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

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FXUS66 KMTR 161815
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1015 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...Dry weather will continue into next week. A mainly
dry cold front will sweep south over the area late Friday night
and early Saturday morning which then increases N-NE winds along
the ridges and peaks today.

&&

.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 10:15 AM PST Saturday...18Z taf package. VFR
conditions will dominate airports around the Bay Area/Monterey Bay
regions. The main story will be gusty northerly winds. Local
observations have already shown increasing winds around the area,
such as gusts to 61 mph east of Livermore. As daytime warming
occurs vertical mixing should become more effective, which will
help to bring some of the higher level winds to the surface. As
such, strong gusty conditions remain in the tafs today. The
strongest winds will remain just off the surface, so wind shear
remains or was introduced for all taf sites. Winds will begin to
ease late afternoon and early evening from north to south.
Confidence in timing is high and moderate to high for wind speeds.
If anything, winds could be a little stronger than forecast.

Vicinity of KSFO...VFR. Northerly winds will continue to increase
today in the wake of a dry cold front. Sustained winds in the 15
to 25 kt range with gusts to 35 kt are expected.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR. Northerly winds will increase midday
into afternoon today. 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...
             SCA...Pigeon Pt to Pt Pinos 0-10 nm
             GLW...Pt Arena to Pigeon Pt 10-60 nm
             GLW...Pt Pinos to Pt Piedras Blancas 0-10 nm
             GLW...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...SF Bay
             SCA...Mry Bay
             SCA...Pt Reyes to Pigeon Pt 0-10 nm

&&

$$

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

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