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

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FXUS66 KMTR 240006
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
506 PM PDT Fri Oct 23 2020

.SYNOPSIS...Seasonably cool conditions will persist through Saturday
with widespread night and morning low clouds. A strong offshore wind
event will develop over the greater San Francisco Bay Area on Sunday
producing critical to extreme fire weather conditions into at least
Monday. Strong and potentially damaging winds will be possible
across portions of the region late Sunday evening into early Monday
morning before wind speeds begin to diminish. High pressure will
bring a warming trend for much of next week with warm days and cool
nights as overnight lows drop into the 30s for interior valleys.

&&

.DISCUSSION...as of 01:55 PM PDT Friday...A deep marine layer
persists over the region which spread further inland across the
interior overnight. Thus, temperatures are a few to as much as 15
deg F cooler compared to 24 hours ago (across the interior). This
will hold afternoon temperatures in the 60s at the coast to 70s
inland. As a weak mid/upper level approaches the Central Coast
overnight, look for the marine layer to deepen further resulting
in widespread low clouds heading into Saturday morning. This will
aid in seasonably cool temperatures with generally a uniform
temperature spread ranging from mid/low 60s at the coast to lower
70s inland. Additionally, onshore winds will increase slightly by
Sunday afternoon.

All attention turns to the upcoming wind and fire weather event
which will begin on Sunday. The forecast models are in good
agreement indicating a dry frontal boundary dropping across the
greater San Francisco Bay Area Sunday afternoon in response to a
mid/upper level trough digging southward across the Northern
Rockies. Atmospheric conditions will rapidly dry from north to south
across the region with north to northeast winds becoming strong and
gusty, even in the lower elevations and along the coastline. This
wind and fire weather event is on track to be the strongest the
region has experienced so far this fall.

Now the details: The first burst of stronger offshore winds will
begin over the interior North Bay Mountains (primarily of Napa
County)late Sunday morning as winds come down the Sacramento Valley.
These stronger winds will then spread across the remainder of the
North Bay and then into the East Bay Hills including the Diablo
Range Sunday afternoon. As these winds will be synoptically driven,
the typical diurnal tendencies are not likely. Thus, strong and
potentially damaging winds will spread over much of the San
Francisco Bay Area during the late afternoon/evening hours on
Sunday. While the strongest winds will occur in the hills, ridges
and peaks, valley and coastal areas will likely experience offshore
winds gusting to 40 to 50 mph. Thus, a High Wind Watch is in effect
for the entire San Francisco Bay Area while weaker winds are
forecast to persist over Monterey and San Benito Counties. From the
previous forecast discussion: "Wind damage will be likely for any
fire weakened trees. Downed trees, branches and powerlines will be a
high possibility. In addition any temporary tents or structures may
be compromised by the strong burst of winds. Make plans now for
possible power outages whether they are planned or unplanned. In
simple terms this event looks to be on par with the 2017 wine
country fires and last years Kincade fire (that occurred Oct 26-27th
2019). Will say the meteorological setup is slightly different. Cold
advection looks stronger which should favor better mixing of winds
to the surface. In addition though the North Bay may take the brunt
of the wind much of the East Bay, SF Peninsula and Santa Cruz
mountains will see high wind potential as well as the Highway 1
corridor Sunday night. The 2017 wine country and 2019 Kincade peaked
between 9 pm and roughly 1-3 am at night. This event may arrive
slightly earlier but should any fires start it will be a long night
of gusty winds. In addition the airmass appears to be much drier
with humidity values forecast to plummet into the single digits and
teens. There will be no marine layer so even the valleys will be
bone dry. And as has been noted throughout the week this will all
occur on top of record dry fuels. So yes it has similarities to the
2018 Camp Fire as well. Trying to think back and this pattern shows
strong signals to some memorable Sierra East Wind events where the
winds howl down the corridors of Interstate 80 and Hwy 50 pouring
out across the Bay Area. Believe the 2004 Fred, Power and Rumsey
fires showed big growth days under a similar setup."

Wind speeds will begin to diminish during the day Monday as the
aforementioned mid/upper level low drops into the Desert Southwest
and then ejects eastward across Texas by the middle of next week.
That said, gusty offshore winds will persist in the interior North
Bay Mountains and potentially the East Bay Hills through at least
Monday evening. Regardless, critically dry conditions will remain in
place region-wide through midweek as high pressure builds in from
the Pacific. This will bring about mild to warm daytime temperatures
while overnights will be cool under mainly clear sky conditions.
Expecting many of the interior valleys to drop into the mid/upper
30s next week, the coldest of the season. Dry conditions then appear
likely to persist heading into November.

&&

.AVIATION...as of 5:00 PM PDT Friday...Stratus has mostly burned
off this afternoon as the marine layer has mixed out. The flow is
becoming more onshore favoring a redevelopment of low clouds
tonight. This is supported by the models which show increasing low
level moisture tonight and Saturday. Coastal areas will see low
cigs first before spreading into the Bay Area after 08Z. Clearing
in the Bay Area mid to late morning with partial clearing in the
Monterey Bay Area.

Vicinity of KSFO...Most of the clouds will be surrounding SFO
with MVFR cigs finally getting in after 13-15Z. Bases 1000-1500
ft. Clearing by 18Z as the entire Bay Area starts to clear out.
Northwest winds to 15 kt decreasing after 04Z.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Still clear over Monterey Bay with only
patchy stratus south of Point Lobos. IFR cigs returning after 03Z
possibly later. With increasing low-level moisture on Saturday
only partial afternoon clearing is expected.

&&

.FIRE WEATHER...as of 01:55 PM PDT Friday...The Fire Weather
Watch has been upgraded to a Red Flag Warning and been expanded
to include the entire San Francisco Bay Area, including valley and
coastal locations.

High confidence that damaging burst of winds arrives as early as
4-7 pm Sunday afternoon rapidly passing from east to west over
much of the Bay Area. Current model solutions show winds peaking
between 7 pm and midnight Sunday (gusts to 70 mph looks likely)
night as this is being driven by the timing of a cold front well
to our east. Point being is this won`t take on the usual middle
of the night timing that often occurs with Bay Area offshore
winds. As noted above for comparison this will be on par with the
2017 Wine Country and 2019 Kincade Fire. Still think the setup is
slightly different and will favor strong wind potential over a
larger region that will include all of the East Bay, SF Peninsula
and much of the Santa Cruz mtns as well as Highway 1 corridor from
Sonoma southward to Half Moon Bay.

Just looked at fuels and ERC readings setting new records. The
stratus coverage the next 1-2 days will temper some of the fine
fuels but it wont matter. Other high confidence element here is
the incredibly dry airmass that will quickly overspread the region
Sunday night into Monday. Expecting widespread RH values in the
single digits and teens at all elevations with no help from the
marine layer.

Any ignitions Sunday evening and night will be very problematic.
Winds will ease Monday morning but low humidity and offshore winds
to continue at lighter wind speeds Monday into Tuesday. High
pressure next week with no rain through the end of the month.

&&

.MARINE...as of 04:37 PM PDT Friday...Winds are mostly light to
moderate, but remain stronger over the  northern outermost waters.
Northwest to north winds will return  Saturday before becoming
gusty and shifting more offshore on  Sunday. Mixed seas persist
with a moderate northwest swell and a  longer period southerly
swell. The next long period northwest  swell arrives Sunday night.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...SCA...Pt Arena to Pigeon Pt 10-60 nm

&&

$$

PUBLIC FORECAST: RGass
AVIATION: W Pi
MARINE: DK
FIRE WEATHER: RWW

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