Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
453 PM PDT Sat Apr 18 2015
Synopsis...a moderately deep marine layer along the coast will
result in cooler temperatures along the coast while inland temperatures
remain above average. Cooler temperatures and unsettled weather
are expected across the district this week as weak disturbances
swing through the region.
Discussion...as of 2:35 PM PDT Saturday...widespread stratus
across the region cleared back to the coast by middle-morning today
leading to mainly clear skies across much of the district with
exception to the coast and coastal waters. Temperatures this
afternoon are running between 5 to 10 degrees cooler than 24 hours
ago with temperatures along the coast seeing the greatest drop.
Current readings show temperatures in the 60s along the coast with upper
60s and 70s inland. Not sure temperatures will reach their
expected highs today with the cloudy start we had this morning.
The shortened allowed heating time will definitely have an impact
on the final numbers today.
A weak short wave approaching the district from the north will
maintain a moderately deep marine layer along the coast. Low
clouds are expected to penetrate well inland during the night and
morning hours as a result. As this short wave slides south models
show a low cutting off over northern California this afternoon.
The low then veers to the west and meanders southward through
Sunday. This system will serve to enhance the marine layer along
the coast resulting cooling along the coast as well as areas
affected by the marine layer. Otherwise conditions are expected to
remain mild and dry.
By Monday afternoon the low is forecast to swing into Southern
California. Models continue to show increased instability over the
Gabilan and Santa Lucia ranges as well as the northernmost
mountains over Napa and Sonoma. However the moisture is not there
to support anything more than an isolated sprinkle or
The chance of an isolated shower or thunderstorm increases Tuesday
afternoon with a slight increase moisture over the region. Have
introduced a slight chance of thunderstorms to the northernmost
portions of Napa and Sonoma as well as the eastern flank of the
County Warning Area and the mountains of Monterey and San Benito counties.
Temperatures across the region will gradually cool through the
remainder of the week as a broad long wave trough sweeps through
the region. A few weak impulses are prognosticated to rotate through the
long wave trough Tuesday and Wednesday which will bring a slight
chance of isolated showers to the region.
Model solutions begin to diverge toward the end of the week.
However it appears the weather pattern will remain unsettled.
The European model (ecmwf) and Canadian depict a moderate cold front sliding
through the region Friday into Saturday. This solution would
bring widespread rain to the district. Made little change to the
extended and maintained an unsettled pattern and slightly below
Aviation...as of 4:50 PM PDT Saturday...marine layer was about
1000 feet deep late Saturday afternoon...but is expected to deepen
slightly overnight and into Sunday. Onshore gradients are slightly
stronger than yesterday. These factors favor earlier development
of low ceilings around sf Bay tonight compared to last night and then
later clearing on Sunday. However...eddy circulation off Golden
Gate may help delay development of low ceilings in sf Bay until late
evening or around midnight. Light to moderate westerly winds this
evening and again by late Sunday afternoon.
Vicinity of ksfo...VFR conditions expected through most of the
evening. Ceilings likely to develop overnight...but confidence is
low/medium on timing. Eddy circulation offshore is expected to
delay IFR/MVFR ceiling onset until 09-10z. Low ceilings expected to clear
around 18z Sunday morning. West winds 16g20 knots this
evening...gradually decreasing overnight.
Sfo bridge approach...similar to ksfo.
Monterey Bay area terminals...IFR ceilings likely to develop by 04-05z
and then persist through 18z Sunday. Light westerly winds early this
evening becoming light and variable by midnight.
Marine...as of 2:34 PM PDT Saturday...northwest flow under high
pressure over the coastal waters tonight. Winds will be strongest
well offshore of point arenas. Elsewhere winds will generally be
light through Monday. Sea conditions gradually improve into
Monday. Winds and seas increase Tuesday through middle week.
... Arena to pt Reyes 10-60 nm until 9 PM
Public forecast: CW
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