Swell Matrix

Area Forecast Discussion

National Weather Service San Francisco Bay Area

520 pm PDT Sun Sep 22 2019


A slight cooling trend will continue into Monday as an upper level trough pushes inland. High pressure will then build inland over northern California as an upper level low persists over the Desert Southwest. This will bring a robust warming trend to the region Tuesday into Wednesday with dry offshore flow and increased fire weather concerns. Temperatures trend sharply cooler late in the week and into next weekend with the potential for unsettled weather conditions as an upper level trough deepens over the West Coast.


As of 1:55 PM PDT Sunday, Increased cloudiness over the area in response to an upper level trough moving into northern California has resulted in much cooler afternoon temperatures across the district. Current readings are generally ranging from the upper 60s at the coast to the 80s to near 90 inland, which are generally 5 to 10 degrees cooler than temperatures from 24 hours ago. The 1 PM reading of 64 degrees at the Monterey Airport is 17 degrees cooler than 24 hours prior.

This upper level trough will form into a cut-off low over the Great Basin, dropping south over the Desert Southwest by late Monday night. A ridge of high pressure will build inland over northern California Tuesday into Wednesday resulting in offshore flow and a warming trend that will extend all the way to the coast. Look for temperatures to warm into the 80s at the coast by Tuesday with widespread 90s inland. Some of the warmest inland locations can expect daytime temperatures to exceed 100 degrees. Similar conditions are likely to persist on Wednesday, although a wind shift to onshore flow could bring cooler temps to near- coastal areas in the afternoon. Given this warm, dry air mass forecast to develop with the offshore flow, the Fire Weather Watch has been converted to a Red Flag Warning for the North and East Bay Hills/Mountains from Monday night through early Wednesday morning.

Fortunately, this heat wave is expected to be short-lived as a rapid cooling trend is forecast to begin after midweek as a deepening upper level trough develops along the west coast. This trough is expected to bring 10 to 15 degrees of cooling by Thursday with additional cooling on Friday and cool and unsettled weather persisting through the weekend.


As of 4:51 PM PDT Sunday, for 00Z TAFs. Onshore flow and a moist boundary layer left in the wake of dry cold front that moved across the region earlier today will allow low clouds to redevelop this evening and overnight. The Fort ord profiler indicates the marine layer has deepened to around 1500 ft over the last few hours. The marine layer is forecast to remains around 1000-1500 feet overnight. Therefore, anticipate moderate inland intrusion across area terminals. Moderate onshore surface flow will gradually diminish this evening and through the overnight hours becoming light and locally variable. 925 mb winds and higher will turn offshore overnight bringing a brief period of LLWS to the North Bay at around 1500-2000 ft. This will become a more widespread issue Monday night.

Vicinity of KSFO, VFR conditions will prevail through late tonight with temporary MVFR cigs possible over KOAK early Monday morning occasionally effecting visuals. Moderate onshore winds will gradually diminish becoming light overnight.

SFO Bridge Approach, Similar to KSFO.

Monterey Bay Terminals, VFR conditions will prevail through this evening with LIFR/IFR conditions expected to return between 05z- 08z tonight. Clearing is anticipated around 17z-19z Monday. Light to moderate onshore winds will gradually diminish this evening becoming light and locally variable overnight.


As of 2:09 PM PDT Sunday, Light to locally moderate northwesterly winds will persist into tomorrow with the strongest winds expected north of Point Reyes where hazardous squared seas are also possible. Winds will increase tomorrow as another trough moves through the region. Gale force gusts are expected starting tomorrow morning over the northern outer waters. Mixed seas will continue with a shorter period northwest swell at 8 to 10 seconds and a longer period southerly swell.

Watches, Warnings, Advisories

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

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