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

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321
FXUS66 KMTR 260330
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
830 PM PDT Fri Sep 25 2020

.SYNOPSIS...Noted warming trend starts by Saturday afternoon as
offshore winds begin to develop. Offshore winds increase Saturday
into Sunday morning with Red Flag Warnings in place for the North
and East Bay hills for critical fire weather conditions. Winds
will ease on Sunday but hot and dry weather across the region.
Expect more offshore winds Sunday night into Monday morning with
very warm and dry air in place. Continued unseasonably hot and dry
Monday and Tuesday with only gradual cooling by midweek. West
coast ridge stays in place into early October.

&&

.DISCUSSION...as of 08:30 PM PDT Friday...The forecast remains on
track. Moderate warming is still expected for Saturday. Coastal
areas are forecast to see highs in the 70s, while inland areas are
set to near the 90 degree mark. Primary concerns in the short
term remain with the Red Flag Warning that goes in effect 8pm
Saturday and lasts into Monday morning. Expect dry air, strong
offshore winds over higher elevations across the North and East
Bay, and poor nighttime humidity recoveries. Additional concerns
will be the increasing heat over the weekend. High confidence
remains for Sunday and Monday offering the warmest temperatures
across the Bay Area and into the interior. Concerns for prolonged
heat exposure are increasing for inland areas and upper altitudes
as models show only slight decreases in temperature into the mid
week. Some of these areas look to only reduce high temperatures by
as little as a degree per day through next week. Long term models
are starting to agree on the next weekend still offering around
590daM heights at 500mb. Neither the GFS nor the EURO offering a
significant change in the upper level pattern for the first few
days of October. For additional information see the previous
discussion.

&&

.PREV DISCUSSION...as of 1:45 PM PDT Friday...The latest water vapor
imagery captures the moisture plume moving into the PAC NW
(including far northern California) this afternoon as a cold front
makes its way ashore. Meanwhile over our region, onshore flow has
allowed for another day of seasonal temperatures. Clear skies are
expected for most of the CWA for the rest of the afternoon, while
high-based clouds occasionally roll their way overhead. For
tonight, rinse and repeat to last night`s conditions.
Unfortunately following this fair weather day, the warming trend
looks set to get going by tomorrow.

The GFS and ECMWF guidance builds the upper-level PAC ridge and
brings it up to the West Coast through Saturday, introducing 591
dm heights back into the CWA. As it does so, high confidence that
it will build to 592-595dm 500 hPA and for its axis to remain just
offshore through early Tuesday. NAM 850 hPa temps quickly build
across the CWA Saturday afternoon, surpassing 20 C over most of
the region. The culmination of the upper-level ridge and the
surface thermal trough will result in hot conditions in the
interior and warm conditions along the coast, with Sunday and
Monday looking like the hottest days during this setup. Expecting
Sunday and Monday temps to range between 90-102 F in the interior
and for coastal temps to stay around the 80s F. It should be noted
that we will once again observe a notable temperature gradient
over SF, as the western side of the city is progged to reach the
upper 70s while downtown and the high-terrain locations of SF
County may make it up to 90 F, especially by Monday.

On top of hot temperatures this weekend, the position of this upper-
level ridge with respect to the deepening upper-level trough over
the center CONUS will play out in the form of this weekend`s
inside- slider.

The latest runs on the NAM have suggests that
northerly/northeasterly offshore winds will begin during late
Saturday evening, with previous local WRF runs depicting the 925 hPa
wind shift just after 2Z Sunday (7PM Saturday) over the North Bay
before permeating and mixing down into the East Bay hills as well,
especially during the overnight period from Sunday night going into
Monday morning. Will be keeping an eye on later runs of the WRF
as we get closer to the event start should any other modifications
be required for our primary areas of concern.

The Red Flag warning is set to start at 9pm Saturday night for
the North and East Bay hills as well as the East Bay interior
valleys. The strongest winds are still looking to remain at higher
terrain locations, and as such locations below 1000 ft will not
experience the same windy conditions. That being said, these winds
aloft will substantially dry out the airmass, resulting in low
RHs all across the CWA. Initially, expecting RH values in the
30-35% range across most of those higher-terrain locations
overnight Saturday into Sunday morning, but conditions will only
continue to dry out through Sunday. As such, high confidence for
there being little, if any, fog Sunday morning across the Bay
Area. Although there may be some patchy spots along the Monterey
Peninsula southward given that the dry air mass will take some
time to get that far into our CWA.

The dry air mass will continue to decimate RH values through the
day, dropping them down into the teens across the higher-terrain
North Bay and East Bay hills overnight Sunday into Monday. It is
this period where the greatest fire weather concerns are currently
progged as such dry conditions associated with this offshore flow
will make for dangerous fire start conditions. As such, the Red Flag
Warning is currently out until Monday morning; offshore winds
will dominate the region until at least Monday morning when winds
area- wide are expected to shift to a more southerly component as
the upper-level ridge settles over the northwestern corner of
Nevada.

