VFR back to the coastline today. The gradient of surface pressure remains weighted to some northerly (ACV-SFO) otherwise onshore per NAM output showing 4 mb to almost 5 mb SFO-SAC during the afternoon and evening.

Depending how well (or not) the coastal stratus to our north holds together during mixing today will determine if coastal stratus, fog and mist return tonight and Monday morning. The NAM has been verifying a little better than the HRRR model on this forecast. The NAM leans toward stratus, fog and mist returning tonight and Monday morning.

Recent GFS/NAM/HRRR output has also showed mid level instability arriving from southern California approx Monday timeframe, but it appears to dissipate while running northward into the strong ridge and mid level dry air just to our north.

Vicinity of SFO...Stratus /IFR/ forecast mix out time is 17z today. VFR for the remainder of the day then stratus is forecast to return 10z Monday. West to northwest wind 5 to 15 knots except 15 to 25 knots this afternoon and evening.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Stratus, fog and mist /IFR-LIFR/ mixing out to MVFR-VFR by late morning. Stratus, fog and mist /IFR-LIFR/ returns tonight and Monday morning. Mainly west to northwest winds 5 to 15 knots.

MARINE

(Today through Friday) Issued at 907 AM PDT Sun Aug 9 2026

Fresh to strong northwest winds persist with rough seas up to 12 feet today through mid week. Northern coastal waters will be hazardous for small craft with occasional near gale force gusts possible through early week. The San Francisco Bay north of the Bay Bridge into the Delta will have gusty onshore winds through late evening. Long period southerly swells at around 15 to 17 seconds continue.

FIRE WEATHER

Issued at 1206 AM PDT Sun Aug 9 2026

Near-critical fire weather conditions persist today across the interior due to very hot and dry conditions. Strong thermal belting, extended burn periods, elevated PIG, and near or record BI/ERC values are expected. Temperatures will be 10 to 15 degrees above normal with a compressed marine layer. Poor minimum daytime relative humidity as low as 10% in the afternoons with little-to-no recovery overnight is expected with nocturnal drying in the highest terrain. Even with critically dry conditions, wind is the limiting factor preventing this from being a Fire Weather Watch/Red Flag Warning event. Outside of the diurnal sea breeze that will bring breezy conditions in the afternoons and evenings through gaps and passes, onshore flow will be light. Additionally, critical conditions are not expected to meet coverage or duration requirements to warrant a product. Localized elevated fire weather conditions in the highest terrain can be expected Monday due to the combination of recent hot/dry conditions and the increase in onshore flow. Depending how the pattern and marine layer evolves, a headline may be warranted and this could last through Wednesday.

MTR WATCHES/WARNINGS/ADVISORIES

CA...Coastal Flood Advisory until 2 AM PDT Thursday for CAZ006-506- 508-529-530.

Heat Advisory until 10 PM PDT this evening for CAZ503-504-510- 512>518.

PZ...Small Craft Advisory until 9 PM PDT this evening for SF Bay N of Bay Bridge.

Small Craft Advisory until 3 AM PDT Monday for Pt Arena to Pt Reyes 0-10 nm.

Small Craft Advisory until 9 PM PDT Monday for Pt Arena to Pt Reyes 10-60 NM.