Swell Matrix

Area Forecast Discussion

National Weather Service San Francisco Bay Area

1034 am PDT Fri Sep 24 2021


Seasonable weather conditions will persist near the coast as a shallow marine layer remains in place through late week. Meanwhile, temperatures across the interior will be above seasonal averages on Friday before cooling occurs during the upcoming weekend. Even cooler, unsettled conditions are possible early next week with rain chances returning to the North Bay.


As of 09:15 AM PDT Friday, A very shallow marine layer, generally around or below 500 feet in depth, has resulted in stratus and fog along the entire coastline this morning. However, the inland extend was limited to coastal valleys in Sonoma and Marin counties and locally into the Salinas Valley. Look for this to retreat back to the coastline through the morning giving way to another warm day across the interior as high pressure remains aloft. Speaking of conditions aloft, smoke from wildfires over central and southern California continue to produce an abundant amount of smoke this morning. This smoke is being transported northward across the Central Coast and will move over the Bay Area through the remainder of the day. However, smoke is forecast to mostly remain loft through the upcoming weekend as onshore flow persists in the boundary layer. That said, the ongoing forecast for the short-term remains on track with no updates anticipated this morning.

Previous Discussion

As of 02:25 AM PDT Friday, Short Term..Today and Tonight, Stratus have been trying to build back over the waters tonight. Off shore winds have calmed down, and are currently light with locally driven directions. The marine layer sits around 1000ft by the coast. Patchy fog will be seen this morning around the Monterey Bay.

Onshore winds will pick back up over the area this afternoon. Temperatures will cool a few degrees inland, remaining fairly warm today, but will cool back to below normal around the coast. The central CA coast will see some elevated haze this afternoon from the fires in southern CA.

..Long Term.. Saturday through Friday, Weak high pressure aloft will persist this weekend. With light steering flow, gradients closer to the surface will dominate. Onshore flow will keep temperatures around normal or up to 5 degrees above for inland areas. The coast will stay cool through the remainder of the period, as we keep onshore flow.

Light zonal flow starts to move over the North Bay coast Sunday morning. Our next system begins to move onshore OR/WA Monday. The surface cold front with this system extends down to south central CA. Models are in agreement on the front weakening as it approaches our area. Overnight Monday into early Tuesday morning, we start to see an increase in low level moisture. Most of this will be in the form of cloud cover. The moisture with this system isn't anything to be excited for, but enough to yield light rain where the dynamics can set up. GFS/ECM PW values have consistently been around 1 inch for the past few days. Early Tuesday morning, positive vorticity advection over the North Bay will bring chances for scattered light rain. Higher terrain will also see better chances of light rain falling at this time. As the trough progresses, another short wave will dig southward over the area right behind the trough. This will bring weaker positive vorticity southward, across the Bay Area and Central CA coast Tuesday night into early Wednesday morning. By the time the second pulse of lift moves across the area we already have drier air. The mid levels stay fairly dry during this system, which wont help that second chance for rain. If the moisture beefs up in the models over the next few days, there could be better chances for rain south of the Bay Area.

That second short wave most east quickly, dropping high surface pressure into the Great Basin. Both global models are hinting at more off shore winds at the end of the period, on Thursday. We will continue to monitor this.


As of 10:34 AM PDT Friday, For the 18z TAFs. A shallow marine layer remains in place as satellite imagery shows stratus along the coast and over much of the waters. Additionally, can see smoke aloft over much of the region from various wildfires. VFR conditions north of the Monterey Bay currently with clearing expected over the next couple of hours. VFR prevailing into the early evening before an early return of stratus is anticipated for coastal terminals. MVFR/IFR conditions overnight (low confidence on cig heights) as the marine layer deepens due to weakening high pressure and an approaching trough. Some patchy fog in the North Bay Valleys and around the Monterey Bay possible again into early Saturday morning. Light winds this morning increasing and turning onshore this afternoon at around 10-15 kt.

The latest HRRR smoke model forecast shows the smoke aloft moving northward throughout the day. This will create FEW to SCT cigs depending on location at about 15,000 to 25,000 ft AGL as well as some slant range visibility issues. The near surface product shows some increase in smoke across the North Bay this afternoon where some haze may occur. Elsewhere, most of the smoke looks to remain aloft.

Vicinity of KSFO, VFR through this evening. Low clouds expected to move into the bay early this evening before expanding into the terminal later tonight. MVFR/IFR cigs then expected through mid Saturday morning. Smoke aloft due to various wildfires to reduce slant range visibility. W/NW winds this afternoon around 15 kt.

KSFO Bridge Approach, Approach may lose visuals this evening before the terminal as stratus moves in. Otherwise, similar to KSFO.

Monterey Bay, IFR currently, though satellite imagery shows some signs low clouds are beginning to dissipate. VFR conditions by the afternoon before stratus quickly returns in the early evening (~01z-03z). IFR/LIFR conditions overnight with some patchy fog possible again late tonight into the early morning. Light winds this morning, W/NW in the afternoon 10-15 kt.


As of 09:15 AM PDT Friday, Generally light southwest to northwest winds over the coastal waters through the weekend. Locally breezy afternoon and evening winds expected near the Golden Gate. Northwest winds will begin to increase across the waters early next week. Mixed seas continue with a moderate northwest swell and a longer period southerly swell. A new longer period northwest swell will arrive on Sunday associated with a developing low over the Gulf of Alaska. Seas will quickly build into Tuesday.

Watches, Warnings, Advisories

Today, sca, SF Bay from 2 pm

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