NWS Forecast Area Discussion - San Francisco Area

FXUS66 KMTR 230353

Area Forecast Discussion
National Weather Service San Francisco Bay Area
853 PM PDT Thu Apr 22 2021

.SYNOPSIS...Mainly dry and seasonable weather conditions will
persist through Friday night with periods of night and morning low
clouds. Cooling will occur on Saturday as both onshore flow and
cloud cover increase region-wide ahead of an approaching weather
system. Widespread rainfall is likely late Saturday night and
Sunday as a cold front sweeps across the region. Showers will
taper off Sunday night, but a few showers may linger into Monday.
A warming and drying trend is then expected beyond Monday.


.DISCUSSION...As of 8:50 PM PDT Thursday...Persistent marine
layer clouds and moderate onshore flow resulted in a cool day
across our region. Highs remained in the 50s in most coastal
areas, while 60s and lower 70s were observed elsewhere. Highs
today were well below normal. For example, today`s high
temperatures of 52 in Downtown San Francisco and 60 at the Sonoma
County Airport were 11 and 10 degrees below normal respectively.

The marine layer remains deep this evening and evening satellite
imagery and surface observations indicate that low clouds have
already advanced well inland, all the way to Pacheco Pass in
southeast Santa Clara County and closing in on Livermore in the
Interior East Bay. Ensemble low cloud probability forecasts have
captured the localized clearing on the Sonoma County Coast and
indicate that area may remain clear overnight, while low clouds
will persist or expand elsewhere. A shortwave ridge, currently
centered a short distance offshore, is forecast to shift eastward
overnight and the likely result will be a compression of the
marine layer by Friday morning. Thus, the expectation is that
clearing on Friday will occur more quickly across inland areas
compared to today, and afternoon clearing will be more
widespread. Consequently, high temperatures should trend at least
modestly warmer for inland areas tomorrow. Coastal locations
likely won`t see much change.

Inland temperatures are then expected to reverse course on
Saturday and cool once again as both onshore flow and clouds
increase ahead of an approaching weather system.

From Previous Discussion...A cold front will then drop into the
North Bay predawn hours on Sunday before progressing
southward/inland through the day. This will provide generally
light to occasionally moderate rainfall along the frontal boundary
with the potential for lingering rain showers in wake of the
frontal passage. Additionally, cannot rule out an isolated
thunderstorm or two over the North Bay as instability increases as
the main mid/upper level low drops into northern California.
Latest ensembles continue to indicate rainfall totals from this
system to range from 0.50"-1.00" for the North Bay hills /Santa
Cruz Mountains, 0.25"-0.50" around SF Bay/Peninsula/North Bay
valleys and 0.10"-0.25" for the interior East/South Bay valleys.
Cooler temperatures are also likely region-wide on Sunday with
daytime highs only reaching into the 50s and colder air advects

Conditions will begin to dry out late Sunday night into Monday
morning along with cooler temperatures, generally in the 40s region-
wide. While a lingering shower or two may be possible through midday
Monday, most precipitation will have exited our region to the east.
A very gradual warming trend is expected through the remainder of
next week as high pressure develops over the region.


.AVIATION...As of 05:10 PM PDT Thursday.....For the 00Z TAFs.
Weather pattern features low pressure exiting the region and being
replaced by a highly amplified ridge. The marine layer deepened
significantly under the exiting trough and has brought MVFR to
coastally influenced TAF sites through the day. Latest satellite
imagery shows a healthy upstream feed with continued deep onshore
advection of the stratus through SSW to NNE coastal gaps. Coastal
surface eddies have also complicated the forecasting of the onshore
advection of the stratus and model data is no longer aligned with
observations thus generally low to medium confidence on marine
layer coverage and timings. That said, building high pressure will
compressed the marine layer tonight into tomorrow morning and
limit the coverage of the stratus deck while lowering ceilings
closer to the IFR level, especially near the coast. Rather breezy
onshore winds of 14 to 22kt are possible through the remainder of
the evening and will gradually taper off overnight. For tomorrow
morning, a compressed yet stubborn marine layer will be present
once again along the coast and through coastal gaps. Continued
onshore flow through the day with advancing high clouds ahead of
the next storm system arriving late Saturday night.

Vicinity of KSFO...MVFR with rather breezy onshore winds this
evening. Satellite trends have been to shift the thick SF
peninsula feed southward over the terminal, thus expecting BKN-OVC
MVFR cigs to persist through the remainder of the day before
lowering down to borderline MVFR/IFR level overnight due to the
influence of the ridge. Patchy cigs will linger through midday
tomorrow as HREF ensemble suggests KSFO will once again be on the
edge of a stratus feed tomorrow.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...MVFR OVC cigs with a very robust onshore
feed through the remainder of the day. Satellite imagery does show
some thinning out of the lower levels of the marine stratus deck
early this evening but the mid/top level stratus deck continues
to push onshore unimpeded. For tonight, the influence of the ridge
will compress the marine layer and force ceilings down to the IFR
range through the middle of the night. Steady onshore flow and
upstream stratus feed will bring MVFR cigs throughout the day
tomorrow, though could see some brief midday clearing.

&& of 8:28 PM PDT Thursday...Breezy to locally gusty
northwest winds will prevail in the waters north of Point Reyes
through this evening. Northwest winds will finally subside to
become light to locally breezy across the waters tonight into
Friday as high pressure moves ashore. By Saturday morning winds
are forecast to turn southerly ahead of an approaching storm
system and weaken further. Southerly winds will then become breezy
to briefly gusty as a cold front is expected to move across the
waters during Sunday morning. Northwest winds will return in the
wake of this cold front later Sunday into next week.


     .Tngt...SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...SF Bay until 9 PM




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NWS MTR Office Area Forecast Discussion