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NWS Forecast Area Discussion - San Francisco Area

000
FXUS66 KMTR 302344
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
444 PM PDT Wed Sep 30 2020

.SYNOPSIS...Very warm to hot and dry conditions will persist over
the region through late week, likely peaking on Thursday afternoon.
This will result an an increase in Heat Risks across interior
locations as well as critical fire weather conditions over the North
Bay Mountains and the Santa Lucia Range where ongoing fires
continue. A region-wide cooling trend is then expected for this
upcoming weekend.

&&

.DISCUSSION...as of 01:25 PM PDT Wednesday...Very warm and dry
conditions persist this afternoon just above the shallow marine
layer currently at around 500-800 ft in depth. Coastal areas and
adjacent valley locations are currently in the 60s to 70s while
far interior locations have warmed into the 80s to lower 90s. Even
some reports in the higher elevations of the Central Coast have
warmed to around 100 deg F, evidence of the hot air mass aloft.
This heat is a result of the building high pressure aloft with
850mb temperatures expected to reach into the mid 20s deg C by
Thursday afternoon. However, with the lack of a more pronounced
offshore flow, confidence is low regarding how much warming will
occur near the coast (including downtown San Francisco) as light
onshore winds are forecast to persist near the surface.
Regardless, triple digit temperatures are likely across the far
interior by Thursday afternoon. Additionally, cooling will be
limited in the region`s higher elevations as overnight lows will
likely only cool into the 70s if not lower 80s. Thus, a Heat
Advisory is in effect for the southern/interior portion of
Monterey and San Benito Counties today and then across all
interior portions of the region on Thursday (including San
Francisco and the Bay Shoreline).

In addition to the heat, critically dry conditions are likely region-
wide from late this evening through at least Friday morning. Given
the sharp temperature gradient from the Pacific to the interior,
onshore winds are likely to become locally breezy in the region`s
higher elevations. These winds combined with ongoing wildfires and
the critically dry air mass will result in localized critical fire
weather conditions, especially over the Dolan Fire and Glass
Incident. Thus, a Red Flag Warning has been posted for these
locations. More details can be found in the Fire Weather Discussion
below.

While the high pressure aloft remains the dominate feature into
Friday, some cooling is likely region-wide as onshore flow
increases. This should bring temperatures back down enough to reduce
the Heat Risks, yet widespread 90s will persist across the interior
with 70s to 80s more common near the coast. Temperatures will cool
further heading into the weekend as the ridge aloft flattens and
beings to weaken. Thus, look for temperatures generally to cool back
to near seasonal averages. Additionally, the return of a deepening
marine layer will bring back low clouds and/or patchy fog during the
overnight and early morning hours. The synoptic pattern does not
change much heading into early next week with weather conditions to
be typical for early October.

Looking farther out, keeping an eye on Tropical Cyclone Marie out in
the eastern Pacific. Operational forecast models continue to show
remnant moisture from this system being advected northward toward
California as an upper level trough digs into the Pacific Northwest
late next week and into the following weekend (October 9th & 10th).
This may bring the potential for precipitation to the Pacific
Northwest and even as far south as northern California. However,
confidence remains low at this time, especially for widespread
precipitation across the greater San Francisco Bay Area. Something
to keep an eye on in the coming days regardless.

&&

.AVIATION...as of 04:42 PM PDT Wednesday...For the 00z TAFs. As
clouds over the ocean erode, there is a shallow layer of a few
clouds at area terminals. Mostly VFR, but linger smoke has dropped
vis below VFR conditions for periods of time. Slant range vis
concerns on approach remain through the TAF period. Expect an
increase in smoke coverage/visibility impacts over the next few
days as northerly winds drive smoke back into the San Francisco
Bay from the North Bay. Winds remain onshore and breezy near
coastal gaps, but will subside overnight. Chances of low level
wind shear concerns remain in the TAFs at times to a number of
terminals across the region. Expect additional low level wind
shear concerns to persist through Thursday night when surface
winds weaken. Localized VLIFR-IFR cigs near the coast tonight. VFR
forecast for Thursday with and onshore seabreeze in the
afternoon.

Vicinity of KSFO...Winds remain around 15kts at this hour, and
will slowly reduce through the evening. Slant wise vis impacts on
approach prevail with chances of reduced vis overnight as winds
aloft may push smoke into the San Francisco Bay. As winds weaken
overnight, chances increase for low level wind shear late tonight
into Thursday and Thursday night. Clouds are expected to remain
mostly near the immediate coastline, and only a few near the
terminal through the TAF period. Winds strengthen Thursday
afternoon.

