Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA

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FXUS66 KMTR 241136

Area Forecast Discussion
National Weather Service San Francisco Bay Area
336 AM PST Wed Jan 24 2018

.SYNOPSIS...Expect increasing rain chances from north to south
through the day as the next storm system impacts the area. Steady
rain quickly turns to showers for Thursday. A round of more
organized showers is possible Thursday afternoon as the upper low
moves inland. Dry weather forecast Friday through the weekend and
into early next week.

&& of 3:28 AM PST Wednesday...GOES-East infrared
imagery shows increasing clouds early this morning across the
North Bay in advance of an upcoming storm system. Regional radar
shows light to moderate rain presently falling in Humboldt County
and locations north. This rain should make it to the North Bay
around or after sunrise on Wednesday, with rain chances spreading
south through the day with the sfc cold front`s gradual
southeastward progression. Models remain consistent with storm
total rain amounts: 1-2" for the North Bay Hills/Mountains, 0.5-1"
for the North Bay Valleys, and around 0.33-0.67" in the San
Francisco Bay Area (with higher amounts possible in the Santa Cruz
Mountains and lesser amounts possible in the Santa Clara Valley).
Farther south, rain amounts will continue to decrease with
generally less than 0.50" expected in Monterey and San Benito
Counties, with the highest amounts along the coast near/around Big
Sur. In addition to the rain, breezy conditions are possible on
Wednesday with southerly wind gusts up to 25 to 35 mph ahead of
the front.

Behind the cold front, the northwest winds will aid in ushering in
cooler air aloft (ie cold air advection). The 00Z Wednesday (4 pm
PST Tuesday) Oakland sounding recorded an 850 mb temp of 9 deg C;
by 00Z Thursday (4 pm PST Wednesday) expect the 850 mb temps to
fall to around 5 deg C with additional cooling to below 0 deg C
Wednesday night and Thursday. A bit higher in the atmospheric
profile at 700 mb, temperatures presently around 0 deg C will
tumble to around -10 to -13 deg C by Thursday. This will result in
steep lapse rates that will provide increased convective
potential, particularly in the northern waters and the North Bay.
Think isolated thunder potential is justified on Thursday
afternoon, and matches with the Storm Prediction Center`s Day 2
convective outlook. With the freezing levels falling to
3,000-4,000 ft MSL, can`t rule out the potential for these
convective showers/storms to produce small hail and brief heavy

Beyond Thursday, forecast models are in good agreement with
building an upper level ridge off the Baja California coast. This
will boost 500 mb heights that will translate to dry weather and a
warming trend at the surface as rain chances are deflected to far
northern California, Oregon, and Washington State. The Canadian
model tries to bring in a weak frontal system as far south as the
Golden Gate by next week Monday or Tuesday, but the GFS and ECMWF
hold the ridge in place with dry conditions prevailing. In fact,
the GFS and ECMWF hold the ridge over the eastern Pacific through
the end of January and into the beginning of February.

&& of 3:36 AM PST Wednesday...For 12z tafs. Satellite
imagery shows mid to high level clouds streaming over the area.
Mixed bag of conditions ranging from VFR to LIFR. Patchy fog in
the North bay is expected to remain in place over the next few
hours or so. Cigs are expected to lower to MVFR as a cold front
approaches the area this morning. Precip is set to arrive to the
SF Bay Area by around 20-21Z. Increasing southerly winds as the
morning progresses with gusts at KSFO up to 25 kt after 21Z. Rain
will turn to showers in the wake of the front tonight and cigs
will rise.

Vicinity of KSFO...VFR through early this morning becoming MVFR
after 15-16Z and light rain after 20-21Z. Light south to
southeast increasing to 15-18 kt gusting to 25 kt after 20Z.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR through late this morning becoming
MVFR after 21Z. Showers to move into the area by around 00-01Z.

&& of 02:13 AM PST Wednesday...An approaching cold
front will result in increasing southerly winds today along with
chances of rain. Winds will turn westerly this afternoon in the
wake of the frontal passage. Instability behind the cold front
will bring a slight chance for thunderstorms across the coastal
waters tonight and Thursday.


     .Tday...SCA...Pigeon Pt to Pt Pinos 0-10 nm from 1 PM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm from 1 PM
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm from 2 PM
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...SF Bay from 9 AM
             SCA...Pt Reyes to Pigeon Pt 0-10 nm from 9 AM
             SCA...Mry Bay from 4 PM




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