Strong Wind in California, November 7 to 8, 2020

Area Forecast Discussion
National Weather Service San Francisco Bay Area
954 PM PST Sat Nov 7 2020

.SYNOPSIS...Gusty northwest winds are expected overnight and
through Sunday morning, especially near the coast. A weather
system approaching from the north will bring scattered showers
late tonight and Sunday, primarily to areas south of San Jose.
Gusty and dry northerly winds are then expected in the hills from
Sunday afternoon through Monday morning, mainly across the higher
elevations of the North and East Bay. Cold overnight lows are
forecast both Sunday night and Monday night with widespread 30s
expected in the valleys, and some upper 20s possible in the
coldest inland valleys. Dry conditions with mild daytime
temperatures and cool nights are forecast for much of next week
with increasing rain chances late in the week.

&&

.DISCUSSION...As of 9:05 PM PST Saturday...Our region is in
between two cold weather systems this evening and the primary
short-term forecast concern is the magnitude of gusty northwest
winds overnight and into Sunday morning. Locally breezy conditions
are currently being observed, mainly near the coast and in the
hills where gusts of 25-35 mph are being observed. The 00Z local
WRF model indicates northwest winds will increase through the
remainder of the evening and peak between about midnight tonight
and mid-morning on Sunday. Strongest winds will be near the coast
and locally in the hills. In these areas wind gusts up to 40 mph
are likely. A few coastal locations, such as Point Reyes, and
windier spots in the hills may see gusts up to 50 mph which is in
Wind Advisory territory. However, expect winds of this magnitude
to be confined to isolated windier locations and not be
widespread, so a Wind Advisory is not anticipated at this time.
If the 00Z models indicate a trend towards stronger winds, a Wind
Advisory will be issued. Northwest winds are forecast to
gradually subside from mid-morning Sunday onward, but breezy
conditions will persist in coastal areas through the day.

Evening water vapor satellite imagery shows the next cold upper
trough dropping south out of the Pacific Northwest and into
northern California. Because this system originated over western
Canada, its dry continental airmass does not contain much
moisture. However, as this system continues to dig south, it will
carve out a bit more over-water trajectory and pick up some
additional moisture. Scattered showers are possible from late
tonight through much of Sunday, but primarily across areas from
San Jose southward. Rainfall amounts are expected to be very
light, generally less than a tenth of an inch, except in the Santa
Lucia Mountains of Monterey County, the Big Sur Coast, and
southern Diablo Range where locally up to a quarter inch, or even
a half inch of precipitation is possible by late Sunday.

Even before scattered shower activity ends across the southern
half of our area on Sunday afternoon, much drier air will begin to
sweep across the San Francisco Bay Area as winds veer from
northwest to north in the hills. Locally gusty, dry north winds
are likely in the hills of the North and East Bay from late
Sunday through early Monday, mainly across the higher elevations
of Napa County. Humidity values will drop rapidly on Sunday and
poor overnight humidity recovery is expected on Sunday night.
These developments will raise fire weather concerns in the North
and East Bay Hills since recent rains mostly skipped this area and
fuels remain very dry. Winds will subside in all areas by midday
Monday.

Skies will clear from north to south from Sunday afternoon through
Sunday evening as drier air spills south. This dry airmass will
also be cold with 850 mb temps forecast to drop as low as -1 deg C
tomorrow. This cold/dry airmass will set the stage for cold
overnight temperatures on both Sunday night and Monday night when
many of our interior valleys are expected to cool to around
freezing. Temperatures in the North Bay Valleys will be tricky to
forecast for Sunday night. If gusty north winds in the hills
manage to mix down into the valleys, then temps won't get all that
cold. But given that moderate magnitude of the expected winds in
the hills, it's unlikely much of that wind will make it into the
valleys. Frost Advisories may be issued for the North Bay Valleys
for late Sunday night and early Monday morning and potentially for
the following night as well. Even colder temperatures are likely
in the southern interior valleys of Monterey County where mid to
upper 20s are possible. In those areas, Freeze Warnings may be
needed starting Sunday night. The coldest nights will be Sunday
night and Monday night with gradual nighttime temperature
moderation expected thereafter.

Besides chilly nights, our weather during the first half of next
week looks to be uneventful, with mostly sunny days and seasonable
daytime temperatures in the upper 50s to mid 60s. Rain chances
return late next week, primarily across the northern portion of
our area. In the longer range, model ensemble means indicate the
development of a longwave trough off the west coast starting
around the middle of the month, which would mean greater potential
for widespread soaking rains.

&&

.AVIATION...As of 09:53 PM PST Saturday...for 06z TAFs. There are
a few clouds over the San Francisco Bay, with a denser patch of
clouds to the South Bay and down the Central Coast. That being
said, cigs are between 2000-3000 feet for VFR/MVFR conditions.
Clouds will increase through the overnight hours but are not
expected to lower more than 1500 feet. The main impacts through
the night will be the strong northwest winds. While strong gusts
have reached KSFO, other area terminals remain fairly light to
breezy keeping the chance for LLWS possible as stronger winds are
aloft. That being said, winds will increase overnight with
stronger gusts, though recent models are trending later that what
was originally forecast. These gusts are forecast to persist
through Sunday, as clouds scatter out after 18z for VFR in the
afternoon. Terminals should see gusts weaken around 00z with
ending Sunday evening.

Vicinity of KSFO...VFR with FEW clouds at 2500 feet. MVFR
conditions forecast as cloud cover increases overnight. A stray
pop-up shower is still possible near the terminal, but confidence
remains low with no impacts expected. Onshore winds are already
gusting between 25 and 29 knots and are only expected to increase
through the night. Strongest wind gusts are forecast to be between
35 to 40 knots. These gusts are expected decrease slightly after
17z but remain through the day. VFR forecast for Sunday afternoon
with wind gusts waning Sunday evening as VFR continues through the
end of the period.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR/MVFR conditions expected through the
overnight hours as cloud cover increases and lowers slightly.
Still a chance for a pop-up shower, but confidence remains low.
Winds are still light to moderate and onshore. Winds will increase
overnight with strong wind gusts expected with peak gusts Sunday
morning between 20-25 kts. VFR forecast for Sunday afternoon as
the wind gusts persist. Expect clearing skies as wind gusts
diminish Sunday evening.

&&

.MARINE...As of 08:30 PM PST Saturday...Strong northwest winds
prevail with gale force wind gusts expected across the majority of
the coastal waters overnight tonight and through most of Sunday.
These strong winds will produce steep wind waves and large
northwest swells through Sunday. There is a slight for a pop-up
shower over the waters, but confidence is low. By Monday, winds
will decrease with mixed moderate period northwest and southerly
swells remaining.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...Wind Advisory...CAZ006-505-507>509-511-512
             GLW...Pt Arena to Pt Reyes 0-10 nm
             GLW...Pt Reyes to Pigeon Pt 0-10 nm
             GLW...Pigeon Pt to Pt Pinos 0-10 nm
             GLW...Pt Pinos to Pt Piedras Blancas 0-10 nm
             GLW...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             GLW...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Mry Bay
             SCA...SF Bay

&&

PUBLIC FORECAST: Dykema
AVIATION: DK
MARINE: DK

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