textproduct: San Francisco Bay Area

KEY MESSAGES

Updated at 1051 AM PDT Tue Sep 15 2026

- Cooler temperatures through Thursday

SHORT TERM

Issued at 1051 AM PDT Tue Sep 15 2026 (This afternoon through Wednesday)

The marine layer has returned, but it's not uniform. A mesoscale, low-level, cyclonic eddy has formed off the coast of San Mateo County. This is producing easterly, offshore winds in the North Bay and westerly, onshore winds across the Central Coast. The morning sounding measured the marine layer at 1,200 feet deep over Oakland. Surface-based marine layer detection shows it's more shallow to the north and deeper to the south. The marine layer in Sonoma County is around 800 feet deep with near normal temperatures and mostly clear skies. Meanwhile it's around 2,000 feet deep across Monterey County, where max temperatures are around 10 degree below normal, and most of the coast was under overcast skies this morning.

The synoptic scale pattern features a trough over the west coast and a ridge over the SE US. At 500 mb, the trough will become cut-off over the next 24 hours and settle somewhere over Northern California. As a result, temperatures will be cooler than normal today and even more so tomorrow. This low will also bring stronger winds Wednesday afternoon, with gusts of 20-30 mph expected across the East Bay Hills, Salinas Valley, and both the San Francisco and Monterey Bays.

LONG TERM

Issued at 1051 AM PDT Tue Sep 15 2026 (Wednesday night through next Monday)

The cut-off low will certainly deepen the marine layer Wednesday night, and it may provide enough lift to support some drizzle Thursday morning. After losing upper level support, the low will gradually weaken through the day. As a result, winds will be much calmer Thursday afternoon. Temperatures will remain roughly 5-10 degrees below normal.

The remnants of the cut-off will gradually move off to the NE and eventually get caught up in the jet stream flow again. We should be on the backside of the trough by Friday, with a weak short-wave ridge building Saturday. This feature looks short-lived as another cut-off low approaches the coast Sunday or Monday. Temperatures should return back to normal Fri-Sun with typical marine layer impacts.

By next week the forecast confidence really falls apart. A few of the deterministic global models show deep, reinforcing trough bringing much cooler weather. The ensembles, however, reveal that this is an outlier in the total guidance envelope. The ensemble mean is much more benign, and there is at least an equal likelihood of warm ridging next week. The ECMWF ensemble actually shows anywhere from 5C to 25C 850 mb temperatures by the middle of next week. That pretty much covers all of the historical possibilities for this time of year. In other words, we don't yet know what's going to happen next week.

AVIATION

(18Z TAFS) Issued at 1029 AM PDT Tue Sep 15 2026

MVFR-IFR conditions continue directly along the coastline with VFR across the interior. MVFR stratus returns again tonight with IFR conditions possible directly along the coastline. Confidence is low to moderate that stratus will not reach SJC or LVK overnight but if it does occur, it likely will not be until after 09Z. Highest potential for IFR conditions directly along the coastline. Onshore winds pick up this afternoon to around 10-15 knots with light to variable southerly winds overnight.

Vicinity of SFO...VFR through the rest of today with MVFR CIGs likely to return overnight. Patchy stratus could arrive as early as 04Z but it more likely will not arrive until 10Z. Onshore winds to around 15 knots expected this afternoon and evening with occasional stronger gusts possible. Stratus clears by late tomorrow morning with a return of onshore winds around 15 knots expected during the afternoon and evening.

SFO Bridge Approach...Generally similar to SFO but gusts may be slightly stronger (closer to 20 knots) over the bay on the Bridge Approach.

Monterey Bay Terminals...MVFR-IFR conditions persist at MRY through the rest of today. Confidence is low that a ceiling will fully mix out at MRY due to persistent onshore push of stratus into MRY from the Monterey Bay region. If stratus does clear, it likely will not be until after 19Z. MVFR-IFR CIGs return to SNS overnight. Winds remain light through mid morning with westerly winds around 12 knots expected today. Winds weaken overnight becoming light to variable.

MARINE

(Today through Sunday) Issued at 1029 AM PDT Tue Sep 15 2026

Northwesterly winds diminish across the inner and southern coastal waters today with a moderate breeze and occasional fresh gusts prevailing through late week. Strong northwesterly winds diminish more slowly across the northern outer waters with hazardous conditions continuing into Wednesday. Rough seas linger through Wednesday before seas abate and become moderate.

FIRE WEATHER

Issued at 345 AM PDT Tue Sep 15 2026

The transport of cooler air on northwest winds relative to sea surface temperatures in the upper 50s to lower 60s continues to help produce coastal stratus early this morning. The marine layer depth varies from 1500 to 2000 feet. Above the marine layer temperature inversion, humidities are currently in the mid 20% to 30% range. The depth of cool air transport will increase which will gradually increase humidities at higher elevations during mid to late week. Before humidities reach a maximum, there may be temporary, small areas of nocturnal drying at highest elevations, coverage though should be minimal.

MTR WATCHES/WARNINGS/ADVISORIES

CA...None. PZ...Small Craft Advisory from 9 AM to 9 PM PDT Wednesday for SF Bay N of Bay Bridge.

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