textproduct: San Francisco Bay Area

KEY MESSAGES

Updated at 1120 AM PDT Mon Sep 21 2026

- Cool today

- Warmer temperatures Wednesday - Thursday

- Cooler weather returns Friday

UPDATE

Issued at 826 PM PDT Mon Sep 21 2026

Stratus is expanding across the inland valleys in the wake of the cut-off low, which is moving across the southern part of the Central Valley into the Mojave Desert. Coastal drizzle was reported in the late afternoon hours, and will be added to the forecast through the night with the upcoming forecast package. No other changes to the forecast.

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SHORT TERM

Issued at 1120 AM PDT Mon Sep 21 2026 (This afternoon through Tuesday)

The dominant weather feature today is a 500 mb cut-off low currently centered over Pt. Conception. The 500 mb height at Vandenberg was 5750m as measured from the 12Z sounding. This is just below the 10th percentile in the climatology for this time of year, so it's fairly notable, but not extreme. The result is cooler temperatures, a deeper marine layer, some morning drizzle, and even a few real raindrops. The marine layer was measured at 2,400 feet deep by the 12Z weather balloon from Oakland. This deeper cloud layer, combined with low-level vorticity and orographic lift allowed some coastal drizzle this morning. Drizzle was reported throughout the typical areas across the Bay Area and Central Coast. The La Honda RAWS in the Santa Cruz Mountains was the winner with 0.05" of precipitation recorded over the last 24 hours. This deep marine layer, high humidity and ample cloud cover will keep temperatures 5-10 degrees cooler than normal this afternoon.

The cut-off low will continue to slide NE over the next 24 hours, loosening its grip on the weather here. Tuesday will be a transition day as the marine layer starts to compress and temperatures increase back to normal.

LONG TERM

Issued at 1120 AM PDT Mon Sep 21 2026 (Tuesday night through next Sunday)

The long term forecast is a little bit of a roller coaster, but like the kind for kids that doesn't do anything too scary. As the low continues to exit the area, the upper level pattern will gradually shift to a strong trough over the Gulf of Alaska and a ridge stretching from Texas to central Canada. By Wednesday the west coast will be in between these two features, with a small positive 500 mb height anomaly arriving Wednesday. While the 500 mb height anomaly is minor, the 850 temperature anomaly will climb to around 5-7C above normal Wednesday - Thursday. This will bring inland temperatures into the upper 80s to low 90s, with 70s along the coast both Wednesday and Thursday. There is some moderate HeatRisk associated with this event, but it's not widespread and has been trending down. With the longer nights, short duration, and continued onshore winds, we are not expecting a heat advisory with this event at this time. The trough over the Gulf of Alaska will move into the Pacific Northwest by Friday, reversing the warming trend and bringing temperatures back below normal for next weekend.

AVIATION

(06Z TAFS) Issued at 1021 PM PDT Mon Sep 21 2026

Extensive MVFR-IFR stratus has flowed into the North Bay and the Monterey Bay region, and continues to develop in the SF Bay region. A deep marine layer should allow many of the inland valleys to see ceilings overnight before the stratus retreats back to the immediate coast late Tuesday morning. Low to moderate confidence in temporary scatterings at HAF and MRY around the 19-22Z timeframe. Gentle to moderate onshore winds will develop Tuesday afternoon and evening. Stratus forecasts for Tuesday night into Wednesday morning are lower confidence, particularly away from the immediate coast, as a compressing marine layer should keep stratus away from many inland regions but lower ceilings and visibilities where it does form.

Vicinity of SFO... A tricky forecast for the next several hours as the stratus feed through the Golden Gate gets deflected to the north, leaving the terminal area VFR. High resolution model output has the terminal getting MVFR ceilings imminently, but the models have not captured the current situation well and have kept the terminal VFR for the time being. If stratus forms over SFO expect the clouds to scatter out late Tuesday morning, with a moderate west breeze developing in the afternoon and evening hours. Low confidence of stratus returning Tuesday night, which most of the high resolution guidance has backed away from and may be impacted by a compressing marine layer.

SFO Bridge Approach... MVFR stratus may arrive before or after it gets to the terminal tonight, depending on how exactly the stratus develops with SF Bay. Otherwise similar to SFO.

Monterey Bay Terminals... MVFR ceilings at present with IFR conditions expected later tonight. Stratus will scatter out Tuesday late morning to the early afternoon, with moderate confidence in a brief scattering period at MRY near 19-22Z. Stratus is expected to return to MRY late in the afternoon and persist into the evening. At SNS, confidence is lower as the compressing marine layer may keep the deck away, but on the flip side, the compression could also lower ceilings and visibilities at MRY on Tuesday night.

MARINE

(Tonight through next Sunday) Issued at 826 PM PDT Mon Sep 21 2026

Moderate to fresh northwest breezes persist overnight with locally stronger winds north of Point Reyes. By Tuesday afternoon, widespread strong northwest winds prevail across the coastal waters, allowing for seas to build from moderate to rough. Northwest winds and seas remain elevated through the week with a brief period of improved conditions Wednesday night into Thursday. By Friday, winds increase with building rough seas.

FIRE WEATHER

Issued at 423 AM PDT Mon Sep 21 2026

An upper-level low will support near to slightly below normal temperatures, increased diurnal winds, a deep (2,500') marine layer. Warming and drying trend kicks off Tuesday with temperatures up to 10 degrees above normal and areas of moderate HeatRisk Wednesday and Thursday. The marine layer will compress below 1,000 feet during this warm-up with nocturnal drying expected above it.

MTR WATCHES/WARNINGS/ADVISORIES

CA...None. PZ...Small Craft Advisory until 5 AM PDT Wednesday for Pt Arena to Pt Reyes 0-10 nm-Pt Arena to Pt Reyes 10-60 NM.

Small Craft Advisory from 3 PM to 9 PM PDT Tuesday for Pigeon Pt to Pt Pinos 0-10 nm.

Small Craft Advisory from 3 PM Tuesday to 3 AM PDT Wednesday for Pt Pinos to Pt Piedras Blancas 0-10 nm.

Small Craft Advisory from 3 PM Tuesday to 5 AM PDT Wednesday for Pigeon Pt to Pt Pinos 10-60 NM.