Wildfire in California, September 1 to 3, 2021

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1113 AM PDT Wed Sep 1 2021

.SYNOPSIS...Cooling trend back to more seasonal temperatures
continues today into Thursday. High pressure builds Friday and
through the Labor Day weekend with a warming trend as inland
temperatures warm back into the 90s and lower 100s by Sunday and
Monday.

&&

.DISCUSSION...as of 10:48 AM PDT Wednesday...Marine layer to
around 2500 feet this morning with low clouds slowly retreating
back to the coast. Temperatures will remain seasonably cool with
widespread highs from the upper 60s to upper 70s coast and bays.
Inland areas will warm into the 80s with steady onshore winds
through the day in response to the approaching 500 mb upper trough
axis. Trough axis will pass over the Bay Area overnight or early
Thursday morning. This will allow for another night of a deep
marine layer and widespread stratus intrusion to start the day
Thursday. By Thursday afternoon weak shortwave ridging starts to
develop with a few degrees of warming and a modest afternoon
seabreeze.

Pattern looks benign Friday into Saturday with a weak low taking
shape well offshore as the desert southwest high starts to try and
build westward. This should translate to near normal temperatures
and persistent night and morning marine layer for the start of the
holiday weekend.

By Sunday and Monday high pressure building out of the deserts and
SoCal will start to build northward bringing warmer 850 mb temps
aloft that will translate to inland areas warming into the 90s and
lower 100s. No real offshore flow of note with the warm ridge
over our region while the previously mentioned weak upper low will
wobble well offshore.

In terms of forecast interest and complexity and possible impacts
we expect a general southerly wind flow to develop over the Bay
Area and Central Coast by Monday in between the two weather
systems. That should be favorable for smoke free skies assuming no
new ignitions. Moisture streaming out of Baja may try and squeeze
in between the high and low. Current model simulations keep that
moisture and instability mainly over the ocean through early next
week but we'll be watching for any deviations in the coming days.
Until then expect dry weather for the holiday weekend with a noted
inland warming trend.

&&

.AVIATION...as of 11:13 AM PDT Wednesday...For the 18z TAFs. The
marine layer has deepened to over 2,500 ft AGL per profiler data
with stratus still enveloping most of Sonoma County late this
morning. Low clouds are also still along the entire Central Coast
and slowly dissipating inland. MVFR/IFR cigs will continue across
the North Bay over the next hour or two with VFR conditions
elsewhere. VFR this afternoon before low clouds return this
evening as a deep marine layer continues. MVFR/IFR cigs to then
prevail overnight and through the rest of the taf period. Breezy
onshore winds this afternoon at 15-20 kt.

HRRR smoke model shows increase smoke aloft beginning this
afternoon across the North Bay creating slant range vis issues.
Some increase in near surface smoke as well.

Vicinity of KSFO...VFR into tonight. Some chance low clouds will
fill into the approach this evening before moving into the
terminal late tonight. Chance of MVFR cigs late tonight into mid
tomorrow morning. W/NW winds this afternoon 15 to 20 kt. HRRR
shows an increase in smoke aloft late this afternoon and tonight
with some reduction in slant range visibility expected.

KSFO Bridge Approach...Similar to KSFO.

Monterey Bay...Satellite shows stratus dissipating down the
Salinas Valley and the Monterey sunbelt with low clouds still
persisting over the bay and peninsula. VFR conditions into late
this afternoon before stratus is expected to return by the early
evening (~01z-03z). MVFR/IFR conditions then to prevail through
the rest of the period. W/NW winds this afternoon 10 to 20 kt.

&&

.MARINE...as of 08:26 AM PDT Wednesday...Light to moderate west
to northwest winds across the coastal waters the rest of the week.
Locally breezy northwest winds will persist over the northern
outer waters through Thursday morning. Gusty afternoon and evening
winds are also expected near the Golden Gate and into the Delta.
Sea state remains driven by steep short period northwest waves at
periods of 8 to 10 seconds along with a weak moderate period
southerly swell.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...SF Bay from 3 PM

&&

PUBLIC FORECAST: RWW
AVIATION: AS
MARINE: AS

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