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NWS Forecast Area Discussion - San Francisco Area

000
FXUS66 KMTR 182353
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
353 PM PST Mon Jan 18 2021

.SYNOPSIS...Warm, dry weather today, along with gusty offshore
winds in the hills of the San Francisco Bay Area. A strong, and
much more widespread, offshore wind event is forecast from this
evening through Tuesday morning. During this time, damaging winds
are possible in the hills and locally in the valleys and near the
coast. Winds will gradually diminish on Tuesday afternoon. A
cooling trend is forecast from Wednesday through Friday and slight
rain chances develop by the end of the week.

&&

.DISCUSSION...as of 01:50 PM PST Monday...Short term weather
impacts for the Bay Area and Central Coast still remain the
same-damaging winds, fire weather concerns, record warmth, and
dangerous surf.

Afternoon satellite shows clear skies across the Bay Area and
Central Coast this afternoon. The clear skies are thanks for a
strong offshore flow and very dry airmass. The offshore pressure
gradient may have weakened slightly since last night, but it still
remain high leading to gusty NE to N winds. Winds are strongest
over the highest terrain with gusts still 40-60 mph. Gusty winds
are mixing to lower elevations with gusts 25-35 mph. A Wind
Advisory remains in effect for most of the Bay Area this
afternoon through Tuesday afternoon. A second, and stronger burst,
of offshore winds are still expected to develop later this evening
and overnight. The second round of wind is in response to an upper
level low forming over NV and deepening and tracking SW overnight.
The upper low eventually becomes cut-off at 500mb off the SoCal.
As this happens the offshore gradient (SFO-WMC) is forecast to
tighten anywhere from -15 to -19 mb tonight and early Tuesday.
Given the forecast areas above 1,000 feet will go to a High Wind
Warning (except interior San Benito) at 4 PM today. Would not be
surprised to see a few high peaks hit 80 mph again. Given the obs
from last night it is totally conceivable that a high peak may get
close to 90-100 mph tonight. Needless to say that damaging winds
are projected with more trees and powerlines coming down again
tonight and Tuesday.

The gusty offshore winds will also lead to above normal
temperatures and very low humidity both day and night today
through Tuesday. A few records have already occurred this
afternoon and the peak heating of the day isn`t over yet. Did an
update to increase today`s and Tuesday`s high temperatures. Windy
conditions and low RH will contribute to Fire Weather Concerns
see below for more details...

Winds will begin to diminish region-wide during the day on Tuesday,
yet remain locally strong and gusty into Tuesday night across the
North and East Bay.

The shortwave trough responsible for the second burst of offshore
winds will quickly exit out of SoCal between Wednesday and
Thursday and mid-level heights will quickly fall all across
California as another shortwave develops and enters the region
during the second half the week. The key difference between this
upcoming shortwave and the one currently pivoting the CWA is its
position as it will be approaching from the PAC and therefore
help introduce more of a maritime air mass into California. As it
approaches the CWA, will see a wind shift to more onshore NW flow
that will promote greater moisture transport all along the
divergent quadrant of the shortwave. As such, GFS and ECMWF are
both hinting at an increased possibility for some showers to roll
in to Northern and Central California by Friday and into part of
the upcoming weekend. This shift in the synoptic trend is
consistent with the long-range outlook for the PNA, which is
progged to be negative through the remainder of the month and into
February. Should this play out, are seeing the possibility for
this winter season to redeem itself. Stay tuned.

&&

.AVIATION...as of 3:52 PM PST Monday...For the 00Z TAFs. Clear
skies across the Bay Area and the Central Coast. VFR is expected
through the entire TAF period at all terminals. The main hazard
for terminals to be aware of is the increasing offshore flow,
which will result in strong wind gusts and LLWS. Observations have
eased slightly for current gusts from 20 to 25 kts, but winds are
expected to strengthen through the evening and into Tuesday
morning. Models consistently showing winds between 1,000 o 1,500
ft between 40-50 kts, with instances exceeding 50 kts. Therefore,
LLWS of about 35-45 kt remain in the TAFs at most terminals. Those
winds are expected to mix down to the surface providing stronger
gusts between 30 to 40 kt, with locally higher gusts possible.
Strong wind gusts are expected to continue into Tuesday, but
gradually diminish late Tuesday morning and into Tuesday
afternoon.

Vicinity of KSFO...VFR. Currently breezy northerly winds. Offshore
winds aloft will increase over the next couple of hours for
concerns of LLWS in the TAFs through the night. These strong winds
are expected to mix down to the surface providing strong wind
gusts between 40 to 50 mph, potentially higher, possible. An
Airport Weather Warning has been issued, valid 06z through 17z
Tuesday morning when the winds are forecast to be their strongest.
Winds are not expected to gradually diminish until late Tuesday
morning and into the afternoon.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR. Winds are breezy at the moment as
strongest winds are still located at and north of San Jose, but
are slowly moving toward the Central Coast. Northerly winds will
increase through the night, increasing the chances for LLWS and
stronger wind gusts to mix down to the surface overnight. Winds
are expected to gradually weaken late Tuesday morning as skies
remain clear.

