Swell Matrix

Area Forecast Discussion

National Weather Service San Francisco Bay Area

147 pm PST Tue Nov 12 2019


Warm high pressure on Tuesday will keep the marine layer shallow with high clouds streaming over the region. The ridge weakens slightly Wednesday with a few degrees of cooling. Further cooling Thursday as an upper trough passes through with a return of cooler onshore breezes and some drizzle for the coastal slopes. The ridge will rebuild over the weekend with a return of seasonably warm and dry weather. Some hints for a more active pattern in the extended but no clear signal for when rain will return to the forecast.


As of 01:45 PM PST Tuesday, Filtered sunshine remained over the region this afternoon with high clouds. The coastal waters however held on to low clouds with the Farallon Islands reporting dense fog mid afternoon. Despite some high clouds the warm airmass and lingering offshore flow over the hills resulted in mild afternoon temperatures. As of 1 PM temperatures are mostly in the 60s and 70s with a few interior spots hitting low 80s - or about 5 to 15 degrees above normal for mid November.

No big changes in the short term. Low clouds and fog will remain along the coast tonight moving locally inland, like at Monterey. Otherwise most locations will hold onto the high clouds. The ridge of high pressure currently overhead will slowly move eastward over the next 12-24 hours leading to increased onshore flow and a cooler airmass. Forecast high temperatures on Wednesday will drop 3 to 5 degrees from today. As the ridge moves east an upstream upper trough will continue to advance eastward as well. The trough passage is still on track for Thursday into Friday. The passing trough will bring widespread cooling the region on Thursday continuing into Friday. Temperatures by Thursday will be close to or slightly below climo for this time of year. As noted on the previous discussion the lingering low level moisture combine with added lift from the trough may be enough to cause some light drizzle early Thursday. Some of the 12z models even generate some light precip during this timeframe with possibly a hundredth or two.

As the trough exits on Friday high pressure begins to rebuild over the region for the upcoming weekend. Medium range models even develop another round of offshore flow - similar to what we just experienced.

The following snippet from the previous discussion still holds though. Increasing confidence that more jet energy will break the ridge down with the pattern showing signs of becoming more progressive by the middle of next week and towards the second half of the month.


As of 9:40 AM PST Tuesday, Widespread VFR with advancing high clouds from the WNW. These high clouds will persist through the TAF period. Winds generally light offshore this morning and tonight with locally 6-12kt onshore winds this afternoon for coastally influenced TAF sites. For tonight, expecting predominately VFR with some rogue marine layer clouds bringing intermittent reductions to visbys/flight rules at KMRY/KSNS (similar to last evening, but less of a chance versus 24 hours ago).

Vicinity of KSFO, VFR. Light winds except occasionally breezy onshore later this afternoon to early evening. High clouds through the day AOA 18000-25000 ft. A few clouds around 500 ft possible tomorrow morning, but not expecting impact to visuals.

SFO Bridge Approach, Similar to KSFO.

Monterey Bay Terminals, VFR through this morning and early afternoon. Low to medium chance of seeing a similar return of low clouds around the terminals after 00z, especially 04-08Z, similar to last night.


As of 09:29 AM PST Tuesday, Predominately light variable winds today. Winds shift towards the south later tonight into tomorrow, with locally breezy southerly winds along the coast by tomorrow afternoon. North winds return late in the week. Wind waves will continue to ease. A series of west to northwest swell will move through the waters this week, with moderate to locally large longer period northwest swell arriving later Thursday.

Watches, Warnings, Advisories

Tonight, none.

Visit this site often? Consider supporting us with a $10 contribution.
Learn more

Share this forecast!