Wildfire and Rip Current in California, September 27 to 30, 2020

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1040 PM PDT Sat Sep 26 2020

.SYNOPSIS...Offshore winds are forecast to develop in the hills
of the North and East Bay tonight and Red Flag Warnings go into
effect starting at 9 PM. The offshore winds will mix down to the
surface on Sunday, resulting in hot and dry conditions across the
entire Bay Area with warm temperatures developing south to the
Central Coast. A second burst of offshore winds is forecast Sunday
night into Monday and Red Flag Warnings remain in effect for the
entire North and East Bay through 9 pm Monday. Sunny and hot
weather will continue into Monday with near record high temps in
the 90s and lower 100s. A Heat Advisory is in effect from 11 am
Sunday to 7 pm Monday for all interior portions of the San
Francisco Bay Area, as well as the San Francisco Bay Shoreline
and City of San Francisco. Coastal areas south of San Francisco
may begin to cool by Monday afternoon as southerly winds develop.
Much cooler conditions will develop in all coastal areas by
Tuesday, with slight cooling expected inland. But inland areas
will remain very warm and dry throughout next week.

&&

.DISCUSSION...As of 9:20 PM PDT Saturday...Evening water vapor
satellite imagery shows an upper trough tracking across the
Northern Rockies and over the Northern Great Plains. In the wake
of this upper trough, surface high pressure has been building to
our north over Oregon and far northern California. The result is a
robust 8 mb surface pressure gradient from ACV to SFO which has
swept low clouds from nearly our entire coast. Coastal profilers
show a well-defined marine inversion at about 800 feet at Fort Ord
and less than 500 feet at Bodega Bay. So it's possible we may see
some redevelopment of coastal stratus/fog overnight in this
shallow marine layer. However, given the strong northerly gradient
down the coast and the shallow nature of the marine layer, it's
likely that low clouds/fog will only impact the immediate coast
tonight, and mainly from Monterey Bay southward.

Primary concerns over the next two days center around heat and
offshore winds. Winds in the hills of the North and East Bay will
increase and veer from northwest to north overnight as the
nocturnal inversion develops and as surface high pressure
continues to strengthen to our north and northeast. Expect winds
gusts as high as 50 mph across the higher ridges and peaks of the
North Bay after midnight tonight, with gusts up to 35 mph in the
East Bay Hills. These offshore winds will bring a drier airmass
into our region starting tonight. The combination of gusty
northerly winds and lack of overnight humidity recovery in the
hills will result in critical fire weather conditions. A Red Flag
Warning goes into effect for the entire North Bay, as well as the
East Bay Hills and Valleys, starting at 9 PM this evening.

A low amplitude upper ridge currently centered offshore along 135W
is forecast to shift east and amplify over the next 36 hours.
Airmass warming and drying under this ridge will contribute to a
strong warming trend across our area on Sunday and Monday. This
airmass warming, combined with offshore flow, will mean very warm
to hot conditions in most areas, even close to much of our coast,
on both Sunday and Monday. Widespread 90s and lower 100s are
forecast across most inland areas on Sunday, with upper 70s to
lower 90s close to the coast. Even warmer temperatures are
forecast for Monday when the heat event is expected to peak. A
Heat Advisory is in effect from 11 AM Sunday to 7 PM Monday for
the entire San Francisco Bay Area, except the San Mateo County
Coast. The Monterey Bay Area southward is not included in the Heat
Advisory for a couple of reasons. First, offshore flow is not
expected to be as strong farther to the south. Also, southerly
flow is forecast to develop along our southern coast by Monday
morning, which will result in cooling from Santa Cruz county
southward on Monday.

