Swell Matrix

Area Forecast Discussion

National Weather Service San Francisco Bay Area

819 pm PDT Wed may 24 2017

Synopsis

A cooling trend can be expected through Friday as high pressure weakens and the marine influence increases. Minor warming is then expected during the upcoming weekend into early next week as high pressure returns to the West Coast.

Discussion

As of 08:09 PM PDT Wednesday, No big changes made to this evening's forecast. Satellite continues to show marine stratus along the central California coast. The Fort Ord profiler indicates the marine layer is just above 2000 ft. Afternoon highs were distinctly cooler across inland locations with many stations at least 5 to 15 degrees cooler than Tuesday. Santa Rosa observed one of the most prominent day-to-day changes: the high on Tuesday was 84 but today it was only 64. The cooler temperatures were a response to the departing ridge that had been parked over the West Coast earlier this week, as well as a weak upper level low off the California coast. The persistent onshore flow and overcast skies helped to minimize the change in afternoon highs along the coast.

Expect additional cooling through the end of the work week with below normal temperatures for many locations. Coastal areas will continue to see low clouds and experience highs ranging from the middle 50s to 60s. Warmer weather will return for the holiday weekend as another ridge builds over the West Coast with 90s returning to the southern Salinas Valley as early as Sunday.

Aviation

As of 5:45 PM PDT Wednesday, It's a chaotic stratus and wind pattern presently with a steadily deepening marine layer responding to ongoing lower level cooling on southerly flow along the immediate coast. Much cooler Pacific air modified by coastal SSTs in the 50s includes modified cooler air arriving from the mid-high latitudes wrapping around a lower level circulation off the CA coast. A mid level 500 mb height low approx 400 miles SW of the Bay Area advances to the coast coinciding with a newly developing weak surface low pressure center remaining nearly stationary just west of the Bay Area Thu-Fri. As a result spotty coastal drizzle or very light rain is possible, none indicated in 00z tafs presently, but will amend as needed.

The marine layer becomes more and more elevated tonight and Thursday. The primary forecast challenge will be coverage and duration of stratus ceilings. 925 mb and 850 mb cooling may reach the point of mixing out the marine layer inversion almost entirely Thursday and Thursday night.

Vicinity of KSFO, S-SW winds 10-15 knots til 06z this evening then probably shifting back toward W-SW by later evening, timing is low confidence. Coordinated with CWSU and we decided to delay the onset of forecast stratus ceiling based on present local stratus coverage and wind pattern. SFO-SAC pressure gradient is a healthy 3.6 mb thus the stratus could suddenly shift inland, but the forecast strongly hedges on the south wind component holding longer; tempo MVFR cig 06z-08z the MVFR prevailing into Thursday morning. VFR forecast from 17z Thursday through 06z Thursday evening.

SFO Bridge Approach, Similar to KSFO. Wind direction forecast is low confidence from the Santa Clara Valley to SMB approach due to recently strong 2 mb to 3 mb southerly gradient KSNS-KSJC; 5 min metar obs show NW wind 10 knots at KSJC. 00z taf KSJC forecasts SE wind developing by 03z.

Monterey Bay Terminals, Tempo MVFR cigs 00z-04z this evening then MVFR prevailing tonight and Thursday morning. At least partial clearing is likely by late Thursday morning depending if the marine layer inversion persists.

Marine

As of 08:09 PM PDT Wednesday, Low pressure off the central California coast will bring southerly winds to the area through Thursday. Local gusty winds can be expected Thursday along the Big Sur and Monterey Bay coastline. Surface high pressure further out in the Eastern Pacific is bringing northerly winds offshore north of Point Reyes. This high will eventually rebuild off the coast resulting in increasing northerly winds by late this weekend.

Watches, Warnings, Advisories

Tonight, sca, pt arena to pigeon pt 10-60 nm sca, pt reyes to pigeon pt 0-10 nm from 3 am sca, pt pinos to pt piedras blancas 0-10 nm from 3 am sca, SF Bay until 9 pm

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