Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
909 PM PST sun Dec 21 2014
Synopsis...high pressure is now building over the eastern Pacific
while a deep...mild and moist air mass covers the district. Aside
from some local night and morning drizzle...dry and mild conditions
are expected through at least the first part of the week. Patches
of dense fog are a concern for the rest of the night and into
Monday morning however. Also...higher than average tides will
result in possible minor coastal flooding over the next couple of
days. A tail end of a storm system brushing through from the
northwest will bring a chance of rain around midweek...and usher
in a cooler air mass.
Discussion...as of 8:35 PM PST Sunday...temperatures remain
remarkably mild across the district for the first evening of
winter. The 8 PM reading of 59 degree at our official site in
downtown San Francisco for example is a remarkable 3 degrees
above the normal high for the date. With an upper level ridge now
building strongly northward along the West Coast...all
precipitation has come to an end aside from some local drizzle.
Have just issued an update to the forecasts to remove any mention
of shower chances...but retained the possibility of drizzle and
extended that into Monday morning. Also...have noted that
visibilities are already locally getting pretty restricted. A
recent spotter report from Redwood Estates...at an elevation of
about 1500 feet along Highway 17...estimated visibility was down
to approx 300 feet. And the 04z metars from kwvi and ksts have it
at 1/4 mile and 3/4 mile respectively. In the update then also
added in patchy dense fog as a possibility across much of the area
for the remainder of the night and into Monday morning. In
addition...made some upward adjustments to forecast low
temperatures tonight and highs tomorrow based on latest model
trends and observed values today.
Will also note that just received new 00z output from the GFS has
ramped up a bit on both the strength of the Wednesday afternoon/
evening system and associated precipitation chances for at least the
northern half of our district. But especially as this is still
several days out...will hold off on making any adjustments to the
forecast until all new model data are in.
From previous discussion...a ridge of high pressure will build into
California over the next several days allowing for warming along
with dryer conditions and fewer clouds each day. Tuesday appears
to be the warmest of the week as 850 mb temperatures jump from by about
8c to the 15-16 range on Tuesday and the ridge axis GOES across.
With plenty of sunshine highs will move into the middle 60s to lower
Cooler weather will return along with a very slight chance for
showers Wednesday (christmas eve) as an inside slider skirts close
to tthe northern portion of the County Warning Area. Any rainfall should be very
light with the larger impact being temperatures cooling back
closer to normal. Overnight lows are also expected to drop due to
drier air. We will likely see 30s back in Interior Valley
Longer range guidance favors drier than normal conditions out to
January 4th. Currently no sign of any major storms impacting our
area the next two weeks.
Aviation...as of 9:07 PM PST Sunday...high pressure will build over
norcal during the period. Fog will form under increasing air mass
stability. Forecast includes IFR ceilings and visibilities. Low level
winds will become more northerly late tonight through Monday which
should help with clearing on Monday.
Vicinity of ksfo...MVFR 03z-09z followed by IFR tonight.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...MVFR and localized IFR through the evening
and overnight. Light winds.
Marine...as of 3:00 PM PST Sunday...northwest winds will
gradually increase over the southern and outer waters through
tonight and persist on Monday as high pressure builds off the
California coast. King tides will impact the coast and inland bays
through Tuesday. King tide impacts for seafaring vessels will
occur primarily during low tides...with the possibilities of boat
keels touching Harbor bottom...strong currents in Harbor
entrances...and additional breaking waves over offshore reefs.
... Flood Advisory...coastal and Bay Shore areas
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm
Small Craft Advisory...pt pinos to pt Piedras Blancas 10-60 nm
Small Craft Advisory...Pigeon pt to pt pinos 10-60 nm from 3 am
Small Craft Advisory...rough bar advisory for sf bar
Public forecast: blier/Bell
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