High Wind and Strong Wind in California, December 7, 2020

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1025 PM PST Sun Dec 6 2020

.SYNOPSIS...An upper low from yesterday's frontal passage will slide
southward tonight and retrograde back towards the coast through
tomorrow causing gusty offshore flow and fire concerns tonight
into Tuesday morning. Warming offshore flow and ridging over
Northern California will cause Monday and Tuesday to be our
warmest days this week in which some records may be broken. Beach
hazards and high surf are also expected Monday into Tuesday.
Afterwards, dry weather with mild daytime highs and cool nights
will persist for the rest of the week.

&&

.DISCUSSION...as of 08:08 PM PST Sunday...No updates to the
forecast for the evening shift. The forecast remains on track so
far with a Red Flag Warning and Wind Advisory going into effect at
11 PM. Latest observations shows the offshore gradient is starting
to ramp up and is currently at -8.3 mb. The peak gradient is
expected to peak close to -15 mb late tonight and early Monday.
The increasing offshore flow is already being observed over the
higher terrain of the N and E Bay - Mt St Helena is gusting to 51
mph and Diablo is gusting to 52 mph. Starting to see some strong
gusting already arriving in the Santa Cruz Mts with gusts to 44
mph at Mt Umunhum. Despite the developing gusty winds relative
humidity is still in the moderate range and not expected to drop
until later. For more details regarding the current fire weather
concerns see the Fire Weather section below.

Winds and fire weather concerns do finally ease Monday evening
and Monday night. Looking farther down the round on the latest
00Z NAM doesn't look good as another offshore flow event will be
possible Wednesday night. Stay tuned...

&&

.PREV DISCUSSION...as of 02:56 AM PST Sunday...Water vapor imagery this
afternoon has picked up on moisture plume over the Sacramento
Valley and Delta making its way south. Surface analysis also shows
the center of the 565dm 500mb cut-off low moving into the Great
Basin this afternoon. The SAC-WMC gradient is currently around 3mb
but will begin to increase during the overnight hours tonight and
into Monday morning. RH values regionwide have also trended
downward in elevated portions of the North and East Bay when
compared to 24 hours ago, with some stations reporting values in
that are 5 to 10 percent drier. Elsewhere, have seen moister
conditions when compared to 24 hours ago.

The return flow of onshore flow following the FROPA yesterday has
aided in increasing moisture content across the CWA. Some locations
reported in isolated drizzle, while morning fog helped to moisten
some of the more immediate fuels across the interior valleys.
Nonetheless, the offshore flow expected to pick up late tonight and
descend over the North Bay Mountains and East Bay Hills through the
pre-dawn hours of Monday before eventually snaking into the Santa
Cruz Mountains and SF Peninsula Coast. Overall, the setup for the
next 12-18 hours will go from moist conditions helping to mitigate
some of the immediate fire concerns in the evening to the offshore
flow clipping the Bay Area. Local WRF and NAM both have trended for
the strongest sustained wind gusts over 55 mph to remain primarily
over the highest peaks. Nonetheless, sustained 15-25mph expected to
mix down to the the foothills and valleys, with gusts up up 45 mph
possible at/above 1000 ft. These winds will decimate overnight
humidity recoveries at the higher-terrain locations within the Red
Flag Warning, while also bringing RH values in areas that today were
shrouded in fog down to the 30-35% range by daybreak tomorrow. The
good news for firefighting operations is that the peak of the
aforementioned wind potentials looks progged to begin immediately
before dawn and through the mid-morning. As such, daylight will be
helpful for firefighting operations should any fires break out. Not
expecting widespread RH values below 20% either, which is also good
news. Nonetheless, cannot overstate that the widespead record-dry
fuels and the lack of sufficient wet raining to get the region out
of extreme drought are both playing a critical role in the issuance
of the Red Flag Warning. Also have Wind Advisory in effect for the
same warning area given the potential for gusty winds down to the
coast. Have also included the City of SF in that advisory given its
proximity to the coastal areas along the windy overnight/early-
morning spots along the Peninsula and the timing of peak winds with
the morning commute.

