textproduct: San Francisco Bay Area

KEY MESSAGES

Updated at 109 PM PDT Tue Aug 4 2026

- Moderate HeatRisk in the interior today decreasing midweek.

- Elevated fire weather threat across the interior North and East Bays, and the Diablo and Santa Lucia ranges through the week.

- Widespread Moderate HeatRisk returns for the end of the work week.

- Critical fire weather conditions develop Friday and Saturday.

SHORT TERM

Issued at 109 PM PDT Tue Aug 4 2026 (This evening through Wednesday night)

Rest of today: Marine layer rolled back to the coast, but is hanging pretty solid along the coastline. Lingering marine layer plus a slightly stronger onshore flow equals an impressive 24 hour trend. Temperatures as of 1 pm are currently running a few degrees cooler to as much a 20 degrees than Monday. Warmest locations in the forecast area are across the interior with temperatures still in the mid 90s to low 100s even with the cooling trend. Will keep Heat Advisory as is for now through 10 PM, but it does appear some spots may fall short of their forecast highs.

Additionally, visible satellite imagery also shows smoke lurking over the coastal waters. This smoke is actually from the PacNW fires. Latest smoke model brings some high level smoke over the Bay Area later today, which may contribute to some haze or just a pretty sunset.

Tonight through Wednesday night: Marine layer along the coast will march inland again tonight. Have added another round of coastal drizzle. Depth of the marine layer will remain around the 1k ft mark. Above the marine layer will remain mild and dry with active thermal belts.

The cooldown experience today will continue on Wednesday thanks to persistent onshore flow, marine layer influence, a weak disturbance overhead riding the periphery of the upper ridge. Event with a "cooldown" far interior locations will be well into the 90s with a few making a run at 100 degrees. High probability of exceeding 100 degree will be interior Monterey/San Benito counties and far interior East Bay. Given the slightly cooler temperatures HeatRisk values decrease. As such, will not be issuing a Heat Advisory at this time for Wednesday.

Wednesday night will feature another solid push from the marine layer, but maybe a tad more compressed given building H5.

LONG TERM

Issued at 148 PM PDT Tue Aug 4 2026 (Thursday through next Monday)

Thursday will be a transition day as marine layer compresses and the upper level ridge makes its presence known again. Temperatures will be moderating with a large difference with cool temps at coast and much hotter temperatures inland. Basically, in the big picture nothing major from what we've been seeing. Just that temps will be cool enough to keep HeatRisk values in the Minor to Moderate categories.

Friday through Saturday...the upper level ridge begins to re-assert its dominance with building 500mb heights and warming 850mb temps back to critical levels. The building H5 will further compress the marine layer. The marine layer will remain however. Really need offshore flow to get rid of the marine layer at this point. 850mb temps soar back into the 28-30C range, yielding inland max temps in the low 100s and overnight lows in the 70s. That combo will increase HeatRisk back into the upper end of Moderate to lower Major. Therefore, we'll likely need a mix of Heat Adv for interior locations Friday/Saturday. Still looking above normal at the coast/bays, but natural AC will keep the real heat inland.

Interesting longwave pattern in the extended as the upper level ridge looks to become more "split" with a system sweeping through PacNW. The easing upper level ridge will allow for subtle cooling across the interior for Sunday and Monday. We'll need to take a closer look if any Heat Adv will be needed on Sunday since heat impacts are cumulative.

AVIATION

(00Z TAFS) Issued at 446 PM PDT Tue Aug 4 2026

A more robust marine layer than the previous few days/nights remains mostly constrained to the coastline this afternoon but will begin to make its way inland around sunset tonight, with all terminals expected to see at least a short period of IFR/LIFR ceilings through Wednesday morning into the early afternoon for some.

Vicinity of SFO...IFR/LIFR ceilings are expected to spread over the Bay Area later this evening into the overnight hours with a more organized marine layer and breezy onshore flow helping stratus to spill over into the Bay Area by midnight. VFR conditions are expected by early afternoon, with a local offshore circulation (Otter Eddy) resulting in more variable wind directions at times through most of the forecast period.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR currently, however stratus is already spilling into the Salinas Valley and the Monterey Peninsula in vicinity of both terminals. Once ceilings settle in, IFR/LIFR is expected to persist through at least late morning, and possibly into early afternoon.

MARINE

(Tonight through next Sunday) Issued at 446 PM PDT Tue Aug 4 2026

Light to gentle breezes will persist across the majority of the coastal waters through late week. A localized gentle to moderate breeze continues across the inner waters. Seas continue to abate through Thursday with wave heights between 4-8 feet expected through Friday. The Pacific High strengthens and shifts eastward late this week into the weekend, resulting in strengthening winds and building seas with a return of 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...Heat Advisory until 10 PM PDT this evening for CAZ510-513>515.

PZ...None.