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

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FXUS66 KMTR 061217

Area Forecast Discussion
National Weather Service San Francisco Bay Area
417 AM PST Sat Mar 6 2021

.SYNOPSIS...Isolated showers may linger over the southern portion
of the region through about sunrise. Otherwise, expect dry and
seasonably cool conditions today along with locally breezy onshore
winds in the afternoon. Similar conditions are expected Sunday,
except with lighter winds. A cool weather system dropping south
offshore early in the work week will produce cool and showery
weather from Monday through about midweek, along with low snow
levels and the potential for thunderstorms on Tuesday and
Wednesday. Drier and warmer conditions are likely later in the


.DISCUSSION...As of 3:45 AM PST Saturday...The cold front that
brought brief heavy rain and strong, gusty winds to the Bay Area
late last evening has moved quickly to the east and has mostly
exited the area. Although rain rates were briefly heavy, rain
duration was brief and so rain totals were mostly less than a half
inch. The Napa Airport recorded a peak wind gust of 54 mph late
last evening, and San Francisco Airport wasn`t far behind with a
gust of 47 mph. Several locations in the Bay Area Hills reported
gusts between 50 and 60 mph as the front swept through.

The front rapidly weakened as it moved quickly to the east after
midnight. Current radar shows only a few lines of light showers
across San Benito County, with only isolated light showers in the
post-frontal environment. Models agree that a few showers may
linger until about sunrise, but dry weather should prevail in all
areas for the remainder of the day. Winds have subsided behind the
front but are still gusting between 30 and 40 mph locally in the
hills. Winds are forecast to continue decreasing through the
morning hours, but then increase in the afternoon when west to
northwest winds are expected to gust high as 30 mph near the coast
and in the hills. The airmass behind the front is quite cool and
afternoon highs are forecast to range from the mid 50s to lower
60s and be about five degrees below normal. Dry and seasonably
cool conditions are expected to continue into Sunday, but winds
will be lighter.

Early morning satellite imagery shows a cold upper trough dropping
south from the Gulf of Alaska. This system is forecast to dig
slowly south over the next few days and begin to impact our
weather early in the work week. Models agree reasonably well in
forecasting the trough to draw close enough to the CA coast by
Monday for shower activity to begin to spread into our area,
mainly into the North Bay. By Tuesday, as a cold upper low
approaches the coastal waters near the CA/OR border, showers are
forecast to become more widespread across our area. The airmass
will become increasingly unstable and isolated thunderstorms are
possible from midday Tuesday through Tuesday evening. Scattered
showers and possible thunderstorms will continue into Wednesday as
the upper low continues to track south just offshore. By
Wednesday the best chance of showers and thunderstorms will be
across the southern half of our forecast area. Rainfall totals
from Monday through Wednesday evening are forecast to range from
0.25-0.75" except locally up to an inch or more in coastal areas
and as little as a tenth of an inch in some interior valleys. In
the hills, rain totals are forecast to range from 0.50-1.50".
Brief heavy rain is likely with heavier showers and thunderstorms,
as is small hail. Due to the showery nature of the expected
precipitation, prolonged heavy rain rates are not expected. Snow
levels are generally forecast to range from 3000 to 3500 feet
during the first half of next week, but may drop as low as 2500
feet at times. Thus, the higher peaks will likely see some snow
accumulation. Winds are not expected to be a significant factor
with this system, although brief gusty winds are certainly
possible near heavier showers and isolated thunderstorms.

The models diverge after midweek. The 00Z GFS forecasts the upper
low to shift rapidly inland by Thursday and a ridge to build over
CA by Friday. Thus, the GFS solution would result in a warming
and drying trend late in the week. Most GFS ensemble members are
also dry in the Thursday-Friday time frame. The 00Z ECMWF
forecasts the upper low to linger over south-central California
through Friday, keeping temps from warming much, and also
maintaining precipitation chances, especially across the southern
part of our area. Several ECMWF ensemble members also maintain
precipitation chances through the end of the work week.
Consequently, the forecast beyond the middle of next week is low

&& of 4:17 AM PST Saturday...For the 12Z TAFs. Mixed
MVFR/VFR across terminals with lingering low cigs in the wake of
the cold front that passed through the region earlier in the
evening. Radar imagery shows some scattered echoes in South Bay,
but the steady rain has moved off to the south and east. Areas of
patchy fog may develop through the early morning. Cigs should
gradually lift this morning to VFR, with mostly clear skies for
the afternoon. Winds have largely settled down, with a few
terminals seeing breezy winds out of the west to northwest. Some
breezy WNW winds expected this afternoon, particularly near the

Vicinity of KSFO...VFR with tempo MVFR cigs possible through the
early morning, and sct low and mid-level clouds. Winds are vrb W
to NW, near 10 kt, though may be breezier at times. Trend is to
lighter winds through the morning. Clearing skies for the
afternoon with moderately breezy WNW winds. VFR through Saturday

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Mix of MVFR/VFR through the early morning
with ovc020 reported at KMRY. Showers have moved to the east.
Cigs lift and scatter bringing VFR conditions from mid-morning
through the day. Some low clouds may return Saturday evening but
forecast is VFR.

&& of 02:39 AM PST Saturday...A cold front has passed
through the area, leaving breezy northwest winds in its wake.
Winds will be moderately breezy throughout the day with gusty
conditions along the Big Sur coast, south of Point Sur. These
moderate to strong winds will last into Sunday. A moderate
northwest swell will subside through Sunday then increase Monday
into Tuesday. A low pressure system approaches from the northwest
with rain Monday through at least the middle part of next week.


     .Tday...SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Pigeon Pt to Pt Pinos 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
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Mry Bay from 12 PM
             SCA...SF Bay from 2 PM



MARINE: Lorber

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