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

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FXUS66 KMTR 251744
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1044 AM PDT Sun Oct 25 2020

.SYNOPSIS...Cloudy skies this morning will turn sunny this
afternoon as drier air filters into the region. Becoming windy in
the North Bay hills before dark then spreading over the rest of
the Bay Area tonight with strong and damaging northeast winds. Red
Flag Warnings and Wind Advisories in effect for the entire Bay
Area while the bulk of the winds will not impact the Central
Coast. Winds slowly ease Monday at lower elevations but it will
remain breezy in the hills with a very dry airmass in place
keeping fire danger high. Building high pressure Monday through
Friday will bring sunny and warm days but with cool nights as
lows drop into the mid and upper 30s for the interior valleys
with 40s elsewhere. No rain forecast through the end of the month
into early November.

&&

.DISCUSSION...as of 09:30 AM PDT Sunday...Starting the morning
off with marine stratus across much of the Bay Area. Bodega Bay
Profiler currently suggesting a depth just under 2500 ft,
translating into high RH values along the coast and into the
coastal valleys of the North and East Bay. Much like what we
observed yesterday, expect for marine stratus to clear out by
midday over much of the area. The axis of the positively tilted
trough goes through our CWA today as offshore (NE) winds begin to
enter our high-terrain locations and quickly mix down to the
surface. The SFO-WMC gradient has rapidly increased to 5.3 mb this
morning and is progged to get up to 17 mb by around this time
tomorrow morning.

Essentially, everything is on track for this offshore wind event.
Only minor changes have been observed in some of the short-range
model guidance, including a slight decrease in the progged
offshore surface gradients through the overnight hours. It goes
without saying, however, that this dry air mass is still set to
bring some of the strongest winds to that the CWA has measured
during this historic fire season. NAM-3km and HRRR continue to
pick up on strong wind gusts in the 40-50mph range at some of our
higher elevations >2000 ft, with the possibility of wind gusts as
high as 70mph in our highest points (e.g. Mt. Diablo). In terms of
RH values, short- term guidance picks up on low-level jet that
will advect the dry air mass all the way into the coast overnight.
RH values are progged to fall into the single-digits in spots
that this morning are currently shrouded under the marine layer
(this includes the NAM run for the overnight hours, which has
historically had a slight precipitation bias). Strong wind gusts
are also progged for much of the Bay Area, including much of the
valleys and coastal spots. From Monterey Bay area southward,
winds will be lighter (with the exception of some isolated pockets
along the northern Big Sur Coast. Winds around the Dolan and
Coleman Fire also expected to be light during the next couple of
days. The NE flow that will mix down to the surface will
predominantly be steered towards the SF Peninsula coast while the
Santa Cruz Mountains will help to naturally steer them away from
the Central Coast as a result. Nonetheless are still expecting low
RH values and next to no overnight humidity recoveries across the
entire CWA.

These critical fire weather conditions will be observed over much of
the State in the next couple of days. As a result, questions have
come up regarding the differences between the Diablo Winds,
Sundowners, and Santa Ana winds. Want to clarify that all of these
naming conventions refer to the offshore winds that develop across a
given region. Here in the Bay Area, strong offshore winds are
referred locally as the Diablo Winds. Regardless of their naming
convention, they refer to the same setup when katabatic
(downsloping) winds descend from higher-terrain locations down to
the coast.

While main focus has been on the North Bay, it should be noted that
the burn scars in the area may actually help mitigate some of the
fire risk there given the amount of burning that has already
happened there this fire season. That being said, any new wildfire
starts, especially under these circumstances, is the last thing that
firefighting operations need at this time. Other critical concern is
for the East Bay hills, as they have historically dealt with
wildfires that have made it into urban areas under critical
conditions. For some historical background on the wildfires that
have occurred under extremely critical fire weather risk events, have
including previous discussion. It also goes without saying that air
quality will be entirely at the mercy of whether or not wildfire
starts occur during this critical fire weather setup. Should any
wildfires develop, will be monitoring smoke plumes closely as well.
As always, be sure to reach out to the Bay Area Air Quality
Management District for any new alerts and AirNow for PM and ozone
concentrations.

&&

.PREV DISCUSSION...as of 3:38 AM PDT Sunday...The forecast remains on
track with no real changes of note in terms of upcoming wind.

