Swell Matrix

Area Forecast Discussion

National Weather Service San Francisco Bay Area

451 am PDT Thu may 24 2018

Synopsis

Cloudy and cool today with increasing chance of showers tonight into Friday as a late season low passes over the region. Shower chances end by early Saturday morning. High pressure builds Sunday through Tuesday with a noted warming and drying trend.

Discussion

As of 3:42 AM PDT Thursday, Satellite shows low clouds covering most of the region with high clouds now starting to increase as well ahead of incoming upper low. Current obs show cloud bases around 1500 feet most locations with another morning of drizzle likely with abundant low level moisture and incoming synoptic lift. Combination of cloud cover and fairly cool airmass will mean another day of afternoon highs mainly in the 60s except some 70s far interior valleys of the Central Coast.

Main forecast item of note will be shower chances with incoming upper low. 06z NAM and latest rap model indicate slight shower chances by late this afternoon/early evening over the coastal hills of Monterey county. Any late day breaks in the clouds will help with some destabilization. Timing of upper low, which has been pretty consistent for days now brings the core of the coldest air through late tonight into early Friday morning. Thus we wont have peak heating for instability. Nonetheless enough instability to generate shower chances across much of the region Friday morning. There remains a slight chance of thunder but for now the forecast will just show showers as confidence for t-storms remains low which is supported by latest spc general convective outlook for Friday. Main impact will be for the Friday morning commute when some rogue showers could produce wet roads with some brief moderate showers possible. QPF numbers should be light, on the order of a few hundredths for areas that receive showers although localized amounts up to 0.25 will be possible.

The main shower threat should end as the Sun goes down Friday night but will hold onto slight chance showers through about sunrise Saturday. Saturday will be a cool start to the holiday weekend as we'll be on the backside of the upper low with continued strong onshore flow and cold air aloft.

Good news for the holiday weekend is building high pressure should bring much more sunshine and warmer temperatures for Sunday and Memorial Day with lots of highs back into the 70s and 80s away from the coast. The surface pattern may even generate some northerly flow which would even clear clouds at most beaches for the second half of the holiday weekend.

Above normal temps to stay in place through Tuesday before the next cool trough arrives midweek with another cooling trend.

Aviation

As of 4:50 AM PDT Thursday, Widespread MVFR cigs forecast to persist thru at least mid morning. Approaching upper trof has allowed marine layer to deepen to approximately 3000 ft, with a depth of around 1000 feet. This is leading to cig bases predominately around 1500-2000 ft. Expect SCT with some patchy BKN clouds to persist in the low to mid level into the late morning or possibly early afternoon at some terminals. Onshore winds locally breezy with occasional gusts to around 20kt at the windiest spots. VCSH at the end of the taf period.

Vicinity of KSFO, MVFR thru 17-18Z this morning, then lingering FEW-SCT low to mid level clouds as upper trof arrives. Breezy onshore winds up to around 15k with gusts to around 20kt thru the afternoon. Borderline VFR/MVFR cigs towards the end of the taf period with VCSH.

SFO Bridge Approach, Similar to KSFO.

Monterey Bay Terminals, MVFR thru the mid to possibly late morning around the Monterey Bay. Onshore winds to around 12 kt with local gusts to around 18kt. Low to mid level few to sct clouds will linger around the terminals thru the afternoon with early return of borderline MVFR/VFR cigs by the evening.

Marine

As of 04:45 AM PDT Thursday, Generally light winds across the waters today and tonight. The strongest winds today will be located in the northern San Francisco Bay. Northwest winds pick up by mid day tomorrow and become locally moderate by tomorrow night through the upcoming weekend. Mixed seas with a longer period southwest swell and a smaller northwest swell. Increasing winds later this weekend will bring locally steep fresh swell.

Watches, Warnings, Advisories

Today, sca, SF Bay from 3 pm

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