Swell Matrix

Area Forecast Discussion

National Weather Service San Francisco Bay Area

1002 pm PST Fri Jan 27 2023


Cooling trend continues into the weekend, with well below normal temperatures then persisting through the first part of next week. An upper level disturbance will bring some showers Sunday and Sunday evening, especially to the southern half of our area. Snow levels will be low, locally down to 2500 ft or even a bit lower. In the colder air mass behind it, low temperatures Monday and Tuesday mornings look to be very chilly. Widespread freezes are then likely away from the coast, potentially even touching some coastal areas.


As of 09:02 PM PST Friday, Another calm evening is in the books with temperatures beginning to plateau in the lower 50s and mid 40s. Lower clouds have moved on shore around the SF and Monterey Bays as well as the San Mateo Coast, while other areas remain clear.

There isn't much to add from the previous forecast discussion below. The forecasting team is keeping an eye on light rain over the weekend, followed by hazardously cold overnight lows into the next work week. Expect only minor adjustments as forecast confidence continues to increase.

Previous Discussion

As of 03:15 PM PST Friday, Latest satellite imagery shows mostly clear skies over our area, aside from areas of stratus along the coast from around Half Moon Bay northward and some high cloudiness passing through from the north. Temperatures around the Bays are running 5 to 10 degs cooler than yesterday, reflecting the end of our recent period of offshore flow and associated compressional warming. The pressure gradient from KSFO to KWMC is now +2.9 mb in contrast to -5.5 mb 24 hours ago.

We're for a bit of a change in our weather over the weekend and into next week. The large scale upper level high centered offshore from the Pacific Northwest is forecast to build strongly northward towards Alaska. That in turn will enable a strong but moisture- limited upper level disturbance to travel rapidly southward from the interior of northwestern Canada down through California. In stark contrast to our extraordinary recent period of wet weather, this is the sort of synoptic scale pattern that's more classically associated with "La Nina" winters.

For tonight through Saturday night, Cooling of the air mass aloft and deepening of the boundary layer ahead of the approaching disturbance will result in some increase in night and morning low cloud and fog coverage along with a few degrees of additional reduction in afternoon temperatures. Highs tomorrow around the Bays look to be largely confined to the mid and upper 50s.

More dramatic cooling is expected for Sunday as the approaching "inside slider" upper level system moves in, with max temps looking to be confined to the lower 50s at lower elevations and 40s in the hills. Although associated moisture will be limited by its inland trajectory, nonetheless looks to be sufficient for showers to develop during the day, especially over the southern half of our area where some additional influx of low-level moisture looks to occur.

Want to note a few other things about our weather on Sunday. Snow levels look to get down to around 2,500 ft, and potentially even locally a bit lower in any heavier showers. Also looks like there could be just enough CAPE for some convection from around the Santa Cruz Mtns southward, though very low topped; probably no more than to 13 to 15 kft. But that could be sufficient to produce some small and even an outside chance of lightning. Expect precip totals to generally be no more than a trace in the North Bay and a few hundredths of an inch in the heart of the Bay Area, but potentially up to 1/4 inch in the Santa Cruz Mtns and Monterey Peninsula area, and locally 1/2 inch in the Santa Lucia Mtns of Monterey County.

Then starting Sunday night into Monday morning, it gets really cold. A Freeze Watch has now been issued for virtually all of our area for late Sunday night into Monday morning (see SFONPWMTR). Widespread sub-freezing temperatures then look possible, and then again Monday night into Tuesday morning which could potentially be even colder. And it will remain chilly in the daytime, with highs confined to the lower to mid 50s.

In the extended, temperatures slowly moderate with dry conditions looking to prevail until rain chances return late in the week.


As of 9:57 PM PST Friday, For the 06Z TAFs. A mix of VFR/MVFR/IFR as patchy stratus has formed in the hills around SF Bay, coastal Monterey Bay and parts of the Salinas Valley. Hi-res models project patchy stratus will continue through the night, with the potential to spread to most terminals. Not seeing any signs of valley fog, but the potential is there for the early morning hours. Winds have weakened and will be light overnight. Stratus clears by mid to late morning, and winds will be onshore in the afternoon.

Vicinity of KSFO, VFR at area terminals, but stratus is observed over the coastal and East Bay hills. Greater potential for impacts to East Bay and South Bay terminals overnight, with IFR/MVFR cigs. Light winds overnight. Clouds clearing by late morning with VFR through the end of the TAF period. Winds becoming breezy late Saturday night with an approaching trough from the north.

KSFO Bridge Approach, Mostly clear skies observed over the approach. Low clouds may develop overnight, but currently low confidence in cig development.

Monterey Bay, VFR/MVFR/IFR under patchy stratus at this hour. Low to moderate confidence in low clouds lingering through the morning. VFR Saturday afternoon with moderate onshore winds.


As of 08:47 PM PST Friday, Strong northerly winds this evening with near gale to gale force gusts, especially in the outer waters north of Pigeon Point. Winds diminish this evening, but remain hazardous for small craft, then increase tomorrow night. Long period northwesterly swell has also arrived and will continue to fill the waters through the weekend before slowly diminishing. Light rain is in the forecast for Sunday as a weak cold front pass over the waters.

Watches, Warnings, Advisories

Tonight, sca, pt arena to pt reyes 0-10 nm sca, pt pinos to pt piedras blancas 0-10 nm sca, pt arena to pigeon pt 10-60 nm sca, pigeon pt to pt piedras blancas 10-60 nm sca, pt reyes to pigeon pt 0-10 nm until 3 am

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