Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
1107 PM PDT Monday Mar 30 2015
Synopsis...a mainly dry cold front will sweep into the area
tonight into Tuesday bringing much cooler conditions through
Wednesday...along with gusty winds. Temperatures will then warm
slightly by the end of the week. Dry weather is forecast at least
through Saturday. Rain may develop by late in the weekend into
Discussion...as of 8:54 PM PDT Monday...cooling trend has already
begun ahead of the incoming cold front as onshore winds have
already increased. The sfo-SAC onshore gradient is just about 4 mb
with the northerly gradient having collapsed. The strong northwest flow
over the ocean the last few days allowed SST to drop from 52-53
degrees at the Bodega Bay and San Francisco buoys. This cool air
is now rushing inland with gusts to around 30 miles per hour last hour at
ksfo and Travis AFB out in the Delta...a classic marine push that
for whatever reason it seems we haven't seen much of lately.
There's only one day left in March and its been another abysmally
dry month across the region. Downtown San Francisco and the
usually wet Sonoma County Airport have reported only 0.12 this
month with a paltry 0.06 for San Francisco Airport.
Anyway we have a well defined cold front approaching the region
tonight but unfortunately little or no precipitation will be associated
with the front. Still think we may squeeze out some coastal
drizzle and the 00z NAM does generate some very light showers over
the Big Sur hills on Tuesday morning...which seems plausible given
the strong onshore flow and cold advection into the coastal hills.
In addition to the dramatic cooling that all areas will feel on
Tuesday believe the big weather story will be the gusty northwest
winds. Conditions looks to remain below advisory but blustery
winds off the ocean and in the hills will make for a chilly day to
end the month with cool afternoon highs in the 60s across most of
the district. Some concern for impacts with many drought stressed
trees and limbs perilously positioned around power lines across
the region. We haven't had many strong cold advection fronts in
the last few Winters so will not be surprised to hear of some wind
impacts along the coast and some of the more remote hillside
communities in the North Bay and Santa Cruz Mountains.
Winds will turn more northerly by Tuesday night as the front moves
into the Great Basin and high pressure noses into the region. Wednesday
will be another seasonably cool day with winds lighter but still
noticeable with 20 knots winds still shown by the models in the hills.
Another upper trough will dive into the Pacific northwest and
eventually the Great Basin later Wednesday night into Thursday. This
will drive a final round of north winds Thursday morning through
midday with a drier airmass filtering over the region. April is
often a windy month as cool late season troughs drop over the
state and now we've got somewhat cooler SST in place to drive
some boundary layer turbulent kinetic energy. Compared to our
recent weather it looks like three fairly windy days in store with a
Load of drought and diseased stressed trees that will be tested.
By Thursday afternoon the dry northerly winds will ease and
temperatures will begin to rebound back to near or above normal
with sunny and dry weather forecast for Friday.
Saturday looks seasonably cool with cyclonic flow aloft ahead of
the next large scale trough. Long range forecast is still very
interesting with the Euro trending wetter than the GFS and the
Gem in between. General idea seems to bring a cold trough over the
state by Sunday with mainly a cool and showery regime. Then a more
potent shortwave will drop into the main trough with the location
of main cyclogenesis in question. Potential is there to spin up a
storm by early next week. No new 00z long range guidance in yet.
Aviation...as of 10:45 PM PDT Monday...a dry cold front over the
eastern Pacific continues to move southeast toward northern California. The cold
front and its accompanying surface trough will move southeast reaching the
Bay area Tuesday morning and continue to move southeast over the central
coast Tuesday afternoon. A few areas of gusty winds have persisted
this evening from Marin County to northern San Mateo County including
from ksfo to the San Bruno gap. The area terminals continue to report
VFR this evening. Some stratus and/or strato-cumulus cloud development
is likely overnight especially right along the immediate coastline with
minor inland cloud intrusion possible Tuesday morning. It's difficult to say
how much areal coverage there'll be over inland areas Tuesday morning since
there's been plenty of mixing so far this evening and low clouds have
been mostly absent...but the thought is that it should be limited.
Overall it's a moderate to high confidence VFR forecast for the
Bay area terminals...and low to moderate confidence VFR/MVFR
forecast for the Monterey Bay area terminals.
Vicinity of ksfo...westerly winds continue to gust to 25-30 knots
late this evening. As the dry cold front/trough arrive very late
tonight winds might settle back a little but probably not much...have
kept west winds with gusts to about 20 knots. Model guidance indicates
a return to similarly gusty westerly wind gusts like we had today once
again Tuesday afternoon and evening.
Sfo bridge approach...similar to ksfo.
Monterey Bay area terminals...areas of MVFR ceilings are likely very late
tonight. VFR Tuesday.
Marine...as of 08:15 PM PDT Monday...moderate to locally strong
northerly winds will continue along the central coast as a cold
front pushes through the region and high pressure builds behind
the front. Winds will be strongest south of Point Sur. A mixed
swell will also continue with a long period southerly swell.
... Arena to pt Reyes 0-10 nm
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm
Small Craft Advisory...Pigeon pt to pt pinos 0-10 nm
Small Craft Advisory...pt Arena to pt Reyes 10-60 nm
Small Craft Advisory...pt Reyes to Pigeon pt 10-60 nm
Small Craft Advisory...Pigeon pt to pt pinos 10-60 nm
Small Craft Advisory...pt pinos to pt Piedras Blancas 10-60 nm
glw...pt pinos to pt Piedras Blancas 0-10 nm
Small Craft Advisory...Monterey Bay until 3 am
Public forecast: rww
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