, 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.
..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.
(12z TAFS) Issued at 523 AM PDT Sun Sep 27 2026 Satellite imagery this morning shows high clouds over the Bay Area and Central Coast, which makes it hard to see stratus development underneath it. The exception to this would be the interior Central Coast, where you can see the stratus slinking down the Salinas Valley. MVFR cigs are expected with any additional stratus develop, likely clearing by mid to late morning for most sites. As the upper low moves across the region, the marine layer is expected to be around 1000-1500ft. We may see stratus return for sites around SF Bay and Monterey Bay, whereas offshore flow might keep stratus at bay for the North Bay.
Vicinity of SFO, MVFR cigs should stick around through mid morning to late morning with onshore flow gradually increasing. Guidance shows an uptick in winds late this afternoon and into the evening hours, where gusts to 20-25kt look possible. Winds look to weaken late into the evening and early overnight hours, leading to the next forecast challenge: stratus. Most models tend to show MVFR cigs returning between 9-12Z, though some show it trying to onset earlier. Opted to keep the previous trend with the stratus being shorter lived into Monday morning.
SFO Bridge Approach, Similar to SFO.
Monterey Bay Terminals, No major changes to the forecast this morning as MVFR/IFR cigs hover around the Bay. Look for these conditions to continue through mid to late morning, with VFR conditions returning for the afternoon. Cigs return potentially as early as 0Z to the taf sites, and should be socked for the rest of the TAF cycle.
(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.
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.