Synoptic pattern features a longwave trough over the Rockies with
a strong jet stream aloft coming down the backside of the trough
into Nevada. This will allow very cold air to spill into the Great
Basin setting up a strong surface pressure gradient across the
Sierra. The SFO-WMC is neutral and will rapidly increase to about
18-19 mb over the next 24 hours. Initial shot of north winds will
work down the Sac Valley today and should reach the Napa hills by
early afternoon. Strong wind gusts in excess of 50 mph will hold
off until 4-7 pm. Wind Advisory officially starts at 4 pm and that
timing looks good. Most of the Bay Area will maintain light winds
through that time. Things will change quickly as the dry cold
front and associated dry air quickly spill into the Delta region
and fans out across the rest of the North/East Bay and quickly
passes over the rest of the Bay Area. We dont often get to see
true synoptic frontal boundaries that are so clearly defined but
this well be obvious for anyone paying attention. The airmass
change will be abrupt as very dry air filters over the region and
it wont discriminate between the hills and valleys.

In terms of winds, everything is still on track. The 925 mb winds
first show gust potential across the Napa hills between 00-03z
this evening. As is often the case as the sun goes down the winds
will increase. Between sunset and the overnight hours the
northeast winds will howl across much of the Bay Area. Models show
strongest wind potential will be with the initial burst of winds.
If things go down hill in terms of new ignitions we should know
before midnight. Model cross sections show very strong winds aloft
with downward sinking air. There wont be any inversion of note to
keep the strong winds from mixing down, though some of the the
valleys of the North Bay and Santa Clara Valley will be more
protected there is too much small scale topographic interaction to
pinpoint that level of detail. In terms of magnitude if memory
recalls the models were showing perhaps stronger wind strength
during the 2017 wine country and 2019 Kincade fires but this event
looks to be more widespread. Another noted difference is the
strong east winds over the Sierra region west of Tahoe. During
these setups those winds take a straight beeline towards Mt Diablo
and then under the right conditions will drop down into the East
Bay hills region above the 880 corridor. Wind damage still appears
likely overnight as the high momentum wind gusts will take out
drought and fire weakened trees/limbs/powerpoles across the
region. In addition temporary tents and other structures will
likely be compromised. Trash and other debris will whip around and
cause hazardous driving conditions.

Of course ironically the fresh burn scars will preclude those
areas from burning but any fire prone locations will be of concern
tonight. At this point looking back at history may be germane.
Recall the 2017 Tubbs Fire took on a similar scar to the 1964
Hanly Fire. More recently of course is the 1991 Oakland Hills
Fire. 1923 Berkeley fire was in September but shows the urban
potential. The Vision fire of 1995 near Pt Reyes will be worth
remembering as the models show strong winds downwind of Mt
Tamalpais. So we can say that winds look especially strong
tonight, fuels are historically dry and humidity is even drier
than normal in association with this airmass. Unlike normal
offshore wind events we dont expect the strongest winds to be
confined to the hills as the atmospheric profile will be conducive
to mixing those winds down. The wildcard of course will be any
new ignitions. Will also mention though we haven`t focused as much
on it do expect strong winds along Skyline Blvd in the Santa Cruz
mtns and down along Hwy 1 so those locations outside the CZU burn
scar will be of concern. Inside the CZU burn scar the prospect of
fire weakened redwood trees falling is of grave concern as well.

As the sun comes up Monday morning skies will be crystal clear
(hopefully) and temps noticeably cooler as the dry airmass
filters in. East to Northeast winds will still be blowing but
should ease by mid morning. Models do show an early secondary
burst of winds for the North and East Bay hills so the Red Flag
Warning will continue. We of course will have to re-evaluate the
Wind Advisory and Red Flags by Monday morning and extensions may
be necessary. For clarity of message no changes have been made to
start and stop times for now.

The dry airmass and offshore winds continue Monday night into
Tuesday so should any fires start containment will be challenging.

Next item of note will be if frost advisories will become
necessary. Winds may be strong enough in the hills to preclude
that and all focus for now is on winds. However, those with ag
interests should be ready for some upcoming cold nights. Will note
the ECMWF mos for Santa Rosa is 32 Tuesday morning. If power
outages occur or linger the cold overnights could become
problematic.

Ironically enough afternoon highs will be showing a warming trend
Monday afternoon though much of next week as high pressure
builds. Offshore winds and the dry airmass will set up ideal
adiabatic compression. Should we get through the wind event
unscathed the weather pattern next week looks quiet with high
pressure and seasonably warm days followed by clear and cool/cold
nights.

High confidence for no rain through the end of the month and
early signals looking dry through the first week of November.

&&

.AVIATION...as of 10:44 AM PDT Sunday...for 18z TAFs. Satellite
imagery shows stratus in the process of dissipating across the
region. VFR conditions anticipated within the next couple of hours
for most terminals with low clouds lingering around the Monterey
taf sites into the afternoon. Light winds this morning will begin
to shift out of the north to northeast this afternoon. Persistent
offshore flow this afternoon and overnight should keep VFR
conditions through the period from then onwards.

