High Surf in California, September 20, 2021

Area Forecast Discussion
National Weather Service San Francisco Bay Area
429 PM PDT Mon Sep 20 2021

.SYNOPSIS...Red Flag Warning in effect through 8 pm tonight for
the North and East Bay hills as well as the East Bay interior
valleys for dry offshore winds. Otherwise sunny and warmer
Tuesday as offshore winds continue to drive hot temperatures
towards the coast. A slight cooling trend Wednesday as onshore
winds return. High pressure rebuilds Thursday with another round
of dry offshore winds focused over the Napa hills. Seasonably warm
and dry pattern continues into the weekend.

&&

.DISCUSSION...as of 2:09 PM PDT Monday...Red Flag Warning to
continue through 8 pm this evening. Skies have cleared as offshore
winds have arrived. We are seeing rapid warm-up of temperatures
throughout the region as downslope winds and a drying airmass
produce ideal conditions for some adiabatic warming even though
850 mb temps aren't too warm. 2 pm temps in the mid 80s will
continue to warm into the upper 80s and lower 90s through 5-6 pm
before slow cooling overnight. Models do show one last burst of
offshore winds across the Napa hills from 03-09z this evening but
not enough coverage or strength to justify an extension of the
warning. Even though winds will become light overnight there will
be little or no humidity recovery thus fire weather conditions
will remain near critical through the overnight hours.

Smoke and haze from the Windy and KNP Complex fires is now
starting to make its way south to north across the Central Coast.
That smoke/haze will continue to push northward into the Bay Area
by sunrise Tuesday morning. This will be impactful for air
quality on Tuesday as we expect temperatures to be even hotter on
Tuesday with some hazy sunshine. The pattern will support rapid
warming of temps under light offshore wind flow and no marine
layer. Similar to today expect temps to quickly warm into the 80s
by lunchtime but then trend even hotter Tuesday afternoon with
widespread temps in the upper 80s and 90s. For locations close to
the coast this will be one of the hottest days so far this
summer/fall. Heat risk will become moderate for sensitive
populations and the smoke/haze likely wont help but the very dry
airmass will allow for nighttime cooling and even getting into
shade will shave several degrees off the hot temps. Even though
there are no Red Flags for Tuesday conditions will continue to be
near critical with the hot temps, low humidity and rapidly drying
fine fuels while the long term drought continues.

By late Tuesday afternoon there are signs of a southerly wind
reversal coming up the coast that should bring cooling as early as
Tuesday late afternoon/evening to the Santa Cruz region, then
those southwest winds should push through the Golden Gate and up
into the North Bay valleys overnight into Weds morning. This
southerly wind reversal will occur as the main ridge weakens and
a shortwave approaches far NorCal on Weds. This will lead to
several degrees of cooling for Weds as onshore winds return with
stratus forecast into SF Bay Weds morning. The amount of cooling
for Weds should be on the order of 5-10 degrees so still warm
inland but not early as hot as Tuesday.

That shortwave then drops into the Great Basin by Thursday. The
circulation around that low, as another ridge of high pressure
builds looks to set up yet another round of offshore winds for the
North and East Bay hills. The forcing for these offshore winds is
slightly different and may not follow the usual diurnal trend we
see of stronger winds at night. Right now expect to see increasing
northeast winds across the Napa hills on Thursday overspreading
the rest of the Bay Area Thursday afternoon and evening. Wind
speeds look moderate but unlike the current event we should go
into that offshore wind with very low humidity values that will
trend as low as 10% by late Thursday afternoon with temps back
into the 90s. The models had been struggling with how the low
would evolve but now seem to be settling on the idea of another
round of offshore winds for Thursday afternoon. Will get through
this current Red Flag but have to quickly reset and see if a fresh
round of Fire Wx Watches will become necessary for the Thursday
event.

Long range remains unseasonably warm and dry into the weekend
with ridging over NorCal.

&&

.AVIATION...as of 04:29 PM PDT Monday...For the 00z TAFs. VFR
conditions through the period as the region remains under the
influence of high pressure and offshore flow. W/NW winds along the
coast this afternoon around 10-15 kt with N/NE winds inland.
Latest HRRR smoke forecast shows increasing total smoke from
wildfires to our SE beginning later this afternoon and progressing
northward overnight tonight. Expect some slant range vis issues
as smoke drifts northward. May also be some surface haze as well
by tomorrow afternoon. Additionally, just at the end of the
current taf period todays local WRF model shows a southerly wind
reversal traveling up the Monterey Coast and moving into the Bay
by tomorrow evening. May or may not see some low clouds into
tomorrow evening, but confidence is low.

Vicinity of KSFO...VFR through the period with increasing smoke
aloft overnight tonight that may create some slant range vis
issues. Onshore winds this afternoon around 15 kt diminishing this
evening.

KSFO Bridge Approach...Similar to KSFO.

Monterey Bay...VFR through the period with increasing smoke aloft
from nearby wildfires. The latest HRRR smoke model shows some
increase in near surface smoke as well that may result in haze. As
mentioned above, possible southerly surge tomorrow afternoon that
may or may not bring in some low clouds into Tuesday evening.
W/NW winds this afternoon 10-15 kt diminishing this evening.

&&

.MARINE...as of 02:32 PM PDT Monday...Winds continue to diminish
and will do so through the night. Light to locally moderate winds
will prevail into mid week. The next chance for increasing winds
over the outer waters will be late in the week. A weak longer
period southerly swell continues before a longer period northwest
swell arrives mid week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...Red Flag Warning...CAZ507-510-511

&&

PUBLIC FORECAST: RWW
AVIATION: AS
MARINE: DK

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