SFO Bridge Approach...Possible slant-range vis issues with smoke
and haze. Otherwise, similar to KSFO.

Monterey Bay Terminals...Stratus remains just along the shoreline
in the bay with breezy onshore winds. Strong winds aloft tonight
bring the potential low level wind shear concerns late overnight
and into Thursday. Low clouds are expected overnight for IFR/LIFR
conditions. Clearing expected after 16z with an afternoon
seabreeze on Thursday.

&&

.CLIMATE...Here are record high temperatures for Wednesday
and Thursday...

.......................WEDNESDAY      THURSDAY         FRIDAY
SANTA ROSA...........102 in 1980......102 in 1980....105 in 1980
KENTFIELD.............95 in 1966.......97 in 2012....100 in 2012
NAPA.................100 in 1980......106 in 1980....102 in 1980
RICHMOND..............96 in 2001.......99 in 1980....100 in 1980
LIVERMORE............100 in 2012......102 in 1952....106 in 1980
SAN FRANCISCO.........96 in 2001.......97 in 1980.....96 in 1980
SF AIRPORT............94 in 2001.......97 in 1980.....96 in 1980
REDWOOD CITY..........95 in 2001......103 in 1980....104 in 1980
HALF MOON BAY.........92 in 2001.......83 in 2014.....88 in 1995
OAKLAND DOWNTOWN......98 in 1980......103 in 1980....103 in 1980
SAN JOSE..............94 in 2001.......97 in 1980.....96 in 2012
GILROY................98 in 2010......104 in 2001....107 in 1980
SANTA CRUZ............98 in 1978......102 in 1965....102 in 1980
SALINAS...............99 in 2001......105 in 1980....103 in 1980
KING CITY............105 in 1980......109 in 1980....107 in 1980

&&

.FIRE WEATHER...as of 9:15 AM PDT Wednesday...Critical fire
weather conditions are expected to develop over portions of the
district Wednesday and Thursday due to hot, dry and breezy
conditions.

A shallow marine layer developed overnight bringing much needed
relief to lower elevations of the district. However, conditions
above 1,000 feet still remain mild and dry this morning. Satellite
imagery still shows some heat associated with the Glass Fire.
Specifically the hills above the community of Calistoga.

The Fire Weather Watch has bee upgraded to a Red Flag Warning for
the following areas:

Glass Fire: Red Flag Warning 1 pm Thursday through Friday 6 pm.
Yet the fire remains active near Calistoga. Conditions shouldn`t
change too much for today. By midday Thursday models bring some
increasing Northwest (onshore) winds, but with lowering humidity
values. Northwest winds to increase by Thursday evening with gusts
25-30 mph out of the northwest which would be the first time the
Glass Fire feels the effects of gusty northwest winds. Any open or
unsecured line in the East Zone would be adversely impacted as
well as lingering heat near the zone break and Div KK. Conditions
to remain hot, dry and breezy through Friday with persistent WNW
winds with highs still in the 90s. Finally some cooling this
weekend.

Dolan Fire: Red Flag Warning 6 pm Weds through 6 pm Friday. As
early as Weds 6 pm 925 mb winds to 20 mph over the Dolan Fire.
Thus Watch starts for Weds night for ridge top winds 10-20 gusts
25-30 mph. Strongest winds above 2000 feet with little or no
nighttime humidity recovery. Winds may ease at times but models
showing a prolonged breezy wind event lasting Thursday and Friday.
It wont mix down to lower elevations but should impact most areas
of the Dolan Fire with 1-2 days of northeast winds and very low
humidity, dropping into the single digits at night. This will
definitely be a wind test for containment lines as well as
indirect lines. Some signs of cooling by this weekend as the
marine layer deepens.

&&

.MARINE...as of 02:05 PM PDT Wednesday...Breezy northwest winds
across the coastal waters for the rest of the day and coming days
with locally moderate coastal jets along the Sonoma coast, Marin
Headlands, near Pigeon Point, and the Big Sur coast. Gusty winds
along the coast can be expected through the end of the week. Seas
will mainly be a mix of moderate northwest swell and a longer
period, light southerly swell.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...Heat Advisory...CAZ516>518
             Red Flag Warning...CAZ517
             SCA...Pigeon Pt to Pt Pinos 0-10 nm until 9 PM
             SCA...Pt Arena to Pt Reyes 0-10 nm until 11 PM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm until 12 AM

&&

$$

PUBLIC FORECAST: RGass
AVIATION: DK
MARINE: DRP
FIRE WEATHER: MM/RWW

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NWS MTR Office Area Forecast Discussion

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