textproduct: San Francisco Bay Area

KEY MESSAGES

Updated at 1204 PM PDT Sat Sep 26 2026

- Cooler conditions through Sunday

- Warmer temperatures return Tuesday through next week

SHORT TERM

Issued at 1159 PM PDT Sat Sep 26 2026 (Tonight through Monday)

A 500 mb open wave trough is approaching the Pacific Northwest and far northern California. The 500 mb trough is forecast to become a closed off 500 mb low Sunday over northern California and continue moving southeastward reaching southern CA/NV later Monday and Monday night. The polar jet and subtropical jet are working in unison helping to drive this low forward. The polar jet is digging the trough southeastward and the subtropical jet is bringing winds to the base of the trough which will move the trough eastward. From ~ 700 mb to 500 mb there is a decent cold pool of air associated with the low, less than 10th percentile for late Sep Oakland upper air climatology. However, humidity levels appear mostly unorganized in the 700 to 500 mb layer and there is minimal potential instability while the low moves over our area Sunday through Sunday night. If development is just right there may be a few spots where mid and high clouds produce virga. The anticyclonic curvature of the polar jet will bring subsidence and lower 700-500 mb layer humidity by later Sunday; e.g. recent HRRR output show mid and high level clouds departing to our east by late morning or afternoon Sunday.

Closer to ground level, from the surface to lower levels cool air advection has already begun, which is helping clouds to develop. There is a slight chance that either light drizzle or very light rain develops within the boundary layer cooling along the coast and inland from San Francisco to the East Bay and areas southeast to the Monterey Peninsula to Point Sur through Sunday morning. This is a complex pattern since the aforementioned 500 mb low has detached from the surface low far to our northwest, however there is still a channel of cooler air arriving on northwest winds along the coastline and inland. The probability of precipitation is low, a slight chance. At this time, measurable precipitation is not in the grids for tonight and Sunday morning. From a fire weather perspective, we've seen observations of nocturnal drying at higher elevations Saturday evening, with substantially lower dewpoint temps and RHs in the single digits. The models are having a lot of difficulty trying to capture this complexity; all of the above described is a good example as to just how complex e.g. Sep-Oct weather can be in our cwa.

With the combined cooling, various cloud layers and slight chance for light drizzle/light rain Sunday morning, daytime highs Sunday will be below late Sep normals with highs mainly in the 60s and 70s to a few areas in the lower 80s in the southern interior. Warmer temperatures revisit our forecast area Monday with 500 mb ridging returning in the wake of the 500 mb low. Lower 80s inland for daytime highs will be more prevalent, closer to the coast and bays 70s to 80 and a mix of mid 60s to 70s at the coastline.

LONG TERM

Issued at 1159 PM PDT Sat Sep 26 2026 (Monday night through next Saturday)

500 mb ridging as well as surface high pressure will build in over far northern California, steepening the ACV-SFO pressure gradient to 5 mb to 7 mb Monday night through Tuesday. Warmer lower level temperatures will connect with daytime surface heating and result in warmer to hotter temperatures, with 90s more prevalent inland Tuesday. The strengthening ACV-SFO will produce offshore winds, how strong these winds get will determine how warm/hot it gets at the coastline beginning Tuesday. Similarly, Wednesday will have offshore wind potential too. Downsloping and drying will increase fire weather concerns, please see more in fire weather discussion.

Going back to how the polar jet and subtropical jet streams are working in unison in the nearest term, this appears to be true as well through next week based on recent ECMWF forecasts. The pattern is progressive, the westerlies will help nudge next week's 500 mb ridge forward, somewhat limiting the strength and potentially the duration of offshore winds. Aside from the early Sep quick hot spell with the retrogressive subtropical ridge and strong compressional warming, and minor offshore winds here and there, so far Sep has featured minimal offshore wind flow.

AVIATION

(06Z TAFS) Issued at 848 PM PDT Sat Sep 26 2026

A mix of VFR and MVFR as the marine layer begins to gradually strengthen out ahead of an advancing upper trough. Moderate confidence that a majority of the terminals see MVFR/IFR ceilings, with plenty of uncertainty in the extent and exact timing for the North Bay and Bay Area sites.

Vicinity of SFO...The KOAK sounding showed a weak marine layer around 1000ft, but should continue to gradually strengthen and organize. There is dry air aloft mixing to the surface which provides uncertainty on the extent and exact timing. Went on the more pessimistic side of most guidance, bringing MVFR ceilings in the in the pre-dawn hours, but may not happen until after sunrise Sunday. Expect some coastal drizzle where stratus develops. Once the stratus lifts by late morning Sunday, VFR for a majority of the forecast period thereafter.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Stratus ceilings will hover around MVFR/IFR until mid/late Sunday morning with possible drizzle, with VFR for the remainder of the afternoon and early evening. MVFR ceilings expected to return around sunset Sunday evening.

MARINE

(Today through Friday) Issued at 256 AM PDT Sun Sep 27 2026

Moderate north winds will persist today, becoming fresh to strong as we head into Monday. These winds hold through midweek, with winds becoming moderate to fresh by the end of the work week. Wind gusts will be strong to near gale force at times, with the strongest gusts occuring over the northern outer coastal waters and from Monterey Bay south to Big Sur. Tropical activity will produce long period southerly swell by the middle of the week.

FIRE WEATHER

Issued at 420 AM PDT Sat Sep 26 2026

Our diffuse cold front continues to slowly move southward through Southern California. In its wake, some areas have benefited from the cooler and slightly more "moist" conditions this morning. The exception, however, is across the higher terrain of the Santa Cruz Mountains as well as the Santa Lucia and Diablo Ranges where RH recovery remains poor (20-40%). Afternoon humidity will remain under 40%, largely for areas away from the coast. A push of onshore flow is expected later this evening into Sunday morning resulting in the marine layer deepening to around 2,000 feet ahead of an upper trough. This moisture push should diminish the areal coverage of poor RH recovery Sunday and Monday morning. A few sprinkles cannot be ruled out along the highest peaks associated with this upper low Sunday AM. Dry and modest offshore breezes will be a possibility Tuesday and into Wednesday, with the potential for wind gusts above 35 mph being greatest across the higher terrain of the North Bay in the wake of this upper low. Farther south across the Central Coast, the primary impact from this low will be a return to widespread poor RH recovery with some locales across the Santa Lucia Range struggling to see max RH above 30% Wednesday morning.

MTR WATCHES/WARNINGS/ADVISORIES

CA...None. PZ...Small Craft Advisory from 9 AM this morning to 3 AM PDT Monday for Mry Bay-Pigeon Pt to Pt Pinos 0-10 nm-Pt Arena to Pt Reyes 0-10 nm.

Small Craft Advisory until 2 AM PDT Tuesday for Pt Arena to Pt Reyes 10-60 NM-Pt Pinos to Pt Piedras Blancas 0-10 nm.

Small Craft Advisory from 9 PM this evening to 9 AM PDT Monday for Pigeon Pt to Pt Pinos 10-60 NM.