Wildfire in California, September 22 to 23, 2021

Area Forecast Discussion
National Weather Service San Francisco Bay Area
137 PM PDT Wed Sep 22 2021

.SYNOPSIS...After a brief cool-down on Wednesday, a warming trend
will return for late week. However, marine influences will keep
conditions cooler near the coast compared to warmer interior
locations. Another cooling trend is then forecast for the upcoming
weekend and likely into early next week as well.

&&

.DISCUSSION...as of 01:35 PM PDT Wednesday...A mid/upper level
short-wave trough pushing into the Pacific Northwest and the
return of a shallow marine layer have resulted in a region-wide
cooling trend this Wednesday. Many coastal areas are as much as 15
deg F cooler compared to 24 hours ago while some interior and
higher elevation locations have noticed little change. With low
clouds hugging the entire coastline, look for low clouds to spread
back inland overnight as the marine layer deepens lightly in the
next 12 to 24 hours.

The cooling trend will be short-lived however as the trough drops to
the southeast and into the Intermountain West through late week.
This will result in brief offshore flow over the higher elevations
of the Bay Area from tonight into Thursday morning. While elevated
fire conditions are possible, winds are not anticipated to be strong
enough to create widespread critical fire weather conditions.
Additionally, high pressure will build in from the eastern Pacific
through late week helping to aid in another warming trend. That
said, marine influences should keep conditions cooler near the coast
while interior areas warm back into the upper 80s to middle 90s both
Thursday and Friday.

The ensembles and deterministic models are in a bit better agreement
heading into the upcoming weekend with brief zonal flow developing
aloft. This should allow for temperatures to cool slightly Saturday
into Sunday as onshore flow also persists and temperatures aloft
cool. Additional cooling looks likely early next week as well in
response to an approaching mid/upper level trough. Ensembles are
also indicating the potential for light precipitation across
portions of the North Bay early next week with the deterministic
ECMWF showing precipitation as far south as the Monterey Bay region.
Having said that, we'll continue to monitor trends in the models
over the coming days as confidence remains low at this time with
respect to precipitation across the greater San Francisco Bay Area
and Central Coast.

&&

.AVIATION...as of 10:31 AM PDT Wednesday...For the 18z TAFs.
Although most cloud cover is being confined to the immediate
coastline, and cigs over terminals is low on Wednesday. Terminals
near the Monterey Bay are still IFR at this hour, while the
remainder of the Bay Area is VFR, which will continue through
Wednesday afternoon. Winds are mostly light, but will switch back
to onshore in the afternoon as a seabreeze develops. Stratus
clouds over the ocean will move back inland this evening. Chances
for advection after 00Z around the Monterey Bay, but models are
struggling how the clouds will move into the San Francisco Bay.
The marine layer will likely remain shallow, below 1000 feet, so
any cloud cover will likely be low. With the model disagreement,
there is low confidence in the inland movement of stratus, but the
return of the coastal stratus and onshore flow gave enough
justification to go again models and assume that IFR/LIFR
conditions are possible overnight Wednesday into Thursday.

Vicinity of KSFO...VFR through Wednesday. Onshore winds increase
to just over 15 knots in the afternoon. Low clouds sit at 300 over
at KHAF. Best chances for inland inundation of clouds is to the
East Bay. Chance for IFR conditions at KSFO, but low confidence
due to shallowness of cigs expected and lack of overnight winds.
Added SCT to the TAF, but will be monitored.

KSFO Bridge Approach...Similar to KSFO.

Monterey Bay...Clearing cigs, but onshore push of winds is keeping
terminals in IFR for approximately the next hour. Seabreeze up to
10 knots expected Wednesday afternoon. VFR conditions expected in
the afternoon as well, but it will be brief. Low clouds return
early Wednesday night for cigs at IFR, sinking to LIFR levels into
Thursday morning. Thursday's cloud clearing not until after 18Z.

&&

.MARINE...as of 08:35 AM PDT Wednesday...Increasing gusts out of
the northwest over the northern outer waters Wednesday afternoon
into Thursday morning. Winds will also increase through the Golden
Gate gap and into the north San Francisco Bay Wednesday
afternoon. A longer period northwest swell arrives Wednesday to
mix in with the weaker southerly swell. The next chance for new
longer period swell to arrive is on Sunday.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...SCA...Pt Arena to Pigeon Pt 10-60 nm until 9 PM
             SCA...SF Bay until 9 PM

&&

PUBLIC FORECAST: RGass
AVIATION: DK
MARINE: DK

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