textproduct: San Francisco Bay Area

KEY MESSAGES

Updated at 148 AM PDT Tue Aug 4 2026

- Moderate HeatRisk in the interior today, becoming patchy 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 looks to return for the end of the work week.

SHORT TERM

Issued at 148 AM PDT Tue Aug 4 2026 (Today and tonight)

The southerly surge of stratus that arrived earlier on Monday, has allowed for stratus to become more widespread this morning. Nearly all of our coastline has low clouds along it, with the stratus penetrating inland. The Monterey Bay along with the majority of the Salinas Valley has filled in, is slowly creeping into Bay Area. Expect the marine layer to about 800-1000ft, which should keep high temperatures for coastal communities and areas in or under the marine layer reasonable. Clouds will begin retreating to the coast by mid to late morning, leaving mainly clear skies across the region. In terms of the upper level pattern, high pressure will slowly take hold of the forecast. 850mb temperatures today, look to to be between 24-29C, warmest across the interior. Expect temperatures to a bit cooler than yesterday for areas influenced by the marine layer, while other locations remain hot. Patchy Moderate HeatRisk exists interior North Bay and interior Monterey, with areas to widespread Moderate HeatRisk for interior East and South Bays. Opted to hold off an issuing a Heat Advisory for these locations tonight until we get a better visual on the marine layer expansion since that could put interior temperatures at risk.

LONG TERM

Issued at 148 AM PDT Tue Aug 4 2026 (Wednesday through Monday)

No major changes to the forecast as high pressure will continue to build into the region and then hold through at least Saturday. Temperatures will may decrease a more degrees on Wednesday, but it will be dependent on the marine layer. Interior locations will still be warm, but likely have temperatures in the 90s, perhaps cracking 100 in a few spots. Warmer and dryer weather starts to return on Thursday as the ridge strengthens. Looking at models, 850mb temps don't really deviate too much. We're still looking at that airmass being warm, around 27-30C, depending on the model. Overnight lows would be mild, especially in the thermal belts/mountains. And day time highs would climb back into the 90s and low 100s. The HeatRisk projection looks to have widespread Moderate HeatRisk for interior slots, with pockets of Major HeatRisk. Heat Advisories may be needed.

AVIATION

(12Z TAFS) Issued at 427 AM PDT Tue Aug 4 2026

Stratus is expanding inland along through the interior valleys and is expected to reach SFO and OAK shortly. Confidence is lower that stratus will reach APC and SJC but the potential for brief overcast conditions cannot be fully ruled out at either airport this morning. IFR to LIFR conditions persist along the coast through late this morning, briefly clear during the afternoon, and returns during the late afternoon/early evening. Stratus looks to be widespread again tonight and will reach most interior sites (excluding SJC and LVK). Diurnally breezy onshore winds continue with light, offshore at times, winds expected overnight.

Vicinity of SFO...Stratus is approaching SFO via the SFO Bridge Approach (southern SF Bay) and is expected to reach SFO between 12-13Z. This is most likely to bring MVFR to IFR conditions with the marine layer between a depth of 500-1000 feet. Any cloud cover that does develop should clear by 17-18Z. Moderate confidence that stratus will return late tonight and persist through the remainder of the TAF period.

SFO Bridge Approach...Stratus is filling in faster over the south SF Bay and gradually building northwards towards SFO. High resolution guidance suggests clearing times for the approach will be similar to SFO but patchy clouds may linger along the approach path for longer than over the terminals. Otherwise, similar to SFO.

Monterey Bay Terminals...IFR-LIFR conditions continue through late this morning. Moderate confidence that a brief period of VFR will return during the afternoon/evening before stratus returns (high confidence) late afternoon/early evening. Confidence is slightly lower as to the exact CIG height but for now kept the stratus arriving as IFR and lowering to LIFR overnight. Not seeing any major impacts to visibility forecasted for tonight.

MARINE

(Today through Sunday) Issued at 427 AM PDT Tue Aug 4 2026

A light to gentle breeze will remain in place 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 with wave heights between 4-8 feet expected through late this week. 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 crafts.

FIRE WEATHER

Issued at 452 AM PDT Tue Aug 4 2026

Elevated fire weather conditions continue through the remainder of the week due to dry daytime conditions across the interior with near critical conditions returning late week into the weekend. 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 (20-30%) 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. 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.

MTR WATCHES/WARNINGS/ADVISORIES

CA...None. PZ...None.