Cold and Frost and Freeze in California, February 23 to 25, 2021

Area Forecast Discussion
National Weather Service San Francisco Bay Area
958 PM PST Mon Feb 22 2021

.SYNOPSIS...High pressure over California combined with offshore
flow will mean a continuation of a warming trend through Tuesday.
Temperatures will then cool back close to seasonal averages during
the second half of the week as a series of systems drop inland to
our east. These systems will generate periods of gusty offshore
winds for the remainder of the week and into next weekend, mainly
in the hills. Strongest and most widespread offshore winds are
forecast to occur Wednesday and Wednesday night and again on
Saturday and Saturday night. During these times, gusty winds may
mixed down to lower elevations.

&&

.DISCUSSION...As of 9:15 PM PST Monday...A broad upper ridge,
extending from north of Hawaii eastward to California, combined
with offshore flow to produce sunny, dry and warm conditions
across our region today. Afternoon highs were mostly in the upper
60s and 70s, and were anywhere from 7-15 degrees warmer than
normal. No daily record highs were tied or set, but San Francisco
Airport came within one degree of a record.

Warmer temps today will mean overnight lows tonight won't be as
cool as last night. Still, mostly clear skies and a dry airmass
will mean some of the colder inland valleys will drop into the
upper 30s by daybreak Tuesday.

The primary forecast concern through the remainder of the week and
into the upcoming weekend is wind, specifically periods of gusty
offshore winds. Winds are currently light across our region, even
in the hills. However, offshore winds are forecast to increase in
the hills of the SF Bay Area later tonight as the first in a
series of shortwave troughs drop inland to our east. North to
northeast winds are forecast to increase in the hills of the SF
Bay Area after midnight and gusts of between 30-50 mph are likely
in the higher hills late tonight and Tuesday morning. Winds will
then subside in the hills Tuesday afternoon. This first inside
slider will be too far east to have a significant impact on winds
at lower elevations. Thus, for our valley, coastal, and urban
areas, expect another day of dry and warm conditions on Tuesday.
In fact, highs tomorrow are likely to be a few degrees warmer than
today given that the airmass aloft is forecast to warm slightly
and as offshore flow brings an even drier airmass into our region.
A few record highs are possible tomorrow. A forecast update was
completed earlier this evening to increase tomorrow's high
temperatures across portions of our area.

Winds will increase once again in the hills on Tuesday night as
the next shortwave trough begins to move south out of eastern
Oregon and Washington. That shortwave is forecast to dig more
directly to the south and drop into the Great Basin by Wednesday
afternoon. This track will weaken the upper ridge over our area
and result in slightly cooler temperatures on Wednesday. More
importantly, this system's more westerly trajectory will result in
even stronger offshore winds across our area by Wednesday
afternoon. And gusty winds by late Wednesday afternoon won't be
confined only to the hills. Cool advection aloft, and a well
mixed boundary layer during maximum daytime heating, will allow
gusty winds to mix down to near sea level Wednesday afternoon.
Thus, gusty offshore winds by late Wednesday will be more
widespread across the SF Bay Area, and extend farther south
locally into Monterey and San Benito Counties by Wednesday
evening. Most models increase the WMC-SFO pressure gradient to
about 12 mb by late Wednesday afternoon and to 13-14 mb by late
that evening. At lower elevations, winds will likely peak between
mid afternoon and early evening Wednesday when gusts of 25 to 35
mph are likely, with a few of the windier locations, such as
areas near the Delta, perhaps gusting 40-50 mph. In the hills,
winds will probably peak later in the evening when gusts of 50-60
mph are likely in the North and East Bay Hills. Wind Advisories
will almost certainly be issued for the North Bay Hills, East Bay
Hills, and the Santa Cruz Mountains from Wednesday afternoon
through Wednesday night. Wind advisories are also possible for the
East Bay Valleys, and potentially for the North Bay Valleys and
SF Bay Shoreline as well.

Winds are forecast to diminish on Thursday morning, but periods of
gusty offshore winds will likely continue through the end of the
week, particularly in the hills. One more system is then forecast
to drop in from the north by Saturday. The exact trajectory of
this system is in doubt due to model differences, but the general
idea is for it to track even farther to the west and potentially
generate widespread strong and gusty winds across our entire
forecast area over the weekend. The 12Z ECMWF tracks this system
far enough west to develop a little over-water trajectory and some
isolated light precipitation in our area on Saturday. However,
mostly ECMWF ensemble members remain dry.

From Previous Discussion...Speaking of precipitation, most
deterministic and ensemble model output maintain dry weather
through the end of February. Both the GFS and ECMWF show a system
moving southward along the California coast in early March with at
least some precipitation. Though as we've seen before, should
this system prove to track farther westward than the deterministic
models currently show it could mean we miss out on any rain as
most of the system stays offshore. For what it's worth some
ECMWF/GFS ensembles are showing such a solution.

&&

.AVIATION...as of 9:58 PM PST Monday...For the 06Z TAFs. VFR with
mainly clear skies overnight into Tue expect for some increasing
high clouds. Increasing offshore winds in the hills will create
some low level wind shear Tuesday morning. Skies to remain clear
through the day Tuesday as dry offshore winds continue.

Vicinity of KSFO...VFR. Light winds overnight but increasing
offshore winds aloft will create some low level wind shear Tue
morning through midday.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR. Light winds turning light offshore
overnight into Tuesday morning with light north by afternoon.

&&

.MARINE...as of 9:00 PM PST Monday...Northerly winds will
increase and seas will build overnight into Tuesday as high
pressure strengthens to the northwest. A Gale Warning is in effect
over the northern outer coastal waters from late tonight through
Wednesday afternoon. Steep wind waves and moderate period
northwest swell will create hazardous seas for smaller vessels
through the week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...SCA...Pt Arena to Pigeon Pt 10-60 nm until 3 AM
             GLW...Pt Arena to Pigeon Pt 10-60 nm from 3 AM
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm

&&

PUBLIC FORECAST: Dykema
AVIATION: RWW
MARINE: RWW

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