Scientific Forecaster Discussion
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Area forecast discussion
National Weather Service San Francisco Bay area
1037 PM PDT Sat Apr 19 2014
Synopsis...dry and mild weather will continue through Monday
afternoon. A weak storm system will impact the area Monday night
through Tuesday with a chance for light rain. Dry weather and
near normal temperatures return for the balance of the work week.
Discussion...as of 8:55 PM PDT Saturday...mostly sunny skies
prevailed across the region today. However...brisk onshore winds
held afternoon temperatures a bit below normal near the coast and
close to normal inland.
Low clouds have already developed along much of the coast and
around sf Bay this evening. Expect low clouds to become more
widespread through the remainder of the evening given the
continued robust onshore flow. Surface high pressure is forecast
to build to our north and northeast late tonight and tomorrow
morning. This will result in a rapid diminishment of onshore flow
late tonight. In fact...both the NAM and WRF show light north to
northeast winds developing near the top of the boundary layer by
sunrise Sunday morning. This should bring about rapid clearing of
low clouds on Sunday morning with mostly sunny conditions expected
in most areas by late morning. Meanwhile...subsidence under a
strengthening upper ridge will warm the airmass at 850 mb close to
15 degree c. The end result will be warmer weather on Sunday with
high temperatures warming at least five degrees above normal.
The warming trend will be short-lived. Temperatures will turn
cooler on Monday as a Pacific weather system...currently seen on
satellite out near 140w...approaches the coast. Onshore flow will
ramp up on Monday and high temperatures will be anywhere from 5 to
10 degrees cooler compared to Sunday.
The incoming system looks reasonably impressive on satellite at
the present time...but the NAM and GFS agree that this system
will be short on moisture and produce only scattered light precipitation
in our area on Monday night...with perhaps isolated light showers
continuing into Tuesday. Rainfall totals with this system still
look to be on the order of a tenth of an inch or less. The 12z
European model (ecmwf) and 12z Gem models are both somewhat wetter than the NAM and
GFS and it will be interesting to see if their 00z runs maintain
this wetter solution. Based on current satellite...this wetter
solution may be correct. But even if the wetter Euro/Gem solutions
were to verify...rainfall amounts on Monday night will still be
The cooling trend that will get underway on Monday will continue
on Tuesday. A much cooler airmass will overspread our area by
Tuesday morning with 850 mb temperatures forecast to drop down as low as
2 degree c. Highs on Tuesday will be only in the middle 50s to middle 60s
across our forecast area.
Temperatures will gradually warm on Wednesday and Thursday as a
low amplitude ridge develops along the West Coast.
The medium range models agree that wet weather will develop across
northern California by next weekend. The specifics are still
difficult to pin down this far in advance and the models are
showing quite a bit of run-to-run differences in regard to timing
and rainfall amounts. But the model consensus points to at least a
reasonably good chance of widespread late season rainfall by the
end of next weekend.
Aviation...as of 10:30 PM PDT Saturday...upper level trough is
moving inland north of the area. A dissipating frontal system has
spread low clouds into the sfo and mry Bay area this evening.
Satellite imagery indicates slow clearing just northwest of the sfo Bay.
Sfo has cleared out while ceilings continue over the East Bay.
These clouds should clear by 08z. Trickier forecast for the mry
Bay area as low level moisture from the front and less impact from
the trough will allow MVFR/IFR ceilings through most of the night.
Vicinity of ksfo...VFR conditions as drier northwest flow settles in.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...MVFR/IFR ceilings.
Marine...as of 8:00 PM PDT Saturday...larger swell with longer
periods still expected later tonight. A high surf advisory has
been issued from 5 am Sunday to 5 am Monday morning for the coast
from Sonoma County south to Monterey County. Latest models
indicate a borderline high surf advisory with northwest swell
reaching 13 to 15 feet with a 16 second period.
... Surf advisory...entire coast from Sonoma County
south through Monterey County starting at 5 am PDT
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm
Small Craft Advisory...pt pinos to pt Piedras Blancas 10-60 nm
Public forecast: dykema
aviation/marine: west pi
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