Swell Matrix

Area Forecast Discussion

National Weather Service San Francisco CA

925 pm PDT Thu Sep 21 2023


Issued at 925 PM PDT Thu Sep 21 2023

Satellite imagery shows increasing areal coverage of stratus clouds over the coastal waters also extending inland across the forecast area including the North Bay, the East Bay, the Santa Cruz Mountains and the northern Salinas Valley. Patchy valley fog reducing visibilities overnight, combining with smoky and hazy conditions into the morning. The 2.4 mb SMX-SFO pressure gradient and wind is strongest relative to northerly and onshore pressure gradients: 0.4 mb ACV-SFO and 1.1 mb SFO-SAC; the mesoscale models NAM, HRRR are under-forecasting the SMX-SFO gradient and southerly wind. For example, SFO Airport has not had a westerly wind this evening, rather there continues a strong southerly component to the wind direction. Also KMRY and KSNS winds are behaving similar to a southerly wind reversal. Stratus and fog mix out by later Friday morning. Cooler than late September normal temperatures are forecast tonight and Friday.


Issued at 831 PM PDT Thu Sep 21 2023

Temperatures remaining below normal through the forecast period with low clouds in the mornings. A rapidly deepening low pressure system off the Pacific Northwest over the weekend will bring a band of rain with a cool front through the Bay Area later Monday into Tuesday. A wet pattern is expected early week next week.


(00z TAFS) Issued at 653 PM PDT Thu Sep 21 2023

Wildfire smoky and hazy conditions continue with surface visibilities MVFR-VFR and slant range visibility reductions ongoing especially at sunset and sunrise. Profiler data and the evening 00z Oakland upper air sounding show a weak lower level temperature inversion. The sounding also shows 0.51" precipitable water, below late September normal. Lower level temperatures are forecast to undergo weak net warming tonight and Friday helping a little to restore the marine layer temperature inversion. Otherwise a drier troposphere tonight to sunrise Friday morning is aside from smoky/hazy conditions now a little more transparent window for outgoing radiative cooling, supporting additional stratus cloud development overnight. Patchy valley fog /VLIFR-IFR/ developing tonight and Friday morning. Morning stratus and fog /VLIFR-IFR- MVFR/ mixing out to VFR by late Friday morning.

Vicinity of SFO, Mainly VFR, surface visibility periodically reduced to MVFR in smoky and hazy conditions later tonight and early Friday morning. Mesoscale model output forecasts MVFR stratus ceiling 13z-16z Friday, mixing out to VFR post 16z Friday. Mainly southwest wind 5 to 10 knots tonight and Friday morning, except southeast near 5 knots 16z-21z Friday, then westerly 10 to 15 knots Friday afternoon and evening.

SFO Bridge Approach, Similar to SFO.

Monterey Bay Terminals, Mainly VFR, surface visibility periodically reduced to MVFR in smoky and hazy conditions tonight and Friday morning. Stratus /MVFR/ redeveloping tonight and Friday morning as well, winds mainly onshore 5 to 10 knots except shifting to southeast direction KSNS and KMRY late tonight and Friday morning. Stratus ceilings /MVFR/ mixing out by late Friday morning otherwise MVFR-VFR surface visibilities in smoky and hazy conditions.


(tonight through next Tuesday) Issued at 820 PM PDT Thu Sep 21 2023

Gusty northerly winds continuing over the northern outer waters tonight, easing Friday. Gentle to moderate northerly breezes are expected elsewhere tonight. A quickly strengthening low pressure system will bring light rain chances to the northern waters by late Monday into Tuesday of next week. Otherwise, northwesterly swell diminishing through the evening with a new swell train arriving late into the weekend.

Watches, Warnings, Advisories

Ca, none. Pz, small craft advisory until 3 am PDT Friday for pzz570.

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