NWS Forecast Area Discussion - San Francisco Area

FXUS66 KMTR 251135

Area Forecast Discussion
National Weather Service San Francisco Bay Area
435 AM PDT Mon Mar 25 2019

.SYNOPSIS...An unsettled weather pattern is expected to persist
through much of the week, with periods of rain forecast from today
through Thursday. Dry weather will likely return by Friday and


.DISCUSSION...As of 2:55 AM PDT Monday...Rain has been slow to
develop early this morning as an incoming frontal boundary
gradually approaches the coast. Latest radar indicates widespread
rainfall should begin to develop in the North Bay within the next
few hours. This is consistent with the latest HRRR which spreads
rain across the North Bay between about 4 am and late morning. Wet
roadways will likely be an issue during the morning commute
across much of the North Bay, but not so in San Francisco or
points south and east of the City. Rainfall is expected to begin
in San Francisco around midday and gradually spread south and east
across the remainder of the SF Bay Area during the afternoon, and
into the Monterey Bay Area this evening. Models indicate the
incoming frontal boundary will produce moderate rain rates in the
North Bay this morning, and also perhaps as far south as San
Francisco and Oakland by early afternoon. But the front is
expected to rapidly weaken as it pushes south of the Golden Gate
and only light rainfall is forecast for the remainder of the
afternoon and evening for areas farther to the south. Rainfall
totals in the North Bay through midnight tonight are forecast to
range from 0.5-1.5". A half inch or less is forecast to accumulate
around San Francisco Bay and in the Santa Cruz Mountains and East
Bay Hills, and less than a quarter inch is expected elsewhere.
Gusty southerly winds are expected to occur just ahead of the
front from mid morning through early afternoon with local gusts up
to 40 mph possible near the ocean and in the hills.

What`s left of the frontal boundary is forecast to stall out along
a line from the Santa Clara Valley southwest to Monterey
tonight. Most shower activity tonight and into Tuesday will
likely be in the vicinity of the stalled front. Both the NAM and
GFS forecast some enhancement along the frontal boundary late
tonight and Tuesday morning as the upper trough offshore begins to
catch up to the surface front. However, the ECMWF completely
washes out the front and forecasts almost no precipitation in our
area from late tonight through Tuesday.

A large upper low currently centered about 800 miles west of the
CA/OR border is forecast to move very slowly to the east over the
next 3 days. As is draws nearer to the coast, a robust shortwave
trough rotating around the low will approach northern California
Tuesday night and produce widespread precipitation across our
area late Tuesday night and Wednesday morning, with scattered
showers continuing into Wednesday afternoon. The airmass over our
area will become increasingly unstable as the low draws nearer and
we may see isolated thunderstorms in our area on Wednesday. Gusty
south to southwest winds are expected late Tuesday night and
Wednesday, with local gusts up to 35 mph possible in coastal and
higher elevation locations.

Shower chances will continue Wednesday night into Thursday,
mainly across the northern portion of our area, as the upper low
finally approaches the Oregon Coast.

Additional rainfall from Tuesday through Thursday is forecast to
range from a half inch to an inch and a half.

Dry weather will likely develop across our entire area by Friday
and persist into at least the first half of next weekend as a
shortwave ridge develops over California. The GFS develops rain in
the North Bay on Sunday as a shortwave trough ripples across far
northern California, but both the ECMWF and Canadian maintain dry
conditions across all areas for the entire weekend.


.AVIATION...As of 04:34 AM PDT Monday...For 12z tafs. VFR
conditions this morning before cigs lower to MVFR into the
afternoon. Cigs then forecast to remain MVFR through much of
tonight. Latest models show showers arriving to the Bay Area
between around 19z-21z. The front is forecast to quickly weaken
once it reaches the Bay Area this afternoon producing lighter
precip into this evening. Shower chances will diminish overnight
with precip lingering a little longer around the Monterey Bay
region. Southerly winds this morning will veer westerly this
afternoon. Southerly winds may become gusty late this morning into
this afternoon around 20-25 kt.

Vicinity of KSFO...VFR into the afternoon with southerly winds.
Gradually lowering cigs becoming MVFR by the afternoon along with
increasing shower chances. Wind gusts 20-25 kt late this morning
before winds gradually shift out of the W/SW this afternoon.
Shower chances will diminish into this evening as the front moves

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR with increasing SE winds down the
Salinas Valley this morning and afternoon. Showers this afternoon
and overnight with lowering cigs. Possible stalling of front
around the Central Coast tonight into tomorrow morning keeping
shower chances in place.


.MARINE...As of 2:11 AM PDT Monday...Southerly winds and rain
will continue through the morning hours ahead of a low pressure
system and cold front. Gusty winds can be expected ahead of the
front. Winds will then gradually become westerly in the wake of
the frontal passage. After a brief break, expect winds to increase
again Tuesday night. Moderate west to northwest swell will
continue through the week.


     .Tday...SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Pigeon Pt to Pt Pinos 0-10 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm until 3 PM
             SCA...Pt Arena to Pigeon Pt 10-60 nm until 3 PM
             SCA...Mry Bay from 9 AM until 4 PM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm from 9 AM until 4 PM
             SCA...SF Bay from 9 AM




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NWS MTR Office Area Forecast Discussion