Winds will quickly calm down across the Bay Area by early-afternoon
tomorrow regionwide as the sting of mid-level offshore winds
associated with the upper-level cut-off low travel southward. This
trough will quickly exit our CWA and center itself somewhere south
of Point Conception by tomorrow evening. Given the strong NE flow
and clear skies during the morning, can expect warm days tomorrow
and Tuesday with the potential for some record-breaking temps across
in both our coastal and interior locations. SF and SJ, for instance,
look set to be flirting with their record highs for the day.
Unfortunately, breezy offshore winds could still pose some concern
over extreme southern Monterey County tomorrow night and into
Tuesday morning as the upper-level trough influence carries into
parts of the Central Coast, but the bulk of the strong winds look
set to be primarily focused down south, with the northernmost extent
of the strongest gusts just south of the Santa Lucia Range.

Mid-range guidance unfortunately showing that another trough will
likely follow a similar path to our current cut-off low by midweek.
But unlike the previous system this one looks set to expand over a
much larger portion of the Great Basin, placing our CWA and indeed
much of Northern and Central California well-within its convergent
upper-level quadrant. For now, looking like this system will
initially introduce a moisture tongue into the PAC NW on Tuesday
which could travel southward and bring us some moisture recovery
akin to what we observed this morning. But following that transport,
dry air mass looks set to once again spill into California from the
NE, which could result in another round of offshore winds starting
sometime after Wednesday afternoon. Will be monitoring this set up
closely.

&&

.AVIATION...as of 10:25 PM PST Sunday...For the 06z TAFs...VFR
across terminals, with a few passing high clouds, and light
surface winds. VFR continues overnight with winds turning
northerly, keeping RH's down. The main concern in the short-term
will be intensifying NNE winds aloft. LLWS is presently in Tafs
for North Bay terminals and expands to all Bay Area terminals by
09z. Winds between 1,500 and 2,000 ft AGL will easily reach 30-40
kt across terminals, with 45 kt possible in North Bay. Surface
winds will initially be moderate but stronger winds will mix down
by the early morning hours - before sunrise in North Bay and
around sunrise for other terminals. Gusts may occasionally reach
25 to 30 kt but 20 to 25 kt will be more common. Fortunately,
winds will significantly weaken by the early afternoon, with clear
skies remaining.

Vicinity of KSFO...VFR continuing through the TAF period. Calm
winds becoming NNE overnight, and gusting to 30 kt at the surface
by sunrise tomorrow. NNE LLWS in TAF beginning at 07z and lasting
through the morning, around 30-40kt at 1,500-2000 ft AGL.
Northerly winds settle down by early afternoon.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR conditions prevail through the
period as drier air moves in overnight. Light to calm winds
becoming SE overnight. Brief LLWS 20-25 kt at 2,000 ft possible
at KMRY Monday morning after 15z as NE winds aloft increase
across the region. Winds settle down by Monday afternoon.

&&

.BEACHES...as of 09:45 PM PST Sunday...A strong storm system near
the Aleutians is forecast to generate a very large, very long
period northwest swell train with long period forerunners at 21 to
25 seconds arriving Monday, and swell peaking Monday Night into
Tuesday along the coast. For Monday, the primary hazard will be on
the beaches where infrequent yet dangerous sneaker waves and
stronger rip currents are expected. By Monday night, the swell
heights are forecast to increase rapidly and the primary hazard
will shift towards large breaking waves of roughly 25 feet at west
and northwest facing beaches prompting dangerous conditions in
the surf zone. A beach hazard statement is in effect 9 a.m. to 5
p.m. Monday for the sneaker wave threat while a high surf warning
is in effect from 5 p.m. Monday through 5 p.m. Tuesday for the
large breaking waves expected. Cold water drownings occur each
year with these type of events but are completely avoidable by
remaining a safe distance from the coastline. If you must visit
the coastline, avoid venturing out on coastal rocks, outcroppings,
jetties, etc, and remain extremely vigilant of your surroundings
at all times.

