Swell Matrix

Area Forecast Discussion

National Weather Service San Francisco Bay Area

915 pm PDT Wed Sep 28 2022

Synopsis

A moderate warming trend is expected for the latter part of the week, as high pressure builds inland and surface flow turns more offshore. A shallow coastal marine layer will continue however, with some areas of night and morning low clouds and patchy fog near the waters. Then a bit of a cooling trend over the weekend as the pressure gradient turns more onshore.

Discussion

As of 09:15 PM PDT Wednesday, Passage through the Bay Area of a dissipating shallow and weak cold front this afternoon has resulted in an increasing north to south pressure gradient along the coast along with diminishment of both the marine layer and associated coastal stratus. At the same time, a wide band of thin high cloudiness is moving through aloft in association with the eastward progression of the trailing upper level trough. Latest profiler data from Bodega Bay and Ft Ord show no marine layer at all at the former and diminishment of its depth at the latter to only around 900 ft.

Forecasts appear on track and no updates presently anticipated.

Previous Discussion

As of 12:32 PM PDT Wednesday, Visible satellite imagery shows a thin veil of high clouds passing over the Bay Area and the Central Coast from northwest to southeast. The convex shape to it represents the leading edge of a dry cold front. Though temperatures are running about three to seven degrees warmer than yesterday, the remain in the 60s and 70s which is five to ten degrees below normal for this time of year. While the stratus has made its usual disappearing trick to the ocean, models are expecting it to return overnight. With the Bodega Bay profiler still showing a far more compressed marine layer between 500 to 1000 feet, it leads to the chance for patchy fog once again Thursday morning.

As the front passes to the east and its driving upper trough moves to the northeast, a ridge of higher pressure will slowly grow over the eastern Pacific. The main impact from this high pressure is a gradual warming trend for the area. Nothing excessive expected, just a return to more seasonal normals where much of the Bay Area will get into the 80s, where the coastline will be in the 70s and interior parts of the region will be in the low 90s. Models are trying to place a weak boundary that will move through on Sunday to drop temperatures below normal for a day, but then return early next week to the seasonal normal conditions. Models still trying to resolve the next upper level trough to move near the Bay Area mid-to-late next week, but too far out for any consistent agreement for confidence.

Last but not least, a mention regarding offshore winds. The track that the trough that is going to exit our area will take is still to head toward Montana before dropping southward down the Rockies. As previously mentioned, this will switch the prevailing wind pattern to the north creating drier, offshore winds Thursday through Saturday, with strongest winds Friday morning and Saturday morning.. The gradient that ensemble models are projecting still appears too weak for strong concern. In fact models today have it slightly weaker than what they were projecting yesterday. Recent rainfall helped wet area fuels which helps qualm all fire concerns for this period of time. However, things are already starting to trend drier. And while these offshore winds are not expected to provide fire concerns in the short term, it is probable that they will aide in drying fuels out again making for a subtle reminder that fire season does not end at the beginning of October.

Aviation

As of 6:57 PM Wednesday, For the 00Z TAFs. Coastal stratus /MVFR-IFR/ returning locally inland tonight and Thursday morning. Conditions lifting to VFR late Thursday morning and afternoon.

Vicinity of SFO, VFR with a gusty west wind this evening. MVFR ceilings developing late tonight into Thursday morning but low confidence in both onset time and coverage. Conditions lifting to VFR late Thursday morning and afternoon.

KSFO Bridge Approach, Similar to KSFO.

Monterey Bay, Coastal stratus /MVFR-IFR/ developing tonight and Thursday morning. Conditions lifting to VFR between mid and late Thursday morning. Onshore winds 5 to 15 knots.

Marine

As of 08:14 PM PDT Wednesday, Gusty northwest winds prevailing to late week, stronger gusts along the coast south of Pigeon Point and over the waters south of Point Sur. Gale force gusts over the southern inner coastal waters. Steep, hazardous seas developing with wave heights reaching 10 to 12 feet by Thursday evening. Winds and seas diminishing Friday through the weekend.

Watches, Warnings, Advisories

Tonight, sca, pt arena to pt reyes 0-10 nm sca, pigeon pt to pt pinos 0-10 nm glw, pt pinos to pt piedras blancas 0-10 nm

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