Winds aloft will begin to increase substantially this evening bringing
LLWS concerns for Bay Area and North Bay taf sites. Models have
925 mb winds (~2,000 ft) between 40 and 50 kt at its peak in the
overnight hours. These stronger winds are forecast to occasionally
mix down to the terminals, so some stronger gusts may occur that
are not prevailing in the tafs. However, widespread gusty winds
are still anticipated at the surface with consistent gusts of
about 25 to 35 kt beginning this evening and continuing overnight.
Winds will gradually diminish tomorrow morning.

Vicinity of KSFO...MVFR cigs around 2,000 to 2,500 ft over the
next hour or two before VFR conditions are expected this
afternoon. Light onshore winds this morning expected to begin to
turn more N to NW late this afternoon before becoming NE this
evening and overnight. LLWS possible this evening as winds aloft
begin to increase before these stronger winds mix down to the
surface later. Conditions will be very close to AWW criteria with
surface gusts expected overnight around 30 to 35 kt with higher
gusts possible. Given how borderline models and guidance look in
regards to gusts, will make the decision whether or not to
increase gusts and issue an AWW in this afternoon`s package.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...IFR/MVFR cigs between 1,000 and 2,500 ft
this morning. Satellite imagery shows stratus across almost all of
Monterey County as well as into the Monterey Bay. Cigs expected to
lift to MVFR late this morning with lingering low clouds into the
afternoon. Clouds will eventually dissipate this evening and
tonight as dry offshore winds arrive. The Monterey Area is
expected to be largely spared from the offshore wind event. Not
anticipating strong surface gusts or significant LLWS around KMRY
or KSNS.

&&

.FIRE WEATHER...as of 4:21 AM PDT Sunday...No changes to Red Flag
headlines. The Warnings officially start at 11 am for the North
and East Bay hills. The timing looks good for the North Bay as
winds will start to hit northern Napa around this time. Admittedly
the strong winds will hold off for the East Bay hills until late
afternoon but for clarity of messaging and to avoid confusion will
keep start times as is for now. The rest of the Red Flags for the
Bay Area start at 8 pm which looks good though some areas may
notice the increased winds slightly earlier.

In terms of overall setup, as noted above not much change in
thinking. Cloud coverage and high humidity to start the day
should fool nobody. Boundary will arrive late this afternoon. Main
event will be from roughly 6 pm (north bay) through the overnight
hours but the strongest initial burst of winds may occur before
midnight. Wind gusts 40-50 mph at lower elevations, downwind of
the hills (East Bay hills, coastal Marin, Pacifica/Half Moon Bay)
with expectation for gusts 60-70 mph for the higher elevations of
of the North and East bay hills, roughly above 2500 feet.

Heavy fuels remain at or near record dry levels. Humidity values
will plummet late this afternoon and overnight with essentially no
recovery. Should any ignitions occur this evening its likely that
all model simulations will fail to capture the true spread
potential. Long range spotting, record dry fuels, wind gusts 50-70
mph and humidity in the teens would all be valid model inputs
along with a highly turbulent/unstable boundary layer. Boots on
the ground should give strong consideration to LCES. Expect egress
to be blocked and normal comms to potentially be disrupted.

Winds will persist all night into Monday morning. Models show
winds slowly easing at lower elevations but increasing again in
the hills with gust potential still to around 50 mph though early
Monday afternoon.

Persistent east flow will continue Monday night into Tuesday
morning with no humidity recovery for all elevations. Its
possible that Red Flags may need to be extended but that will all
be re-evaluated Monday morning.

Long range shows dry with high pressure next week, no rain. Warm
days and cool nights. No marine layer expected for most of next
week.

&&

.MARINE...as of 08:28 AM PDT Sunday...Generally light north to
northwest winds across much of the waters this morning and into
the afternoon with locally stronger winds still over the northern
outer waters. Winds will increase this evening and shift out of
the northeast. Gale force gusts are forecast from then until
tomorrow morning, particularly over the San Francisco Bays and
through the Delta as well as along the Sonoma Coastline, Bodega
Bay, and down the San Mateo coast to Pigeon Point. Winds will
gradually diminish tomorrow morning. These winds will generate
steep fresh swell resulting in hazardous seas, especially for
smaller crafts. Mixed seas will persist with a moderate northwest
swell and a longer period southerly swell.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...Wind Advisory...CAZ006-505>513-529
             Red Flag Warning...CAZ507-511
             GLW...SF Bay from 5 PM
             SCA...Pt Arena to Pigeon Pt 10-60 nm until 3 PM

&&

$$

PUBLIC FORECAST: Diaz/Dykema/RWW
AVIATION: AS
MARINE: AS
FIRE WEATHER: Dykema/RWW

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