Frost Advisory 3, NWS San Francisco Bay Area, CA

Area Forecast Discussion
National Weather Service San Francisco CA
931 AM PST Sun Jan 12 2025

...New AVIATION...

.SYNOPSIS...
Issued at 1209 AM PST Sun Jan 12 2025

Continued dry weather this week with chilly mornings and mostly clear
skies.

&&

.UPDATE...
Issued at 853 AM PST Sun Jan 12 2025

The Frost Advisory has been allowed to expire and the Wind Advisory
was allowed to be cancelled an hour early as winds outside of Mt.
St. Helena have diminished below Wind Advisory criteria. The targets
of opportunity for the upcoming forecast will be minimum
temperatures and the potential for Frost Advisories as well as wind
and the potential for Wind Advisories. Outside of that, it'll be
another mild and dry day.

Sarment

&&

.SHORT TERM...
(Today and tonight)
Issued at 1209 AM PST Sun Jan 12 2025

The atmosphere is very dry. The 00Z sounding found a 0.15" PWAT.
That's in the bottom 0.2% across 56,000 weather balloon launches
going back to 1948. While there's no direct reason to discuss the
LCL on this cloudless night, the high temperature and low dew
point are demonstrated by the 3353m surface based LCL, which is
89m higher the previous record for the month of January
(1/20/1963). With this exceptionally dry air mass, there isn't a
cloud in the sky anywhere over California. The dry air and clear
skies will allow much more efficient radiative heat loss
overnight, and temperatures are generally dropping much faster
than last night. The temperatures are a bit of a mixed bag
however, with some downsloping flow keeping some areas warmer (and
drier) for longer. A good example is Napa, where it's 54 degrees
with a dewpoint of 19F, compared to Santa Rosa, where it's 39
degrees with a dewpoint of 37F. I reverted the temperature
forecast back to the standard NBM, but since it's so sensitive to
wind direction, the uncertainty is high even for the minimum
temperatures this morning. Nevertheless, it will be a chilly start
for most. Winds are still strong across higher elevations,
especially in the North Bay. We are continuing to rely heavily on
the local WRF winds in the official forecast, which suggests the
10 AM expiration time on the Wind Advisory is appropriate for all
but the North Bay Mountains, where winds will likely pick up to
advisory level Sunday evening, and persist through the day Monday.
Otherwise, Sunday will be another very nice day with comfortable
afternoon temperatures generally in the 60s, and plenty of
sunshine.

&&

.LONG TERM...
(Monday through Saturday)
Issued at 1209 AM PST Sun Jan 12 2025

On Monday, a fast moving short wave trough will move from N to S
across California, eventually becoming cut-off west of Southern
California. This will transform the pattern from an Omega block
to a Rex block (high pressure north of low pressure), which will
be with us through the majority of the week. While there is some
discrepancy in the ensemble clusters, the most likely outcome is
that the low will eventually dissipate or perhaps hitch a ride to
the East, as the ridge retreats back to the Eastern Pacific. In
other words a return to a very stable Omega blocking pattern by
next weekend. All this means that there is still no rain in the
official forecast through the 18th and the global ensembles show
this dry period has a 90% chance to extend through the 25th and
likely beyond. The pattern this week will also support some
offshore winds, though the strength should decrease substantially
by mid-week. With enhanced radiational cooling, morning
temperatures will be chilly. The 850 mb temp is hovering between
the 50th and 75th percentile, so we are not expecting wide-spread
freezing temperatures, but most inland areas will drop into the
30s before the sun comes up.

&&

.AVIATION...
(18Z TAFS)
Issued at 931 AM PST Sun Jan 12 2025

VFR through the TAF period. Northerly winds will increase this
afternoon and continue into the early evening across across most Bay
Area terminals. Winds ease into the evening to become
light/variable. Southeast drainage winds look to affect the Monterey
Bay terminals into early Monday with offshore flow returning to the
region by late Monday morning.

Vicinity of SFO...VFR through the TAF period. Light/variable winds
through late morning then will become more northwesterly by this
afternoon. Expect light/variable winds tonight, but winds look to
turn northeast and become moderate to breezy into early Monday.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR through the TAF period. Southeast winds
last through the morning before becoming light and variable at MRY
while SNS sees moderate northwest winds into the afternoon. Winds at
SNS reduce into the night but southeast drainage winds build over
both terminals early Monday morning.

&&

.MARINE...
(Today through Friday)
Issued at 853 AM PST Sun Jan 12 2025

Strong winds continue across portions of the northern outer
waters with gale force gusts. Breezy to gusty winds continue for
the waters through the day, but look to ease into the work week.
Hazardous conditions for small crafts will persist through Monday
with elevated wave heights between 10 to 15 feet expected.
Significant wave heights will remain elevated through the early
work week but will subside below small craft criteria by mid
week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Coastal Flood Advisory until noon PST today for CAZ006-506-508.

PZ...Small Craft Advisory from 3 AM to 3 PM PST Monday for Pt Reyes
     to Pigeon Pt 0-10 nm-SF Bay N of Bay Bridge.

     Small Craft Advisory until 3 PM PST Monday for Pt Arena to Pt
     Reyes 0-10 nm.

     Gale Warning until 9 AM PST Monday for Pt Arena to Pt Reyes 10-
     60 NM.

     Small Craft Advisory until 9 AM PST Monday for Pigeon Pt to Pt
     Pinos 10-60 NM.

&&

SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...RGass
MARINE...RGass

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