Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA

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FXUS66 KMTR 211614
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
914 AM PDT Mon Sep 21 2020

.SYNOPSIS...A cooling trend will begin today as an upper level
trough approaches the coast, and continues Tuesday. Not much
change is expected Wednesday and Thursday, though a second and
somewhat deeper trough could enhance the marine layer enough
Wednesday night and Thursday morning to produce more widespread
coastal drizzle. Dry offshore winds and much warmer temperatures
are projected by Saturday and Sunday.

&&

.DISCUSSION...as of 9:14 AM PDT Monday...No significant forecast
updates this morning. Marine layer deepening to in excess of 1200
feet but morning stratus/fog is quickly burning off. Low vsbys
currently from Half Moon Bay down to Monterey/Salinas Valley
should lift though 10 am. Incoming trough will bring increasing
onshore winds that will help to push any smoke from the August
Complex mainly out of the North Bay and into the Central Valley.

Weather pattern this week looks seasonable with no hot temps of
note. Next front moves into the far North Bay by early Thursday
morning and could generate some very light qpf over northern
Sonoma County. Behind that front temps to remain seasonable with
fairly moist conditions Friday. All eyes on the weekend for
potential strong offshore wind event and hot temps. Will check out
the 12z guidance and update forecasts for the afternoon package.

&&

.PREV DISCUSSION...as of 3:55 AM PDT Sunday...Latest water vapor
satellite imagery depicts an upper level shortwave trough
advancing towards the Pacific NW coast, while the fog product
shows marine stratus blanketing our coast from about Pt Reyes
southward. Latest Bodega Bay and Ft Ord profiler data indicate a
marine layer depth of about 500 ft and 1000 ft, respectively. The
combination of the shallow depth and coastal winds out of the NNW
will limit its inland extension both around San Francisco Bay and
down the Salinas Valley. It will also locally result in surfacing
of the cloud deck and have noted the ASOS at both Watsonville and
Salinas Airports are reporting fog with visibilities restricted
to the 1/4 to 1/2 mile range. Not presently expecting any
widespread dense fog (visibility less than 1/4 mile) but will need
to monitor over the coming hours. Smoke has moved into the Bay
Area from the August Complex fires to the north, with diminished
air quality. Latest near-surface smoke forecasts from the NOAA
ESRL Experimental HRRR-smoke model projects improved conditions
by later in the day.

A cooling trend will begin today as an upper level trough approaches
the coast, deepening the marine layer and increasing the onshore
flow. Highs this afternoon look to be 3-5 deg cooler around the
Bays and 5-8 deg cooler inland, with some additional cooling for
Tuesday. Not much change in temperatures is then expected through
Thursday, but do want to note that a second and somewhat deeper
trough could enhance the marine layer enough Wednesday night and
Thursday morning to produce more widespread coastal drizzle. Think
locally measurable amounts up to around 0.05 inches are possible.

It`s in the extended portion of the forecast, however, that
attention will need to increasingly focus. Longer range
deterministic and ensemble model output remains in excellent
agreement that a strong upper level ridge of high pressure will
build northward along the West Coast late in the week and over
the weekend, with 500 mb heights potentially reaching to near 600
dm offshore from the California-Oregon border by Sunday afternoon.
This will be accompanied by significant air mass warming, and
also lower tropospheric flow turning offshore over our area as
surface high pressure builds inland through the Pacific Northwest
into the Great Basin while an inverted surface trough develops
off the coast.

The net result looks to be a major warming trend, starting on
Friday and then really ramping up over the weekend. It`s still
quite a ways out, but basic projected synoptic picture is
reminiscent of the October 4-5 1987 heat wave, when Downtown San
Francisco reached a high of 102 degrees, third highest of all
time. And, of course, the combination of hot and dry conditions
and possibly gusty offshore winds, is of significant concern from
a fire weather perspective. We`ll be monitoring this closely over
the coming days.

&&

.AVIATION...As of 04:30 AM PDT Monday...for 12Z TAFS. The marine
layer currently sits at around 1,000 ft AGL per the Fort Ord
Profiler while satellite imagery shows stratus along the coast
south of Point Reyes and through the Golden Gate Gap. IFR
conditions may develop over the next couple of hours around the SF
Bay, though confidence is low. VLIFR/IFR conditions this morning
at the Monterey Bay sites due to low clouds and fog. VFR
conditions expected by late morning and to then prevail into the
evening. An approaching upper trough may act to deepen the marine
layer tonight allowing for more extensive stratus coverage
overnight, but confidence is low on timing and extent. Some smoke
and haze across the North Bay this morning may continue to create
slant range vis issues, though the latest HRRR smoke model
forecast shows improving conditions this afternoon with increased
vertically integrated smoke south of San Jose. Not expecting major
surface vis issues. Light winds this morning increasing and
turning onshore this afternoon 10-15 kt with locally higher winds
possible at KSFO.

Vicinity of KSFO...Currently VFR with satellite showing stratus
through the Golden Gate gap and along the coast to the west of
the terminal. Occasional IFR/MVFR cigs may be possible early this
morning though confidence is low. With a deeper marine layer
expected tonight MVFR/IFR conditions are forecast to develop
around the Bay this evening. W/NW winds this afternoon 15-20 kt.

SFO Bridge Approach...Possible lowered slant-range vis from haze,
otherwise similar to KSFO.

Monterey Bay Terminals...VLIFR to IFR conditions in stratus and
fog into mid to late this morning. VFR conditions expected late
this morning. Models/guidance suggest an early return of stratus
this evening.

&&

.MARINE...as of 09:03 AM PDT Monday...Northwest winds will
continue to be breezy over the northern  waters through Monday
night. Winds along the inner coastal waters will increase later
today before breezy winds subside over the  outer waters tomorrow.
Mixed seas will persist with a shorter  period northwest swell and
a long period southwest swell. A more  robust northwest swell will
arrive across the waters late this  week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...SCA...Pt Reyes to Pigeon Pt 0-10 nm from 1 PM
             SCA...Pigeon Pt to Pt Pinos 0-10 nm from 1 PM
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm from 3 PM

&&

$$

PUBLIC FORECAST: RWW/Blier
AVIATION: DK
MARINE: DK

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