Strong Wind in California, December 16, 2017

Area Forecast Discussion
National Weather Service San Francisco Bay Area
932 AM PST Sat Dec 16 2017

...Locally breezy conditions likely through Sunday morning...
...Elevated fire danger particularly for higher elevation
locations for the same time...

.SYNOPSIS...Gusty north winds will prevail today through early
Sunday in the wake of a dry cold front.  These winds will also bring
critical fire weather conditions to many higher elevation locations.

&&

.DISCUSSION...as of 09:30 AM PST Saturday...An elongated
vorticity lobe pushed inland from the coast overnight, passed over
our region early this morning, and is now moving eastwards beyond
the Sierra Nevada. In the wake of this disturbance, northerly
winds have drastically increased resulting in a wind advisory and
red flag warning for higher elevations as well as gale warnings
over the waters.

Earlier this morning, the strongest winds were confined to the
higher elevations and coastal waters due to the presence of a
nocturnal inversion decoupling the lower atmosphere from the
stronger winds aloft. These stronger winds aloft began to mix down
to the surface as the sun rose and mixed out the nocturnal
inversion, with several spotters (from Santa Cruz, Santa Clara,
and Napa county) calling in reports of the initial strong gusty
north winds between 7 and 9 am.

So far this morning, the strongest winds (42 mph sustained/ 61mph
peak gust) were (and currently are) at the Los Vaqueros RAWS at
1100 feet, located southeast of Mount Diablo and north of the
Altamont pass. Elsewhere, gusts in excess of 45 mph has been
reported at many diverse locations, including: Mt Diablo, Hawkeye
RAWS (NE of Geyeserville), Mt Tamalpais, Bodega Bay, Point Reyes,
Oakland Hills, Half Moon Bay, inland San Benito county, through
the Carquinez Strait/inland Delta, portions of the Big Sur
mountains, and the coastal San Mateo mountains.

Our main forecast challenge of the day will be to continue to
monitor the critical fire weather conditions both in terms of wind
and minimum relative humidity values. Please be mindful of the
environmental conditions today in you are planning any out door
activities that could result in unintended fire ignition. The
current forecast looks on track so no changes are planned until
the afternoon package is issued early this afternoon.

.PREVIOUS DISCUSSION...as of 3:00 AM PST Saturday...Have been
keeping a close eye on surface observations during the midnight
shift as as a system crossed our region yesterday evening.
However, so far only a few locations have been gusting over 30 mph
with always windy Mount Diablo back under 40 mph. Models continue
to show stronger winds through the morning with greater pressure
gradients and continued CAA. 850 MB values off the latest NAM have
a bullseye of nearly 60 KT over the North Bay Mountains close to
noon. 850 MB speeds for other spots show the potential for 40 to
50 mph. Not clear that we will see speeds that high based on how
current values compare to expected numbers, but feel that caution
remains the best course of action. Therefore, will keep the wind
advisory going into Sunday morning.

Other concern for higher elevations remains for fire weather with
relative humidity readings expected to quickly drop through the
day and barely recover tonight. Would expect spots above 1,000
feet to drop into the 10 to 20% range later today with many likely
to be under 30% for most of the night. Will keep the red flag
warning going.

Synoptically we will see a ridge of high pressure attempt to
rebuild into our region from the west, but that will quickly
flatten as a system approaches from the north. Models have been
flipping between some moisture and no moisture depending on the
expected track. Models are trending farther to the east which
favors drier weather, so PoPs were decreased. Still looks like an
outside chance for light rainfall mainly over the North Bay
Tuesday night. Any amounts will be very light -- less than a tenth
of an inch.

After that a cool, dry, northerly flow will take shape as the
ridge becomes extremely amplified off the coast. That should lock
into place at least through Christmas Day. For those with travel
plans the end of next week, driving conditions should be ideal.

&&

.AVIATION...As of 3:50 AM PST Saturday...For 12z TAFs. Patchy low
clouds and fog are developing over inland valleys this
morning...otherwise mainly clear skies expected to prevail in the
post frontal environment. Strong and gusty northerly winds
expected today. Windy conditions will persist through the weekend.
Sustained winds in the 15 to 25 kt range with gusts from 20 to 30
kt expected across area terminals. Gusty winds also expected over
mountain ridges and through coastal gaps.

High confidence.

Vicinity of KSFO...VFR. Northwesterly winds will strengthen
later this morning in the wake of the front that moved through
last night. Sustained winds in the 15 to 25 kt range with gusts
from 25 to 30 kt expected.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR. Winds will strengthen and become
gusty later this morning. Sustained winds in the 10 to 15 kt
range with gusts around 20 kt expected.

&&

.FIRE WEATHER...as of 3:15 AM PST Saturday...A Red Flag Warning
remains in effect through 10 AM Sunday. Offshore flow is forecast
to strengthen through the day. Latest models continue to show RH
values dropping through day and poor recoveries Saturday night.
It's fairly common to have lower RH on night number two behind a
front with developing offshore flow. May not technically hit RH
criteria the first part of today, but decided to keep fire
weather concerns going given gusty winds, lack of recent rain,
very low RH (single digits) the last few nights and near record
values of ERCs for the N Bay Mts and Santa Cruz/Diablo PSAs. Areas
of greatest concern will be above 1000 feet. Winds may be a tad
weaker on Saturday night, but RH recoveries worse. For what it's
worth, fire weather concerns are not that unusual for this time of
year. December 16, 2013, a wildfire broke out in Big Sur called
the Pfeiffer Fire.

&&

.MARINE...as of 08:52 AM PST Saturday...Strong and gusty north
winds will prevail in the post frontal environment through today
and Sunday. The strong and gusty winds  will create very steep
wind waves and fresh swell, creating very  rough and hazardous
seas through the majority of the weekend.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...Wind Advisory...CAZ507-511-512-517-518
             SCA...Pigeon Pt to Pt Pinos 0-10 nm
             GLW...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm until 9 AM
             GLW...Pt Pinos to Pt Piedras Blancas 0-10 nm from 9 AM
             GLW...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm from 9 AM
             GLW...Pt Arena to Pt Reyes 0-10 nm until 9 AM
             SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...SF Bay from 9 AM
             SCA...Mry Bay from 9 AM

&&

PUBLIC FORECAST: DRP/BELL
AVIATION: CW
MARINE: CW

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