In terms of air quality, these offshore winds will help to also
introduce smoke from the August Complex back down to the North Bay
and even parts of the SF Peninsula. Thankfully, the smoke canopy
we are dealing with right now is nowhere near as large as what was
observed during our prolonged smoke situation from a couple of
weeks ago.

For the latest on air quality concentrations in the Bay Area, be
sure to follow the Bay Area Air Quality District. And for all other
locations within the CWA, including the Central Coast, be sure to
refer to AirNow. For the latest on active wildfires in the area,
please refer to CALFIRE and your local Sheriffs Office.

Looking ahead, GEFS and ECMWF ensemble members are in good agreement
that this weekend`s upper-level ridge over the West Coast will
settle over the PAC NW, weaken, and amplify over most of the
Mountain West. As such, temps will likely remain above average
through the end of the month, with the ensembles suggesting that
temps for those days will remain below what we are expecting for
this upcoming weekend. Given that the California coast will also be
on the side of the upper-level ridge with southerly flow, cannot
rule out the slight potential for the development of a southerly
surge towards the end of the month. Will keep an eye on it following
the inside-slider event, but for now there is low confidence on
that. Beyond the end of the month, long-range guidance shows that we
may not receive much, if any, precipitation through the first week
of October either as this Western US Ridge- Eastern US Trough set up
looks set to continue for quite some time. Temps will likely remain
above average for some time as well, assuming there is no notable
southerly surge. Stay tuned.

&&

.AVIATION...As of 04:45 PM PDT Friday...For the 00z TAFs. VFR.
Winds are currently onshore with occasional stronger gusts. The
marine layer will be shallow overnight, with limited impacts
around the region. As the next air mass moves over the Pacific
Northwest, the prevailing pattern changes to stronger offshore
flow. As the wind shifts direction from W to NW to NNE overnight
and into Saturday morning, increasing chances for LLWS,
particularly in the North Bay possible. As stronger NNE winds are
expected through Monday morning there is a chance for more
widespread LLWS impacts across the Bay Area terminals that will be
monitored through Monday morning.

Vicinity of KSFO...VFR with strong onshore wind gusts up to 30 kts.
As winds reduce overnight, but remain breezy, the marine layer
will be shallow for limited cloud cover expected, lingering
around 1000 feet. Best chances for thicker clouds early Saturday
morning between 10-16z. VFR forecast for Saturday as winds with
stronger gusts are forecast yet again.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR with onshore, breezy winds. Cloud
cover returns this evening after 3z for IFR conditions, with less
reach farther inland. VFR forecast after 17z with breezy winds.

&&

.CLIMATE...Here are the record high temperatures for Saturday
through Monday...

......................SATURDAY.........SUNDAY..........MONDAY
SANTA ROSA...........103 in 2009....103 in 2010.....104 in 2010
KENTFIELD............ 99 in 2016....103 in 1921.....102 in 1921
NAPA.................105 in 1963....101 in 1958.....102 in 1966
RICHMOND..............97 in 2019.....97 in 2010......97 in 2010
LIVERMORE............103 in 1952....105 in 1963.....105 in 2010
SAN FRANCISCO.........94 in 1992.....93 in 2010......95 in 1966
SF AIRPORT............95 in 1958.....96 in 2010......95 in 2010
REDWOOD CITY..........99 in 1963.....97 in 2010......98 in 2010
HALF MOON BAY.........90 in 1970.....89 in 1966......91 in 1958
OAKLAND DOWNTOWN......93 in 2016.....95 in 1973......92 in 2010
SAN JOSE..............98 in 2016.....98 in 1921......99 in 2010
GILROY...............104 in 1963....108 in 1963.....101 in 1963
SANTA CRUZ...........101 in 1970....103 in 2010.....100 in 1917
SALINAS..............101 in 2016....102 in 2010..... 99 in 1970
KING CITY............105 in 1963....107 in 2010.....102 in 1973

&&

.MARINE...as of 08:26 PM PDT Friday...Prevailing northwest winds
will remain gusty across the coastal  waters through Saturday,
making conditions hazardous for small  craft vessels. A long
period northwest swell continues to decrease as it moves through
the waters tonight and through Saturday.  Winds will diminish
Sunday across the waters but remain breezy with occasional
stronger gusts. Winds are forecast to shift  offshore on Monday
with locally gusty offshore winds through  coastal gaps.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Pigeon Pt to Pt Pinos 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Mry Bay

&&

$$

PUBLIC FORECAST: Diaz/BKM
AVIATION: DK
MARINE: DK

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