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

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FXUS66 KMTR 250539

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1039 PM PDT Sat Oct 24 2020

.SYNOPSIS...Quiet weather will continue through Sunday morning.
Dry north winds will arrive between late morning and late
afternoon on Sunday and these offshore winds will increase
significantly on Sunday evening. Strong and gusty offshore winds
will impact the San Francisco Bay Area Sunday night into Monday
morning. Red Flag Warnings and Wind Advisories are in effect for
the entire Bay Area. Breezy and dry conditions will continue in
the hills through Monday night. High pressure next week will bring
sunny and warm afternoons, but cool nights are expected with
interior valleys forecast to drop into the mid and upper 30s
Monday night through Wednesday night.


.DISCUSSION...As of 9:30 PM PDT Saturday...Currently a weak upper
low is off the central and southern California coast. Marine
layer is not very evident on the wind profilers but there is
abundant low level moisture up to 2500 ft. This will allow low
clouds to spread inland tonight which will keep cool and moist
conditions through Sunday morning.

Meanwhile an upper level trough is dropping into the northern
Great Basin. This trough is forecast to dig south into Utah by
late Sunday. As the trough sinks south, surface high pressure will
build in to our northeast and east from Sunday into Sunday Night.
A dry cold front is forecast to sweep from north to south across
our area on Sunday. North winds will develop behind the front
bringing in much drier air. Low clouds will clear rapidly from
north to south across the SF Bay Area from late morning to early
afternoon. Low clouds may linger well into the afternoon around
the Monterey Bay as northerly flow will be later to arrive.

Latest NAM is in and the forecast is still on track for an abrupt
transition to a dry northeast flow Sunday afternoon and evening.
The 00Z NAM winds show 45-55 mph in the North and East Bay above
1000 feet after 00Z (5 PM PST) so there could be gusts up to 60-70
mph. Across the Bay Area into the Santa Cruz Mountains the NAM
shows winds in the 35-45 mph range above 1000 feet. At lower
elevations the winds are expected to be sustained in the 20-30 mph
range but the strong winds aloft will frequently mix down to the
surface producing gusts of 40-50 mph. From Monterey Bay southward,
winds are expected to turn offshore Sunday night, but mostly
remain light. Humidities are still forecast to drop way down into
the teens and 20s. This combination of strong winds and dry air
will create critical fire conditions. Please refer to the Wind
Advisory product and the Red Flag Warning product for more

From previous discussion...Winds will subside across the SF Bay
Area by late Monday morning, and the Wind Advisory is due to
expire at 11 AM Monday. However, dry offshore winds will remain
gusty in the North and East Bay Hills through Monday night.
Although winds in the hills on Monday night are not expected to be
nearly as strong as on Sunday night, humidity values will be even
lower and critical fire weather conditions will persist. Winds
will continue to subside on Tuesday, but very dry conditions will
persist through at least midweek.

Although the airmass arriving behind the dry cold front tomorrow
will be cooler, drier air will mean slightly warmer days through
much of next week, but also cooler nights once winds subside.
Daytime temperatures will be slightly warmer than seasonal
averages through much of the week, but overnight lows will cool
into the mid to upper 30s in our inland valleys starting Monday
night and continuing through Wednesday night. Have decreased
valley minimum temps below the NBM for this time period.

&& of 10:40 PM PDT Saturday...Good inland push of
stratus this evening. Late burnoff is expected on Sunday with
clouds in some areas not clearing until early afternoon. But a
transition to a dry north to northeast flow will take place later
Sunday afternoon and clouds will rapidly dissipate. North to
northeast winds becoming gusty Sunday evening in the North Bay and
Bay Area with strong LLWS possible.

Vicinity of KSFO...MVFR with bases 1200-1500 feet. Clouds scatter
out after 18Z but do not completely clear until after 21Z.
Northwest winds 10-15 kt in the afternoon turning north to
northeast Sunday night. LLWS.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...MVFR cigs with only partial clearing in
the afternoon. Cigs returning early Sunday evening but dissipating
during the night as dry offshore winds arrive.


.FIRE of 2:15 PM PDT Saturday...Quiet fire weather
tonight through Sunday morning.

Red Flag Warning starts for the North and East Bay hills at 11 am
Sunday. Timing may be a bit early, especially for the East Bay
but initial winds will start to increase over Napa hills by
midday. By late afternoon and before sunset northeast winds
continue to increase over the North Bay hills with gusts 40-50 mph
before dark. Those winds will overspread the East Bay region
rapidly late Sunday afternoon and evening. We often have to wait
for humidity to lower several hours after the winds. In this
situation there will be a clear dry air boundary that will cause RH
to plummet well into the teens and possibly single digits. This
dry boundary will help to propagate strong winds from the hills
down into the valleys. Those locations on the lee of the hills
(North Bay Valleys, Berkeley hills) will be susceptible to strong
downslope winds Sunday evening. Expecting numerous reports of
wind gusts in excess of 50 mph with local gusts in excess of 70
mph very likely at the highest elevations in the North and East
Bay. Even places like the City of San Francisco and coastal
locations from Bodega Bay, Stinson Beach, Pacifica and Half Moon
Bay will see gusty northeast wind gusts from 40-50 mph from Sunday
evening through early Monday.

Any new ignitions Sunday evening will likely see dangerous spread
rates as all fire weather parameters will be off the charts
(fuels, wind, humidity). Long range spotting of up to 1 mile would
certainly be possible. ERC charts still setting records for the
Diablo and Santa Cruz mtns with much of the North Bay and lower
regions of the Bay Area critically dry.

The most critical wind period will ease by mid morning Monday.
However a second burst of gusty winds is expected in the hills by
Monday afternoon/evening with the very low RH staying in place.
Red Flag Warning continues through Tuesday morning for the North
and East Bay hills. May eventually need to extend in time the
warning for the N and E Bay Hills but will leave in place for now.
High pressure will bring warm days and lighter winds by midweek,
but a very dry airmass will persist. No rain is in site through
the end of the month and dry conditions will likely persist for
the first week of November.

&& of 09:00 PM PDT Saturday...Light winds prevail
through Sunday early afternoon with stronger northerly winds over
the northern outer waters. Winds will shift to the north-
northeast Sunday afternoon into evening and become gusty. Gale
force gusts are expected Sunday overnight into Monday morning,
particularly over the San Francisco Bays and through the Delta.
Winds will gradually diminish on Monday. Mixed seas will persist
with a moderate northwest swell and a longer period southerly
swell. A longer period northwest swell will arrive tomorrow night.


     .Tngt...SCA...Pt Arena to Pigeon Pt 10-60 nm




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