Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
434 am PDT Friday Oct 24 2014
Synopsis...mostly sunny conditions are expected today...aside
from some morning fog and low cloudiness near the coast and in the
North Bay valleys. However...increasing rain chances will then
rapidly spread in from the northwest later tonight and Saturday
morning as a stronger and wetter Pacific weather system moves
into northern and central California. Residual showers are
expected to taper off Saturday evening...with dry weather returning
districtwide Sunday and continuing into at least the first half of
Discussion...as of 3:35 am PDT Friday...the weak frontal boundary
that draped down to near the northern border of our district is
now lifting northward as the next and somewhat stronger and
wetter Pacific weather system begins to approach. Even with a lot
of searching...largest rainfall total found for Thursday was 0.04
inches at a mesonet site at Sea Ranch...on the northern Sonoma
County coast. No measurable precipitation was reported at any of our ASOS
or RAWS sites.
For today...latest fog product satellite imagery shows low
cloudiness along virtually all of our coast and locally in the
North Bay valleys...and beginning to extend through the San Bruno
gap...but otherwise skies are mostly clear. Have noted that just
within the last hour the visibility reported by The Half Moon Bay
AWOS has dropped to under 1/4 mile and will monitor to see if
persists and spreads to the point that a short term forecast or
dense fog advisory is needed as dawn approaches. Aside from the
aforementioned morning low cloudiness and fog...mostly sunny
conditions are expected today with afternoon highs around the bays
generally in the lower 70s.
Tonight and Saturday...rain chances will rapidly develop from the
northwest during the overnight hours as a cold frontal rain band
ahead of the next and stronger Pacific low pressure system approaches.
General trend of recent model runs has been to speed things up a
bit. Consensus timing of latest output from the NAM...GFS and
European model (ecmwf) deterministic solutions has rain beginning in Santa Rosa
around a couple of hours before sunrise on Saturday...San
Francisco during the first part of the Monterey...and with a good
chance south to near Monterey by middle-to-late morning. And then
after a short break...another round of precipitation...this time more
showery in nature...is likely as the cool upper level trough axis
quickly follows through. Rainfall amounts through Saturday evening
could potentially be as much as 0.75 to 1.0 inches in wettest spots of
the coastal North Bay hills...but otherwise generally 0.25 inches
or less aside perhaps from a bit more at some locations in the
Santa Cruz Mountains and should note that some portions of interior
Monterey and San Benito counties may well not receive any rain at
As this will be the first significant precipitation event in some
time across much of the area...roadways will become especially
slick as accumulated surface oils mix in especially during the
first hour or so of precipitation. Also...have included mention of a
slight chance of thunderstorms over the North Bay on
Saturday...given substantial surface-based cape values evident in
model output soundings...especially those from the NAM. 500 mb
temperatures really don't get all that cold...only down into the -18c to
to -20c range even with the upper level trough passage. But
surface temperatures and especially dew points are projected to be high
enough to counterbalance that...the latter in particular
reflecting the unusually warm sea surface temperatures that continue in
our coastal waters. There may be a few residual showers Saturday
evening...but dry conditions are projected to return districtwide
by the late night hours.
Sunday through Wednesday...dry conditions are expected districtwide
aside from just a slight chance of some light precipitation in the North
Bay as midweek approaches and longer range model soultions then
begin to diverge a bit as to whether the precipitation band starts to
sag back down towards our northern boundary.
In the even longer term...have noted that the European model (ecmwf) in particular
has remained consistent over its last several runs in projecting
another round of wet weather for our area around next weekend.
Aviation...as of 4:30 am PDT Friday...stratus along the coast
remains disorganized with only patches getting in to the sfo Bay
area. The lack of an inversion layer should keep stratus patchy in
nature. Therefore any ceilings affecting the sfo approach will clear
out by 16z.
Vicinity of ksfo...patchy MVFR ceilings through 16z.
South winds 5-10 knots switching to southwest-west after 20z.
Ksfo bridge approach...similar to ksfo.
Monterey Bay area terminals...sattelite image shows stratus over
mry Bay making little inland progress. Still believe IFR ceilings will
reach sns and linger through 17z. IFR ceilings through 18z at mry.
Marine...as of 3:11 am PDT Friday...high pressure off the
Southern California coast will weaken as a deepening low pressure
system approaches from the west. The low will lift northeast on
Saturday moving inland over the Pacific northwest Saturday night.
South winds will increase today through Saturday then switch to
northwest Saturday night and Sunday. The low will bring a slight
chance of thunderstorms to the northern waters Saturday.
Public forecast: blier
aviation/marine: west pi
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