Freeze Warning 3, NWS San Francisco Bay Area, CA

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1007 AM PST Sat Jan 28 2023

.SYNOPSIS...Cooling trend continues into the weekend, with well
below normal temperatures then persisting through the first part
of next week. An upper level disturbance will bring some showers
Sunday and Sunday evening, especially to the southern half of our
area. Snow levels will be low, locally down to 2500 ft or even a
bit lower. In the colder air mass behind it, low temperatures
Monday and Tuesday mornings look to be very chilly. Widespread
freezes are then likely away from the coast, potentially even
touching some coastal areas.

&&

...New Forecast Discussion Update...

.DISCUSSION...as of 08:56 AM PST Saturday...

With daytime mixing fog should mix out shortly. However extended
the Dense Fog Advisory until 10 AM to allow time for this to
occur. No other changes this morning.

&&

.PREV DISCUSSION...as of 06:22 AM PST Saturday...

Given quickly falling visibility the Dense Fog Advisory was
expanded to include the Bay Shoreline and San Francisco. Expect
sudden visibility changes this morning so travel carefully. No
other changes to the forecast this morning.

&&

.PREV DISCUSSION...as of 03:50 AM PST Saturday...Another night of
relatively light winds, but satellite fog product reveals a slight
summer vibe with patchy stratus and fog. Stratus is covering the
Santa Clara Valley, Salinas Valley, and portions of SF Bay. Patchy
dense fog is being reported at both Sonoma County Airport and
Napa Airport. Even with some patchy stratus being reported it is
another chilly to cold night with lows bottoming out in the mid
30s interior and 40s closer to the coast and bays.

For the rest of the weekend: Morning fog/stratus will slowly
dissipate through the morning. Mostly sunny this afternoon with a
few high clouds ahead of the next storm system. Temperatures will
be slightly cooler than Friday, but still pleasant and in the 50s
to lower 60s today. Change in the longwave pattern is still on
track for Sunday. As a highly amplified ridge nudges into AK an
upper level trough to the east deepens allowing a disturbance to
move southward to CA. This upper level feature will ultimately
form a cut-off low over central CA Sunday afternoon before
drifting SW. At the surface a weak cold front will push through
from the north Sunday afternoon. The fropa will bring a chance
for precip to the Central Coast Sunday afternoon and Sunday night.
There still remains some uncertainty on where precip will fall,
but for now precip chances are mainly from South Bay southward.
Given the lack of moisture tap rainfall amounts Sunday into Monday
will be limited and generally a few hundredths to one quarter of
an inch. The passing front will usher in some rather cold air
making it feel blustery on Sunday. Snow levels will plummet
2,500-3,000 feet Sunday. Not a big snow event, but a coating to a
few inches are possible over the higher terrain. As noted before,
there is some weak instability on Sunday allowing for marginal
convection. The more likely scenario is no lightning, but
stronger cells producing small hail. Speaking of blustery
conditions, latest hi-res models and Grand Ensemble guidance
suggest a pretty good push of wind with and behind the front. The
winds will get some help from the cold air advection too.
Therefore, in coordination with neighboring offices, a Wind
Advisory was issued for interior North Bay Sunday afternoon
through Monday. Wind gusts of 40-50 mph will be possible with the
stand out locations like Mt St Helena gusting 70 mph. Despite
some wind Sunday night the cold airmass behind the front will set
the stage for a cold night with lows in the upper 20s and 30s. A
Freeze Watch is still in effect Sunday night for the entire
forecast area. As the forecast gets fine tuned this watch may be
converted to a Freeze Warning on the next forecast update.

Early next week: The bigger story early next week will be the cold
weather. Several nights of near freezing to sub-freezing
temperatures are possible. The climate site in San Francisco may
actually record the first temperature in the 30s for the season.
Interior locations have a high likelihood of seeing sub-freezing
temps with frost. Needless to say, impacts will be to those
without adequate shelter. Would expect a few nights with Freeze
Warning in our future.

Long range forecast keep rain returning around Feb 02/03. Not a
blockbuster AR set up, but additional rain nonetheless.

&&

.AVIATION...as of 10 AM PST Saturday...For the 18Z TAFs. Dense FG
persisting around the Bay Area this morning, showing only slight
improvements over the last hour. Expecting much more improvement
by 19Z when mostly clear skies are expected to persist into this
afternoon. Deep upper low dropping into the area this afternoon
expected to bring breezy N/NW winds and more uniform cloud cover.
Expect at least MVFR cigs later this evening and more widespread
by Sunday morning at most terminals. Scattered showers enter the
area by Sunday morning as well.

Vicinity of KSFO...Dense FG persisting at the terminal and
surrounding area this morning. RVR showing slight improvement as
of writing. Expect improvements in vis to continue through to the
19Z hour, when mostly clear skies are expected through the rest of
today. Cloud cover increasing overnight with MVFR cigs likely by
sunrise Sunday morning. Chance of scattered showers enters by
Sunday morning with -DZ expected for an extended period through
the middle of the day Sunday.

KSFO Bridge Approach...Similar to SFO.

Monterey Bay...Mostly clear skies through the day today with cloud
cover increasing this afternoon due to an approaching upper low.
Moderate confidence in MVFR cigs late this afternoon into the
evening, persisting through Sunday morning with a chance of
showers Sunday morning.

&&

.MARINE...as of 02:30 AM PST Saturday...Breezy winds have
generated steep wind waves north of the Monterey Bay. While
northerly winds across the waters have diminished slightly
overnight, winds are expected to remain breezy through the
weekend. A burst of stronger wind gusts will arrive this evening
and tonight as a cold front is expected to pass through. Light
rain will be associated with this frontal passage.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...Dense Fog Advisory...CAZ006-506-508
             Marine Fog Advisory...SF Bay until 10 AM
             SCA...Mry Bay from 3 PM
             SCA...Pigeon Pt to Pt Pinos 0-10 nm from 3 PM
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm from 3 PM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm until 3 PM
             GLW...Pt Arena to Pigeon Pt 10-60 nm from 3 PM
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm

&&

PUBLIC FORECAST: MM/SPM
AVIATION: Behringer
MARINE: McCorkle

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