&&

.FIRE WEATHER...as of 3:54 PM PST Sunday...First and foremost - after
coordination with Sacramento WFO decided to extend Red Flag
Warning through 5 PM Monday to capture lowering RH and peak
heating of the day. That being said, a Red Flag Warning is now in
effect 11 pm Sunday through 5 pm Monday afternoon for the entire
North Bay as well as the the East Bay hills and interior valleys.
In addition the Santa Cruz mtns and coastal San Mateo are included
in the warning.

Not a lot has changed from the previous office thinking except the
extension. Latest hi-res models continue to suggest this will be a
borderline event relative humidity-wise, but gusty winds and
record low fuels remain.

Projected offshore gradient is still expected to hit -15mb from
SFO-WMC. A gradient of that strength will translate to peak gusts
45-60 mph around Mt St Helena and possibly Diablo. Timing of the
peak gradient is still expected to be early Monday morning. Until
then winds will increase across the N and E Bay higher terrain
early tonight before spreading across the rest of the Red Flag
Warning area during early Monday. Lower elevations remain in play
as well thanks to some mix down of the winds. A cross section
extending from near Pt Reyes ne through Mt St Helena shows a small
window for a mountain wave at the onset of the peak wind push.
Shortly there after winds have a better shot at mixing to the
valley floor.

Previous discussion- Fuels are at or near record dry levels
across nearly all fuel classes. The fog and low clouds this
morning along with the return of humidity have of course impacted
the 1 and 10 hour fuels and that matters. However in looking at
the EDDI (evaporative demand drought index) over the last month we
can see a bullseye over the Bay Area. This is of course supported
by the latest Drought Monitor and the massive rainfall deficits
going back to last winter. The Hot/Dry wind index also has has
much of the Bay Area exceeding the 75th percentile suggesting its
going to be a rare combination of strong winds and warm
temperatures. Even tough temps are not explicitly used in the Red
Flag Matrix, we are forecasting record highs on Monday when the
downslope winds will bring record warmth to much of the region
(keep in mind record heat for December is a relative term with
upper 60s and 70s enough to break records). So the combo of strong
winds, record warmth and record dry fuels is just too much to
ignore. Any new ignitions sources could lead to rapid fire growth
overnight into Monday morning. Should we get to lunch time Monday
with no new starts the threat looks to greatly decrease through
the afternoon as winds slowly ease, though humidity values will be
much lower by Monday afternoon, to around 30 percent with little
or no recovery Monday night. Mid and long range forecast remains
dry so any new starts will have room to grow before mother nature
is able to help out.

In terms of Red Flag history, we last issued winter time Red Flags
in December of 2013 and 2017. In addition January of 2014 (peak
of the drought years) had 3 Red Flag events. The last significant
December Fire was the Pfeiffer Fire near Big Sur in Dec 2013.

&&

.MARINE...as of 09:44 PM PST Sunday...Gusty northerly winds
continue overnight across the coastal  waters with gusty offshore
winds spilling over the nearshore  waters tonight and throughout
the day tomorrow. Moderate period  west to northwest swell will
continue overnight into Monday morning and will be overtaken by a
much larger, longer period northwest swell. The combination of
breezy winds and steep seas  will result in hazardous seas
conditions, particularly for smaller vessels. The much larger
northwest swell will peak on Tuesday,  and bring potential hazards
along the immediate coast in the surf  zone.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...Red Flag Warning...
             Wind Advisory...
             SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             GLW...SF Bay from 11 PM

&&

PUBLIC FORECAST: MM/Diaz
AVIATION: Lorber
MARINE: Lorber
FIRE WEATHER: MM/RWW

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