Swell Matrix

Area Forecast Discussion

National Weather Service San Francisco Bay Area

508 am PDT Tue Mar 26 2019

Synopsis

Lingering showers, mainly this morning south of San Francisco. Next front arrives tonight with another round of showers overnight into early Wednesday morning. Shower chances linger into Thursday, especially from the Bay Area northward as the final trough passes through. Drying and warming trend Friday into the weekend.

Discussion

As of 4:03 AM PDT Tuesday, Last nights slow moving front dropped some moderate rainfall across the Bay Area. Lingering showers continue early this morning as the surface front has stalled out across the South Bay and Central Coast. Dewpoints in the lower to mid 50s south of the boundary with 40s to the north. Models have backed off on widespread shower chances today so have trended PoPs lower for today with the best chances for showers across the East Bay, South Bay and Central Coast. Elsewhere dry but cloudy weather will persist for SF and the North Bay.

Attention quickly turns to next organized front due to arrive later tonight into early Weds morning. This will bring a more well defined frontal boundary through the region with the best precip chances focused over the coastal ranges and North Bay later tonight into early Weds morning. Cold advection aloft and pva associated with the incoming upper trough supports higher PoP chances later tonight through early Weds.

The frontal boundary will hold together better than last nights front and expect some showers to work towards the Central Coast on Weds afternoon. Have retained t-storm chances across most of the district to account for steeper lapse rates aloft associated with cold upper trough and strong late March Sun angle.

Final shortwave drops down from the north by Thursday morning and looks to bring a band of organized showers across the North Bay and towards the Bay Area through midday Thursday. Precip looks to be light but a cold rain with 1000-500 mb thickness values around 537 dm keeping temps on the cool side of normal and briefly dropping into the 50s during any convective showers. While PoPs look likely for the Bay Area on Thursday, chances really drop off from San Jose southward so have lower PoPs for the Central Coast.

Models continue to build high pressure Friday into the weekend with a noted warming trend and increasing sunshine with noted 500 mb ridge.

Ensemble and national model blends suggest higher pops for late in the weekend or early next week but thats not supported by operational runs or climatology so have kept pops lower.

Aviation

As of 5:00 AM PDT Tuesday, Frontal system has washed out over the southern half of the district. Areas of low clouds are forming this morning due to abundant low level moisture but are not expected to impact SFO and the approach. A few showers may develop along the old frontal boundary this afternoon so have included VCSH for the southern terminals SJC LVK and the MRY Bay terminals. Rain and lower cigs return late tonight as another frontal system approaches.

Vicinity of KSFO, VFR. West winds in the afternoon.

SFO Bridge Approach, Similar to SFO.

Monterey Bay Terminals, MVFR to IFR cigs through 16Z. A few showers possible this afternoon.

Marine

As of 04:58 AM PDT Tuesday, Low pressure off the Oregon coast will keep light to moderate southerly winds through Tuesday night. Stronger winds can be expected over the northern waters Tuesday night and early Wednesday morning as a frontal system moves through. Winds will switch to southwest Wednesday and Thursday as high pressure builds off the California coast. There is a slight chance for thunderstorms Wednesday.

Watches, Warnings, Advisories

Today, none.

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