Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
1111 PM PDT Monday Jul 6 2015
Synopsis...a deep marine layer will remain in place through
midweek and result in widespread night and morning low
cloudiness...patchy fog and local coastal drizzle. An upper low off
the coast will move inland Wednesday night. Associated upper level
moisture may produce isolated showers and thunderstorms in the
Wednesday evening through Thursday night time frame. Temperatures
through the week will generally be near seasonal norms near the
coast and a bit cooler than normal inland.
Discussion...as of 09:15 PM PDT Monday...mostly clear skies
prevail across the district aside from a few passing thin high
clouds. Some shallowing of the marine layer and weakening of the
capping inversion allowed for greater mixing out of the coastal
stratus today. Latest model output continues to indicate its
redevelopment as the evening progresses however...extending
locally inland around the bays and into the coastal valleys later
tonight and Tuesday morning. Temperatures this afternoon were
generally a bit warmer than yesterday in the valleys...and pretty
similar elsewhere. Overall weather conditions on Tuesday look to be
similar to today with low clouds mixing out during the latter part
of the morning.
Latest water vapor satellite imagery shows the upper level low now
centered about 400 miles west of San Francisco. New 00z runs of
the NAM and GFS are in very good agreement in bringing it inland
over the central California coast in the Wednesday evening time
frame. Its approach on Wednesday is expected to result in some
deepening of the marine layer and slower clearing of the stratus
during the day...and some middle and high cloudiness...and thus
somewhat cooler afternoon temperatures.
Primary forecast concern continues to be the potential for high-
based showers and/or thunderstorms in the latter part of the week.
Quick examination of elevated convective parameter output from new
00z model runs suggests a bit later timing than before. The
initial risk now looks to be in the Wednesday evening to Thursday
morning time frame and focused around the landfalling upper low
center itself...so over more southern portions of our district.
And then as the low progresses farther inland over south-central
California Thursday and Thursday night...models suggest
significant middle-level moisture and cloudiness wrapping around it
westward from the Great Basin and into northern California will
then drop progressively southward into the northern half of our
area...along with shower and isolated thunderstorm chances.
Previous discussion...mid-level moisture may linger into Friday
morning with a slight chance of showers over the North Bay. The
middle/upper level system will then lift to the northeast and become
absorbed in the mean flow Friday into Saturday. This should put an
end to precipitation chances region-wide Friday night into the
upcoming weekend. The forecast models generally keep a broad
middle/upper level trough along much of the West Coast late in the
period and will likely result in dry weather conditions and
temperatures near seasonal averages.
Aviation...as of 10:59 PM PDT Monday...a gradual return of
stratus and fog is expected tonight. Ksfo reports a broken MVFR ceiling
at this time.
Vicinity of ksfo...MVFR ceilings tonight. Clearing forecast by 18z.
Westerly wind gusts reaching 20-22 knots 22z-04z.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...VFR continues but still expecting
the stratus and fog to redevelop overnight. IFR is forecast very
late with VFR returning by late Tuesday morning. The marine layer
has compressed through the evening usually not a good sign for
patchy light drizzle...but left it in as tempo groups for ksns and
kmry early Tuesday morning.
Marine...as of 8:52 PM Monday...a weak low pressure system over
the coastal waters will lead to generally light winds and seas
through the period. Winds are expected to be strongest in the
northern San Francisco Bay during the afternoon. Also added a
Small Craft Advisory for San Francisco Bay north of the Bay Bridge
for Tuesday afternoon and evening.
Public forecast: blier/rgass
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