Offshore winds in the hills of the North and East Bay may mix into
the valleys by late Sunday morning as the nocturnal inversion
breaks. Although winds aren't expected to be all that strong at
lower elevations, they will likely be strong enough to result in
critical fire weather conditions when combined with the hot
temperatures and very low relative humidity. This is why the North
and East Bay Valleys are included in the Red Flag Warning. Winds
will then increase in the hills again on Sunday night and veer
more to the northeast and east and develop close to the North Bay
coast. Winds will finally subside in all areas by late Monday
morning. However, critical fire weather conditions will persist
through late in the day on Monday due to hot and very dry
conditions.

As mentioned previously, this upcoming heat event is expected to
peak on Monday in most areas, except southern coastal areas due to
development of south winds there. By Tuesday, southerly winds are
forecast along our entire coast and onshore flow will begin to
redevelop at the surface as the upper ridge axis shifts to our
east. Thus, all coastal areas are expected to see significant
cooling by Tuesday. Inland areas will likely cool too on Tuesday,
but by only a few degrees.

Models agree that the upper ridge will remain anchored slightly to
our east through the remainder of next week. This will maintain
very warm temperatures inland. Weak onshore flow will maintain
more mild conditions near the coast.

&&

.AVIATION...As of 10:40 PM PDT Saturday...For the 06z TAFs. VFR
with high clouds still moving overhead for much of he region.
Surface winds continue to decrease while winds aloft increase and
turn to the NNE for chances of LLWS for KSTS, KAPC, and KLVK
overnight and early Sunday morning. Limited to no marine layer
influence is forecast overnight. Smoke from Northern California
pushed into the North Bay provides chance for slant range vis
issues to increase overnight, but not currently anticipating
significant dense smoke issues. VFR is forecast through the TAF
period with breezy winds in the afternoon with a few stronger
gusts. Winds decrease again Monday night and little cloud cover.

Vicinity of KSFO...Remains VFR with passing high clouds. Winds
are still breezy, but continue to reduce. Haze is possible
tonight and Sunday with the northerly wind shift moving smoke into
the Bay Area. VFR expected through the TAF period with limited
marine layer influence Winds will become breezy with occasional
stronger gusts Sunday afternoon before easing Sunday night.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR through the TAF period. Low
confidence in slant range vis issues beginning late tonight, but
with winds aloft becoming NNE, smoke might move near terminals at
higher elevations. Winds are light, yet breezy in the Salinas
valley. VFR forecast Sunday with breezy onshore winds in the
afternoon. Winds ease Sunday night with little cloud cover.

&&

.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

&&

.FIRE WEATHER...As of 9:15 PM PDT Saturday...Northeast winds
first impact the Napa hills after 9 pm tonight with initial burst
of offshore winds staying in the hills. By late Sunday morning the
winds may start to mix down into the valleys of the North and
East Bay. A second burst of gusty offshore winds are expected in
the North and East Bay Hills on Sunday night when very dry
conditions will be occurring across the entire region. Northeast
to east winds Sunday night into Monday morning will drive all the
way to the coast of Sonoma County southward to Half Moon Bay as
dry offshore flow spills towards the ocean. Offshore winds are
expected to ease in all areas by midday Monday. Heads up on the
Dolan Fire Monday as southerly winds are forecast to develop along
the Big Sur Coast and Los Padres Forest on Monday. Long range
trends remain very warm and dry through early October.

&&

.MARINE...as of 08:30 PM PDT Saturday...North-northwest winds
with strong gusts continue this evening, making for hazardous
conditions for small craft vessels. Winds will decrease over the
southern waters by Sunday afternoon with locally breezy conditions
across the northern outer waters into Sunday evening. By Monday,
high pressure will shift east allowing the winds to subside. Wind
direction will pivot to the north and east Sunday evening before
turning southeast on Monday. Northwest swell will gradually
subside over the weekend while a light southerly swell also
continues.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...Red Flag Warning...CAZ505>507-510-511
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm until 3 AM

&&

PUBLIC FORECAST: Dykema
AVIATION: DK
MARINE: DK
FIRE WEATHER: RWW/Dykema

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