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

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FXUS66 KMTR 210551

Area Forecast Discussion
National Weather Service San Francisco Bay Area
951 PM PST Mon Feb 20 2017

.SYNOPSIS...The cold frontal rain band is now moving southeastward
through Monterey County, with locally gusty winds and scattered
showers left in its wake. Flooding remains a concern as water
continues to flow down the slopes from the recent heavy rains, but
will diminish later tonight and Tuesday aside from some of the
slower to respond mainstem rivers. Showers and winds will
gradually taper off over the next 24 hours, with mainly dry though
cooler conditions expected for Wednesday through Friday. More
active weather looks to return to our area next weekend however,
with a cold and wet storm system potentially moving in.

&& of 09:35 PM PST Monday...The strong storm system
and Atmospheric River event that battered our area over the past
24 hours is now finally weakening and moving out. Rainfall totals
generally ranged in the 1.25 to 2.5 inch range at lower elevations,
and to over a half a foot at several locations up in the coastal
hills. Our site in Downtown San Francisco received 1.86 inches of
rain, with the wet spot thus far being Mining Ridge in the mountains
above Big Sur, with 7.24 inches as of 8:30 pm. Will also note that
Mining Ridge had the maximum 7-day rainfall total for our area,
14.91 inches.

Winds also got quite strong, especially along the coast and up
in the hills. At the Los Gatos RAWS site at 1842 ft up in the
Santa Cruz Mountains for example, noted a maximum sustained wind
of 46 mph with a peak gust to 77 mph, both reported at 5:32 pm.

Current radar and satellite data show the primary cold frontal
rainband extending northeast to southwest through southern Santa
Clara County and northern Monterey County, and weakening as it
progresses inland. Aside from these two counties and San Benito
County, significant rainfall has now pretty much ended, but
scattered showers will continue tonight and Tuesday in the cool
and moist air mass left in its wake.

No lightning reports were noted today over either our land or
water areas. Will nonetheless retain slight chance of thunderstorms
in the forecast for tonight and Tuesday. Have noted some lightning
detection in showers well offshore (and a lightning strike just
now off the Mendocino County coast) and latest model and MOS
output continue to indicate marginally favorable convective
conditions, especially more northern portions of our area Tuesday

For Wednesday through Friday, generally dry weather should
prevail, though new 00Z NAM in particular does now indicate a very
weak disturbance sliding south down our coast on Thursday along
with the possibility of a few associated showers. Temperatures
will be significantly cooler however for the latter part of the
week. The 850 mb temperature in the 4 pm Monday KOAK sounding was
6.15 deg C, but with both the 00Z NAM and GFS bringing it down to
around -2 deg C by 12Z Wednesday and -3 to -4 deg C by 12Z
Thursday. Daytime temps up at higher elevations especially will be
chilly, with night and early morning frost becoming possible in
the valleys.

Then further into the extended portion of the forecast, as we
head into next weekend, inclement weather conditions look to
return. Latest runs of the GFS (00Z Tuesday) and ECMWF and
Canadian models (12Z Monday) are in good agreement in bringing in
a cool and wet weather system to our district. Although quite a
ways out, and thus significant changes in solution could well
occur, present model output indicates potential entrainment of
subtropical moisture as the cold upper low drops southward
towards us from the Pacific NW.


.AVIATION...As of 4:15 PM PST Monday...Frontal system currently
moving through the SFO Bay Area and extends off the coast of MRY
Bay. Surface low behind the front will lift northeast into
northern California tonight. Rain will turn to showers tonight and
continue through Tuesday. Southerly winds have been gusty in
certain areas but have been less than expected in others. This
makes for a tricky wind forecast for tonight. Still expecting
gusty winds up to 40 kt as the surface low will be at its closest
to the area. Winds aloft are strong with sw winds up to 60 kt
at around 2000 feet. Therefore low level wind shear remains in
the tafs.

Vicinity of KSFO...MVFR for cigs and/or vsbys. Rain turning to
showers tonight with continued wet runways. Southeast winds
switching to southwest and increasing rapidly after 01-02Z to
25-30 kt gusting to 40 kt. Winds decreasing after 12Z.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...MVFR with rain turning to showers
tonight. Southeast winds gusting to 30 kt decreasing slowly after

&& of 09:04 PM PST Monday...A 999 mb low centered 200
miles west of Cape Mendocino is moving northeast. This will bring
gale force winds to much of the coastal waters through Tuesday
morning. There is a slight chance for thunderstorms through
Tuesday. High pressure will build over the Eastern Pacific
Wednesday through Friday bringing light to moderate northwest


     .Tngt...High Wind Warning...Higher terrain Bay Area to Monterey
 and San Benito counties.
             Flood Watch...Entire forecast area.
             Wind Advisory...Entire forecast area.
             GLW...Pt Reyes to Pigeon Pt 0-10 nm
             GLW...Pigeon Pt to Pt Pinos 0-10 nm
             GLW...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm from 1 AM
             GLW...Pigeon Pt to Pt Piedras Blancas 10-60 nm until 1 AM
             SCA...Rough Bar Advisory for SF Bar
             SCA...Mry Bay from 10 PM
             GLW...Mry Bay until 10 PM
             SCA...SF Bay from 11 PM
             GLW...SF Bay until 11 PM




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