textproduct: San Francisco Bay Area

KEY MESSAGES

Updated at 1201 AM PDT Mon Sep 21 2026

- Cool conditions today

- Warming trend resumes Tuesday, cooler again Friday into the weekend

SHORT TERM

Issued at 1201 AM PDT Mon Sep 21 2026 (Today and tonight)

The local weather pattern remains dominated by a cut-off low with a broad rotation centered to the west of Monterey and San Luis Obispo Counties. The satellite imagery reveals scattered to broken high clouds associated with the low pressure system, some low clouds developing across the inland valleys, and some convective activity south of the Monterey County coast that has been potent enough to show up on the radar returns. One of those cells even lit up LightningCast (long story short, a satellite based algorithm for lightning probability) with a 50% probability of lightning but fell apart before reaching the southern Monterey County shore. However, most of them haven't corresponded to any rainfall at the surface, as the 00Z (5 PM PDT) sounding from Oakland revealed that the lower layers remain very dry. But could we still see a high based thunderstorm through the night? It's unlikely, even if the HRRR continues to hold on to K-Index values up to 22C, which would correspond to isolated thunderstorms, and some lightning flash signals near the Santa Lucia range around 4 AM to 8 AM. CAPE values are still the limiting factor; the 00Z OAK sounding continued to show a conditionally unstable layer with lapse rates around 8.5 C/km, but this layer was still below the moist air. This is supported by the reflectivity values at -10 C and -20 C, which remain below the values that would be necessary to support the ice particles that would be necessary for thunderstorm development. We will continue to monitor the cells as they develop, because while the risk of lightning is very small, the chance that it would be accompanied by wetting rains (at least 0.1" of rain) are zero, thus posing an obvious concern for dry lightning-induced fire starts.

As the low moves onshore later this morning and passes through to our south, cool conditions will continue with highs ranging from the middle 70s to lower 80s in the inland valleys, the middle 60s to the middle 70s near the Bays, and the upper 50s to middle 60s along the Pacific coast. Typical diurnal onshore winds develop this afternoon and evening with gusts reaching 20 to 30 miles per hour through gaps and passes, and within the northern Salinas Valley.

LONG TERM

Issued at 1201 AM PDT Mon Sep 21 2026 (Tuesday through Sunday)

The cut-off low will continue to move to the east and will be out of our area by Tuesday morning, degrading into a shortwave trough as it moves into the Great Basin. This will allow more zonal flow to develop over the region and start a warming trend through Wednesday and Thursday. The word "ridge" might be a little strong for what the models are expecting, but there is definitely a northward bump in the flow that develops by Wednesday. Temperatures on Wednesday and Thursday will range from the middle 80s to middle 90s in the inland valleys to the middle 70s to middle 80s near the Bays, and the middle 60s to lower 70s along the Pacific coast. There are some pockets of Moderate HeatRisk (moderate risk for heat-related illness among sensitive populations) possible on those two days in favored interior locations, but widespread heat impacts are not expected. As the previous forecaster noted, Heat Advisories in late September typically happen with offshore flows and longer duration events.

By Friday, another trough arrives from the eastern Pacific and reverses the warming trend, with temperatures closer to seasonal averages into the weekend. Longer-range outlooks show a slight lean towards temperatures warmer than seasonal averages and precipitation totals above seasonal averages for the last few days of September into the first few days of October.

AVIATION

(12Z TAFS) Issued at 423 AM PDT Mon Sep 21 2026

Currently LIFR-MVFR. An upper-level low is bringing high clouds and elevated rain showers with a low probability for any thunderstorms to our area. Stratus on the cusp of IFR/MVFR will persist through late-morning before retreating to the coast. Reasonable worst case scenario is bayshore (OAK and SFO) and coastal (HAF and MRY) terminals remain MVFR all day due to an increase in onshore flow this afternoon as stratus would be retreating. Drizzle will be possible through the morning, especially at HAF and MRY, which would lead to further reduced ceilings/visibilities. Diurnal winds will prevail.

Vicinity of SFO...Currently MVFR with westerly flow. Moderate confidence in the overall forecast and 12Z TAF. Reasonable worst case scenario is MVFR stratus lingers through the TAF period with increasing onshore winds this afternoon. The best chances for VFR will be this afternoon (19Z-00Z) with an early return of MVFR stratus likely. Diurnal winds will prevail.

SFO Bridge Approach...The San Mateo Bridge Approach will likely be MVFR through at least late-morning, improving before the terminal.

Monterey Bay Terminals...Currently MVFR with southwesterly flow at MRY and VFR with westerly flow at SNS. High confidence in MVFR stratus through late-morning. As it retreats to the coast this afternoon, MRY will likely remain MVFR with an increase in onshore flow. Nonetheless, another early return of IFR-MVFR stratus is on tap tonight. Diurnal winds will prevail.

MARINE

(Today through Saturday) Issued at 423 AM PDT Mon Sep 21 2026

Moderate to fresh northwesterly breezes will prevail Monday. Locally strong breezes across the northern waters will generate hazardous conditions for small craft. Northwesterly breezes increase Tuesday to become fresh to strong, bringing hazardous conditions for small craft across the coastal waters. Conditions begin to improve Wednesday with strong gusts likely through Friday. Moderate seas Monday will build to become rough Tuesday and Wednesday before subsiding to become moderate Thursday.

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 marine layer, and a chance for drizzle today. It is also bringing elevated rain showers, fortunately there is a low probability for thunderstorms as little to no rainfall is expected to reach the ground. 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 from 3 PM this afternoon to 3 PM PDT Tuesday for Pt Arena to Pt Reyes 0-10 nm.

Small Craft Advisory until 3 PM PDT Tuesday for Pt Arena to Pt Reyes 10-60 NM.