Swell Matrix

Area Forecast Discussion

National Weather Service San Francisco Bay Area

520 pm PDT Sat jul 20 2019


A deep marine layer will maintain cooler than average temperatures today, especially inland. A warming trend will begin on Sunday and continue through the middle of next week as high pressure centered over the Four Corners area builds westward into California.


As of 02:00 PM PDT Saturday, A weakening coastal trough is continuing to bring southerly flow along the coast this afternoon. This has prevented clouds from fully clearing out of parts of Sonoma and Marin counties but expect clouds to continue decreasing early this afternoon. Elsewhere, stratus has retreated to the coast with mostly sunny skies. Temperatures are down several degrees versus 24 hours ago at most locations due to clouds lingering into the late morning and early afternoon. The biggest decrease in temperatures are in the North Bay, East Bay, and South Bay where temps are as much as 5-10 degrees below this time yesterday. This will result in highs several degrees below yesterday's observed highs in these locations. Elsewhere, high temperatures will be similar to yesterday.

Northwesterly flow is expected to replace southerly flow along the coast over the next 24 hours as the offshore surface trough weakens. An upper ridge will develop over the Four Corners region on Sunday and begin to expand northwestward over California through next week. This will act to compress the marine layer, and less inland intrusion of stratus can be expected in the coming days. The building ridge will also begin a warming trend that will last through the middle of next week. 850 mb temperatures will begin at about 20C today and are forecast to warm approximately a degree C each day Sunday through Wednesday. Most of the warming at the surface will be felt at inland locations, with highs rising a few degrees each day. Inland valleys are expected to reach the 80s and 90s, with the hottest locations reaching the upper 90s to lower 100s by the middle of next week,

An upper trough will move off the coast early next week and will prevent the ridge from building more strongly across California. Cool onshore flow is expected to prevail, and only slight warming is expected at coastal locations. Coastal highs through next week are forecast to remain in the 60s and 70s.

Late next week, ensembles depict the upper trough lifting to the north and the Four Corners high gradually shifting westward, becoming centered over the Desert Southwest next weekend. This would result in further warming of inland areas, while continued onshore flow keeps coastal areas cool.


As of 05:15 PM PDT Saturday, For 00z tafs. VFR conditions prevail at all terminals, along with light to locally moderate onshore winds. Latest satellite imagery shows stratus blanketing the coast from around Half Moon Bay northward, and pulled back a bit offshore farther south. The marine layer remains on the deep side, with reports from coastal profilers indicating a depth varying from around 2000 to 2500 ft. Latest model and model guidance indicates widespread MVFR/IFR development during the evening and overnight hours as the low cloudiness pushes back inland.

Vicinity of KSFO, Stratus is trying to push through the San Bruno Gap but believe coastal flow is too light and with too much of a southerly component to enable it to reach the terminal area until after the evening rush. Moderate confidence. MVFR ceiling development is then expected during the overnight hours, clearing mid-late morning Sunday. West Northwest winds 15 to 20 kt will begin diminishing within the next couple of hours.

SFO Bridge Approach, similar to KSFO.

Monterey Bay Terminals, Leading edge of stratus is now off the coast out over the waters, but has already begun pushing back towards the shoreline. Moderate confidence MVFR ceiling onset will occur by midevening at both KMRY and KSNS. Ceilings will likely diminish to IFR during the late evening and overnight hours.


As of 02:13 PM PDT Saturday, Light southerly flow along the coast and northwesterly winds over the outer waters will persist through tomorrow. Locally gusty winds will continue today across the northern outer waters and over the northern San Francisco Bay. Winds will shift back out of the northwest on Sunday.

Watches, Warnings, Advisories

Tonight, sca, pt arena to pigeon pt 10-60 nm until 3 am sca, SF Bay until 9 pm

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