Small Craft Advisory 209, NWS San Francisco Bay Area, CA

Area Forecast Discussion
National Weather Service San Francisco Bay Area
910 PM PDT Mon Oct 7 2019

.SYNOPSIS...Cooling is forecast on Tuesday as onshore flow
returns. Tuesday's cooling will be most pronounced near the
coast. Strong, gusty and dry northerly winds are then forecast to
develop across the greater San Francisco Bay Area by late Tuesday
night and continue through Thursday. These conditions will result
in critical fire weather conditions from Wednesday into Thursday,
especially across the North Bay, East Bay, and Santa Cruz
Mountains. Warmer daytime temperatures are expected late in the
week, followed by modest cooling during the upcoming weekend.

&&

.DISCUSSION...As of 9:10 PM PDT Monday...Light offshore flow
persisted longer than expected today and it wasn't until late in
the day that a weak seabreeze finally developed. Consequently,
most locations reported high temperatures today that were as warm
as yesterday, or even warmer. Only a few locations very close to
the ocean experienced appreciable cooling, such as Half Moon Bay,
Sea Ranch, and San Francisco Oceanside. Models indicate that
onshore flow will remain weak through Tuesday morning and only
begin to increase on Tuesday afternoon. Thus, most cooling on
Tuesday will be in coastal areas where temperatures are forecast
to cool by anywhere from 7 to 15 degrees. Farther inland, the
seabreeze probably won't arrive early enough to cool temperatures
in the interior valleys by more than 5 degrees or so.

Latest satellite imagery shows an upper trough centered along the
coast of British Columbia dropping south into Washington and
Oregon. Models have been consistent in digging this trough to the
southeast and into the Great Basin by late Tuesday night and
Wednesday. A dry cold frontal boundary associated with this
trough is expected to sweep from north to south across our area
late Tuesday and Tuesday night. Increasing northwest winds can be
expected after frontal passage on Tuesday evening, mainly along
the coast and in the hills. Winds are then forecast to veer to
the north and increase in the hills after midnight Tuesday night,
and become locally strong and gusty by late Tuesday night,
mainly in the North and East Bay Hills. Strong and gusty north to
northeast winds are then expected to persist in the North and
East Bay Hills through Wednesday as surface high pressure builds
strongly over the Great Basin. Gusty winds are also expected to
mix locally down into the valleys of the North and East Bay by
Wednesday afternoon.

North to northeast winds are expected to peak in intensity
between Wednesday evening and Thursday morning when offshore
gradients will reach their maximum values. This is also the time
frame when strong and gusty offshore winds will develop across a
more widespread area including the Santa Cruz Mountains. In
addition, the local WRF model also generates localized gusty winds
in coastal areas such as western Marin County near Stinson Beach,
western San Mateo County near Half Moon Bay, and northwest
Monterey County between Carmel and Point Sur. Wind Advisories will
likely be issued for the North and East Bay Mountains -
geographic areas where the strongest, most persistent, and most
widespread winds are expected. Will hold off on issuing Wind
Advisories for the time being and instead focus on what will
clearly be the greatest impact from this upcoming offshore wind
event: the critical fire weather conditions that are expected
across the North and East Bay, as well as in the Santa Cruz
Mountains. A Fire Weather Watch continues for these areas from
late Tuesday night through Thursday (see Fire Weather Discussion
below for details).

Temperatures will initially cool on Tuesday night and Wednesday as
a cooler airmass spills south into our area. But temperatures will
quickly rebound by Thursday when the trough over the Great Basin
shifts eastward and a ridge develops just off the West Coast.
Also, the very dry airmass that will be in place across our area
by late Wednesday and Thursday will allow for large diurnal
temperatures variations later in the week with high temperatures
warming back into the 80s in most areas by Friday, while valleys
cool into the upper 30s at night. Modest daytime cooling is then
expected over the upcoming weekend when a weak upper trough is
forecast to move inland across the Pacific Northwest and northern
California.

&&

.AVIATION...as of 4:47 PM PDT Monday...For 00z TAFs. Mainly clear
skies will prevail through the forecast period with the exception
of a few high clouds passing overhead. Moderate onshore flow will
continue through early evening then decrease becoming light to
locally variable overnight.

Vicinity of KSFO...VFR through the period with the exception
of a few high clouds passing overhead. West winds 10 to 15 kt
will ease overnight becoming light to locally variable overnight.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR through the period. Moderate onshore
flow around 10 to 15 kt will ease overnight becoming light to
locally variable overnight.

&&

.FIRE WEATHER...As of 4:40 PM PDT Monday...Confidence continues
to increase regarding the development of critical fire weather
conditions midweek. A Fire Weather Watch is in effect from late
Tuesday night through Thursday for the North and East Bay Hills,
the North and East Bay Valleys, and the Santa Cruz Mountains.
While the strongest winds will likely occur in the North and East
Bay Hills, locally strong and gusty winds are also likely across
the higher ridges and peaks in the Santa Cruz Mountains. Also,
locally gusty winds may reach the valley floors at times in both
the North and East Bay. Strong and gusty offshore winds will
combine with critically dry conditions. The main message is to
prepare for the strongest offshore flow we've seen this season,
critically low daytime humidity with poor overnight recoveries,
and dry fuels, all resulting in critical fire weather conditions.

South of the SF Bay Area, winds during this upcoming event are
expected to be mostly light. However, locally gusty northeast
winds may develop in the northern Santa Lucia Mountains near the
Big Sur Coast for a brief period of time from late Wednesday night
into Thursday morning. Because these winds will be localized and
of short duration, this area has not been included in the Fire
Weather Watch.

The Fire Weather Watch will likely be transitioned to a Red Flag
Warning sometime on Monday, most likely in the early morning
hours. Once the watch transitions to a warning, the timing of the
event will be fine-tuned.

&&

.MARINE...as of 2:02 PM PDT Monday...Generally light west to
northwest winds will continue through tonight before an upper
ridge over the eastern Pacific and a trough along the West Coast
strengthen and increase winds over the northern outer waters. Gale
force conditions will develop over the northern waters Tuesday
night as a strong upper low moves into the Great Basin. As winds
increase early this week, expect higher seas to build. The
combination of strong winds and higher seas will create hazardous
seas conditions over portions of the coastal waters through mid-
week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...SCA...Pt Arena to Pigeon Pt 10-60 nm from 9 PM

&&

PUBLIC FORECAST: Dykema
AVIATION: CW
MARINE: AS
FIRE WEATHER: RGass/Canepa/Dykema

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