Scientific Forecaster Discussion
fxus66 kmtr 251145
Area forecast discussion
National Weather Service San Francisco Bay area
445 am PDT Wed Apr 25 2018
Synopsis...a cooling trend is forecast to persist through late
week and into the upcoming weekend as an upper level trough
approaches from the Pacific. This system may produce a few showers
over the North Bay late Friday into Saturday as it pushes inland
north of our region, yet widespread rainfall is unlikely.
Discussion...as of 3:00 am PDT Wednesday...onshore flow is
directly east-west as the sfo-SAC gradient is 2 mb and the sfo-acv
gradient is neutral. This gives the marine air the maximum effect
over the district. With the marine layer holding around 1500 feet
night and morning low clouds can be expected around much of the
mry and sfo Bay area while temperatures will remain as cool as
An upper low centered 750 miles west of Point Arena will drift
slowly east-northeast. As it gets closer the marine layer will deepen even
more Thursday through Saturday bringing cool marine air into the
interior valleys. Models showing night and morning low clouds
spreading into places like Livermore and Concord Thursday night.
Eventually the marine layer will mix out but the airmass will be
colder so temperatures will continue below normal. By Saturday
highs around the entire County Warning Area will range from the upper 50s at the
coast to the lower 70s in the far interior. Models continue to
show the upper low moving into northern California on Saturday.
Precipitation chances appear to be very slim and confined to
widely scattered showers in the North Bay.
Medium range models now show a slow warming trend for the
beginning and middle of next week as the low moves further inland.
Temperatures should return to near their normal values by about
Aviation...as of 4:45 am PDT Wednesday...for 12z tafs. Widespread
stratus event along entire California coast bringing generally IFR
to MVFR cigs to terminals this morning. Expect cigs to linger
slightly longer than yesterday morning, primarily 17-18z, up to
19z for some locations nearest to the coast. Winds breezy onshore
along the coast and just inland during the afternoon, otherwise
mostly light winds. A few passing high clouds today. Stratus is
forecast to return again late this evening through tomorrow
Vicinity of ksfo...MVFR thru 16-17z despite unfavorable stratus
feed direction (southwest to northeast) as band of bkn015 now
permeating throughout the South Bay. Breezy onshore winds in the
afternoon up to 15kt. Stratus returns tonight/tomorrow morning.
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...IFR cigs will persist into the mid to
late morning (up to 19z or even later if there is a stubborn patch
over kmry airport). IFR cigs will return tonight/tomorrow
morning. Generally light winds through the period.
Marine...as of 04:36 am PDT Wednesday...light to locally moderate
west to northwest winds will continue through the week. Locally
gusty conditions possible near coastal gaps or prominent points
such as the Golden Gate and Point Sur. Light to moderate seas
will also persist through the forecast period with a mixed south
and northwest swell developing late in the week.
Public forecast: west pi
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