, Issued at 1204 AM PDT Tue Sep 15 2026 (Today and tonight)
Interesting to see some high clouds continue to stream over the heart of the Bay Area early this morning. In fact, the stream of clouds have been like a conveyor belt of upper level moisture over the last 36-48 hrs. Why the conveyor belt like feed? While the longwave pattern shows broad troughing over the West (including CA) it also shows a weak upper level disturbance parked well NE of the Hawaiian Island. The sub-tropical jet is the connection between that weak disturbance and CA. Impactful? No. Interesting to see a feed for a longer period of time? Yes. Now onto the forecast.
Marine layer struggled to fully form last night due to dry air and offshore flow. Tonight we're dropping the offshore flow, but dry air remains. As of midnight stratus still remains patchy, but as the night moves on there is higher conf that stratus will become more widespread over the coastal waters and move locally inland. The limiting factor will be the lingering dry air.
Monday was a warm up day, but Tuesday will be a transition day in the other direction. Broad upper trough will get a boast from a highly amplified jet nosing into AK and the downstream trough deepening. Onshore flow will kick a little more today as well. Winds will be strongest along the coast, inland valleys, and inland gaps/passes (Altamont). Onshore flow and dropping 500mb heights will bring a general cooling of 3 to 5 degrees nudging much of the region to below normal for this time of year.
Moistening in the low level and falling 500mb heights will once again promote better stratus development Tuesday night into Wednesday with the depth increasing of 1000-1500 ft for the marine layer.
..issued at 1204 AM PDT Tue Sep 15 2026 (Wednesday through Monday)
How a about a little taste of late fall to begin the Long Term. The aforementioned amplified jet will to drive another upper low down the PacNW coast and into NorCal/OR. Latest guidance actually cuts off the low from the main flow pattern. Not only are we getting another upper low, but this one will be chilly for this time of year with 850mb temps dropping close to 10C, which is an anomaly of 5 to 7. So what does this mean for overall sensible weather? The cooldown in temperatures will continue with highs dropping another 3 to 8 degrees. Highs will be mainly 60s coast and upper 60s to mid 80s interior. Onshore flow will bring gusty conditions again. The marine layer will be a tricky one. Generally speaking one would expect a deeper marine layer and that's what the forecast has Wednesday into Thursday. However, one always has to watch stratus development as the airmass aloft begins to approach the current SST. If they're too close, stratus production generally declines. Confidence is too low at this point to remove stratus, but will monitor nonetheless. Given that the stratus mid- week is tricky and not classic, impacts from extreme nocturnal drying events are less likely.
The upper level low departs eastward late Thursday into Friday with shortwave ridging to follow. Subtle warming trend develops for next weekend with interior locations solidly returning to the 80s and far interior mid 90s.
(12z TAFS) Issued at 503 AM PDT Tue Sep 15 2026
Coastal stratus and fog /MVFR-IFR-LIFR/ are developing with a channel of cooler air moving in on northwest winds, including development benefiting from radiative cooling to space through daybreak. The marine layer depth varies from 1500 to 2000 feet. Sep solar angle heat input and vertical mixing will slow stratus and fog development during the day, then model forecasts show greater areal coverage of stratus and fog /MVFR-IFR/ returning tonight and Wednesday morning. Patchy light coastal drizzle is a possibility, however drizzle is not currently included in the 12z TAFs.
Vicinity of SFO, VFR except tempo stratus /IFR/ 13z-17z this morning. Stratus /MVFR/ redevelops by early Wednesday morning. West wind 5 to 10 knots, increasing to 15 to 20 knots during the afternoon and early evening. West wind decreasing to 5 to 10 knots tonight and Wednesday morning.
SFO Bridge Approach, Similar to SFO.
Monterey Bay Terminals, Stratus and fog /MVFR-IFR-LIFR/ during the morning, ceilings lifting to SCT vs BKN /MVFR/ today. Stratus /MVFR-IFR/ moves inland early this evening, with stratus prevailing tonight and Wednesday morning. Mainly onshore winds 5 to 15 knots.
(today through Sunday) Issued at 248 AM PDT Tue Sep 15 2026
Northwesterly breezes will diminish across the inner coastal waters and the southern coastal waters today. Hazardous conditions for small craft will linger across the northern outer waters through Wednesday morning. Moderate to rough seas will prevail through today before subsiding to become slight to moderate Wednesday through Friday.
Ca, None. PZ, Small Craft Advisory until 9 AM PDT this morning for Pigeon Pt to Pt Pinos 10-60 NM-Pt Arena to Pt Reyes 0-10 nm.
Small Craft Advisory until 9 AM PDT Wednesday for Pt Arena to Pt Reyes 10-60 NM.
