Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
328 am PST Thursday Dec 12 2013
Discussion...as of 2:10 am PST Thursday...freeze warning remains
in place through 9 am for the North Bay valleys. Compared to
previous nights temperatures are running much warmer but ksts is already
down to 29 so will keep final freeze warning in place through the
morning hours. Dewpoints are now in the upper 20 and lower 30s so
not expecting too much more cooling.
After a chilly start look for a mostly sunny and seasonable
afternoon. The Bay area will be in-between an upper low over
Southern California and a weak incoming front that will move into
far northern California by this evening. Unfortunately it doesn't
look like any rainfall associated with the front will make it
south of Mendocino County. The only real impact will be to turn
winds briefly onshore later tonight into early Friday. Synoptic
flow off the ocean should trend dewpoints slightly higher tonight
with lows generally above freezing all areas. So it looks like the
first night without a freeze warning since last Friday nights rainfall.
By this weekend the models build a strong ridge of high pressure
from the eastern Pacific...that will slowly spread eastward
through early next week. 850 mb temperatures around 9 celsius on Saturday
morning will warm from 15 to 17 celsius by Monday and Tuesday per
the latest European model (ecmwf) solutions. Often times during this time of year
the 850 mb temperatures aren't good proxies as those air parcels wont mix
to the surface due to strong surface based inversions or the
presence of Tule fog that gets advected westward under offshore
flow. It appears at this time that offshore flow will indeed
produce seasonably warm and possibly some record breaking warmth
by early next week. In particular locations around Monterey that
are protected from any cold Central Valley air and benefit from
local down-slope flow could see temperatures well into the 70s by Monday
or Tuesday. For many Bay area cities record highs are only in the
70s for the middle of December so should the 17 celsius air verify
at 850 mb it will be possible to flirt with some record warmth.
The Prospect of some sunshine and warmth probably sounds nice
after such a cold start to the month. Our nights will remain
mostly clear and seasonably cool but not expecting any more freeze
headlines for the foreseeable future.
At this time there is no real threat of any rain for the next 10
days. Other model data in the 15 day time scale including the long
range GFS and cfs models portend dry weather through the end of
Aviation...as of 3:25 am PST Thursday...VFR conditions will
prevail through the forecast...except near ksts...where MVFR visible
will be present through the middle morning hours. Moderate
confidence. Otherwise...light offshore flow this morning will
become light onshore late this morning into the afternoon hours. A
few of the short term models hint at a deepening marine layer and
possible patchy low clouds Friday morning...very low confidence so
it was not added to the forecast.
Vicinity of ksfo...VFR conditions with mainly light and variable
winds will prevail through the forecast.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...VFR conditions will prevail through
the forecast. Moderate confidence. Otherwise...moderate offshore
flow at ksns will become onshore early this afternoon.
... Warning...North Bay valleys through 9 am
Public forecast: rww
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