&&

.BEACHES...as of 02:04 PM PST Monday...A large, very long period
northwest swell has arrived to the coastal waters, creating
hazardous conditions along the coast through the day today. Large
swell heights are expected with breaking waves of 18 to 22 feet,
locally higher at favored breakpoints, along west to northwest
facing beaches. As a result, a High Surf Advisory is in effect
through this afternoon for west and northwest facing beaches while
a Beach Hazard Statement is in effect for the northern Monterey
Bay including Santa Cruz. With warm temperatures present over a
holiday weekend, continuous vigilance is advised. Never turn your
back to the ocean.

&&

.CLIMATE...Record Type: Highs (Old Record Temp/Year)

                    Jan 18th

Santa Rosa          74/1976
Kentfield           70/2014
Napa                76/1920
Richmond            71/2018
Livermore           74/1919
San Francisco       70/1920
SFO                 68/1948
Redwood City        74/2009
Half Moon Bay       72/2009
Oakland Downtown    73/2014
San Jose            74/1920
Gilroy              80/2014*
Santa Cruz          83/1920
Salinas Airport     81/1994
King City           83/2009

*Monthly Records

&&

.FIRE WEATHER...as of 12:30 PM PST Monday...First and foremost,
the Fire Weather Watch was upgraded to a Red Flag Warning earlier
this morning. An updated fire weather forecast was also sent out
to highlight the Red Flag Warning. Otherwise, forecast remains on
track. A very dry airmass continues to settle over the district
with impressive RH readings - a few sensors are showing humidity
readings in the teens and upper single digits. Additionally, the
very gusty offshore flow continues with gusts 40-65 mph. Needless
to say, the combination of winds and low RH would normally
heighten fire weather concerns. Fortunately, early season rains
and some "green up" has tempered a widespread fire weather threat.
However, Monterey/San Benito counties received much less rainfall
and elevations above 1,000 feet will be at critical levels.
That is why a Red Flag Warning has been issued for the Central
Coast. Winds may ease a tad on Tuesday, but low RH will remain
keeping the fire weather threat through the afternoon on Tuesday.
Conditions will continue to improve on Wednesday. A storm system
will bring cooler temperatures and possibly some precip by next
weekend.

&&

.MARINE...as of 02:00 PM PST Monday...Northerly winds will
continue to increase over the waters into tonight with occasional
low end gale-force gusts possible. Winds will pick up
substantially tonight as a burst of strong offshore winds over
land spills over into the waters. Widespread gale force conditions
will then persist through tomorrow morning over the entire
coastal waters including the San Francisco and Monterey Bays. This
will result in widespread hazardous conditions. A large, very
long period northwest swell will continue to impact the waters
into this evening bringing additional hazards. The next long
period northwest swell will arrive mid-week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...Red Flag Warning...CAZ517-518
             Wind Advisory...CAZ518-528-530
             High Surf Advisory...CAZ006-505-509-530
             High Wind Warning...CAZ507-511-512-517
             Wind Advisory...CAZ507-511-512
             Wind Advisory...CAZ006-505-506-508>510-513-529
             SCA...Mry Bay until 9 PM
             SCA...Pt Arena to Pt Reyes 0-10 nm until 9 PM
             SCA...Pigeon Pt to Pt Pinos 0-10 nm until 9 PM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm until 9 PM
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm until 9 PM
             GLW...Mry Bay from 9 PM
             GLW...Pigeon Pt to Pt Pinos 0-10 nm from 9 PM
             GLW...Pt Pinos to Pt Piedras Blancas 0-10 nm from 9 PM
             GLW...Pt Arena to Pt Reyes 0-10 nm from 9 PM
             GLW...Pigeon Pt to Pt Piedras Blancas 10-60 nm from 9 PM
             SCA...Pt Arena to Pigeon Pt 10-60 nm until 3 AM
             GLW...Pt Arena to Pigeon Pt 10-60 nm from 3 AM
             SCA...Pt Reyes to Pigeon Pt 0-10 nm until 6 PM
             GLW...Pt Reyes to Pigeon Pt 0-10 nm from 6 PM
             SCA...SF Bay until 5 PM
             GLW...SF Bay from 5 PM

&&

$$

PUBLIC FORECAST: MM/Diaz
AVIATION: DK
MARINE: AS
FIRE WEATHER: MM

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