Swell Matrix

Area Forecast Discussion

National Weather Service San Francisco Bay Area

359 pm PST Mon Nov 28 2022

Synopsis

Cool and quiet through mid week with frost and freezing temperatures possible inland. An unsettled pattern develops thereafter with rain chances increasing for late week into the weekend.

Discussion

As of 02:50 PM PST Monday, Weather impacts to the Bay Area and Central Coast over the next few days will be focused on: cold temps (day and night), precip, lower snow levels, and gusty winds.

Current longwave pattern shows a deep upper low anchored near Winnipeg with a broad upper trough extending SW to the West Coast. At the surface there is a low pressure also near Winnipeg with a series of weaker lows and fronts extending SW to the West Coast. One of these fronts is currently moving though the Bay Area. This passing front is a dry front, but is causing a change in the temperatures nonetheless. Latest 24 hour trends show many locations running a few degrees colder than yesterday afternoon. The front will continue to move S and E through the day. Behind the front high pressure will build in from the north. In fact, weak to moderate offshore flow will develop tonight. Winds will be breezy over the higher terrain of the North and East Bay. Offshore flow will also limit clouds. In the valleys, winds will be weaker under clear skies setting the stage for radiational cooling. Temperatures Tuesday morning will be cold and below normal, but far from record values for most areas (see below). Morning lows will be widespread 30s with a few 40s near the coast/bays and upper 20s far interior valleys.

Mostly clear skies expected on Tuesday and Wednesday. Despite some afternoon sunshine, temperatures will be below seasonal averages. Highs will be mainly in the 50s and lower 60s. Another night of good radiation cooling is expected Tuesday night into Wednesday morning. It does appear that Wednesday morning will likely be the coldest morning of the next few. Again, record values are still pretty low for most areas so not expecting record lows per SE. Nonetheless, lows will be a 3-5 degrees colder than Tuesday morning. A few more 20s across the interior, but still mainly in the 30s to low 40s elsewhere. Borderline for any Frost Advisory for coastal locations.

By Thursday the ridge that brought cold and clear weather will be replaced by a digging upper level trough. Longwave pattern by early Thursday shows a tilted upper level trough across the PacNW. This upper level feature is on track to sweep through the region Thursday and Friday. At the surface, a cold front will approach from the north Wednesday night. The Bay Area will be in the warm sector with weak isentropic lift resulting in increasing clouds and shower activity overnight. By Thursday, dynamics increase with a cold front and upper level jet support. Additionally, PWATs increase to near one inch. Light to moderate rain will impact the Thursday AM commute with heaviest rainfall initially over the N Bay and then progressing south through the day. Some of the hi- res models are even showing some hints of a NCFR Thursday afternoon. Needless to say, will need to monitor for minor flooding potential (ponding of roadways). Steadier precip will ultimately transition to showers from S to N Thursday into Friday. Rainfall totals have not changed much from Wednesday night through Friday: N Bay/Coastal Mts 1-3", Bay Area 1-2", Interior Monterey/San Benito 0.5-1". In addition to precip, the passing front will result in a 6-12 hour window of stronger winds. Gusts of 30-40 mph with some high spots to near 50 mph. Not enough confidence for Wind Advisory yet, but something to watch. Finally, the fropa will usher in another shot of colder air, which will lower snow levels and bring some frozen precip to the higher terrainThursday. One challenge will be the temperatures Thursday night into Friday. They'll depend on how much clearing is seen and what the winds will be like. For now, forecast is cold with temperatures dropping back into 30s. It's possible that some patchy black ice could occur.

Rain chances remain in the forecast for the upcoming weekend as another system moves in from the PacNW. Another round of precip is possible, but not as much as the first system.

Aviation

As of 04:00 PM Monday, For the 00Z TAFs. Dry frontal boundary made its way through the area this afternoon, leaving behind gusty northwest winds. NW winds will prevail for the rest of the day today. As of latest observations, wind gusts have diminished and sustained winds at the surface are decreasing. Winds aloft will stay breezy, so minor LLWS at KMRY and KAPC possible tonight. Valleys/low lying areas embedded within higher elevations such as over the Diablo Range and the Santa Lucias are already seeing low cloud development and will be enhanced overnight with radiational cooling. However, these low clouds expected to stay confined to higher elevations, keeping terminals VFR though the period.

Vicinity of SFO, VFR through the period. Northwest winds peaked at around 35 knots earlier this afternoon but are beginning to diminish and will continue to decrease through the evening. Winds will further diminish after 03z. Light to moderate winds are on track to shift N/NNE early morning Tuesday before shifting back to onshore later in the afternoon.

SFO Bridge Approach, Similar to KSFO.

Monterey Bay, VFR through the period. Gusty surface winds will diminish through the evening. Above the surface, around 500 to 2000 feet, however, will remain breezy through the evening so expect some minor LLWS through 06z. Winds shift southerly early tomorrow morning and will switch back to onshore in the afternoon.

Marine

As of 02:24 PM PST Monday, Dry cold front is currently moving through the waters, bringing strong northwest winds that will persist into Tuesday which has made conditions hazardous for small craft vessels. Low-end gale force gusts are expected south of Point Sur coast through the afternoon. Could see brief, localized gale force gusts across the central outer waters this evening. A moderate period northwest swell of 11 to 13 seconds prevails. Winds will diminish Tuesday night into Wednesday morning before next weather system arrives later Wednesday.

Watches, Warnings, Advisories

Today, sca, SF Bay sca, mry bay sca, pt arena to pt reyes 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 sca, pigeon pt to pt pinos 0-10 nm glw, pt pinos to pt piedras blancas 0-10 nm

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