Rip Current and High Surf in California, September 12 to 14, 2021

Area Forecast Discussion
National Weather Service San Francisco Bay Area
455 PM PDT Sun Sep 12 2021

.SYNOPSIS...Seasonable conditions today and into tomorrow,
locally cooler near the coast and warmer across the interior.
Light concentrations of smoke arrives with north winds Monday into
Tuesday. Warmer conditions Tuesday and into Wednesday. Cooler
Thursday and into the weekend.

&&

.DISCUSSION...As of 01:15 PM PDT Sunday...The San Francisco Bay
Area is nestled in the buffer region between a weak shortwave
trough 500 miles to the west and a weakening high pressure ridge
500 miles to the southeast. Consequently, seasonable conditions
are underway today with slightly below normal temperatures
alongshore and bayside and slightly above normal temperatures in
the interior. A fragmented 1000 foot marine layer is evident
alongshore on satellite with mostly clear skies elsewhere (aside
from locally smoky conditions and a few high clouds along a dry
cold front farther north). A locally moderate afternoon to evening
seabreeze should kick in as the day progresses. In short,
gorgeous late summer weekend weather.

For late tonight (and again tomorrow night), northerly winds will
develop through the Sacramento Valley along the periphery of the
North and East Bay interior ranges. These winds are expected to
pick up primarily between 3 and 9am each of the next two mornings
and peak around the sunrise hours. Peak wind gust speeds over
ridges and crests will run in the 15-25 mph range, with lighter
speeds at lower elevations. Winds will be sub criteria for a fire
weather watch locally but neighboring offices may be issuing and
we will continue to monitor. These northerly winds will also
advect fairly light concentrations of smoke into the region from
the north late tonight through Tuesday.

Tomorrow will feature the weak shortwave trough axis sweeping
through the forecast district, though influences of ridging
farther to the southeast will remain. Thus, only locally marginally
cooler conditions are expected tomorrow alongshore, with similar
temperatures across the interior where warmer mid level air
lingers.

By Tuesday, a high pressure system that has been developing in the
near eastern Pacific will transition towards the coast and push
ashore, rapidly ejecting any lingering troughing, and advecting in
warmer, drier air. Thus, expect a marginal warming and drying
trend from Monday to Tuesday that will then linger into at least
midday Wednesday. Tuesday will be the warmest day of the week with
afternoon temperatures in the interior in the mid 80s to mid 90s,
though locally extreme interior locations such as Berryessa, Pope
Valley, Pinnacles, etc could approach the upper 90s. Cooler, more
seasonable conditions will exist nearshore though bayside areas
will still push to the mid 70s to low 80s, particularly near
Silicon Valley and Sausalito.

Another shift in the weather pattern is anticipated to arrive late
Wednesday at the earliest, or by early Thursday at the latest.
This will come as a stronger pulse within the broader polar low
descends down and combines forces with a cut off low off of
central/southern California to suppress the heat domes along our
south to southeast periphery. Consequently, a trend towards
cooler, less dry weather is expected to arrive from Thursday into
next weekend. Forecast ensemble certainty has been wavering
regarding the weather pattern by next weekend, but has shown a
recent trend towards the earlier GFS solutions featuring deeper
troughing, though the details still are in question. That said,
the latest ensemble member trends are towards introducing a slight
chance of rain for the extreme northwestern periphery of the North
Bay as the deeper troughing arrives late this week into the
weekend, though confidence remains very low at this time. For now,
we will continue to evaluate the ensemble and model trends and
adjust accordingly.

At the much broader scale, this meteorological ebb and flow is
occurring due to our positioning in the buffer region between the
very broad fledgling polar low along the upper CONUS/Canadian
border and the series of heat domes across the lower CONUS. As
waves within the polar feature pass over us or just to the north,
they act to suppress the southern heat domes and consequently
bring the ephemeral swings in the tangible weather conditions at
the surface. Long story short is that seasonable conditions will
persist, with a swing towards warmer conditions Tuesday, and
cooler conditions from Thursday and beyond.

&&

.AVIATION...As of 4:55 PM PDT Sunday...For the 00z TAFs.
Widespread VFR this afternoon with mild to moderate winds and some
breezy conditions around at SFO. These winds will begin to weaken
after 05z, with mild to moderate winds expected overnight.
Stratus looks to favor only the coastal areas as well as the
Monterey Bay terminals this evening and through the night. This
will lead to IFR in the evening followed by LIFR in the late night
for those areas. Patchy fog will also form around the North Bay
as well as the Monterey Bay in the late night. The lowered CIGs
and patchy fog look to linger into the mid to late morning, but
widespread VFR looks to return before Monday afternoon.

Vicinity of KSFO...VFR through the TAF period. Expect Northwest
wind around 17 kts with 24 kt gusts through 05z. Winds will back
off into the night, but will still remain moderate. Status will
struggle to filter into the SF Bay tonight, but could see some
lowered CIGs around KOAK. Winds will increase again into Monday
afternoon, with similar speeds to today.

KSFO Bridge Approach...Similar to KSFO.

Monterey Bay...VFR conditions will continue to prevail with
moderate winds though this evening. Expect IFR in the late
evening/early night as stratus moves inland. Winds look to become
light and variable in the late night with areas of fog forming as
terminals fall to LIFR. Cloud cover and fog will be slow to thin
around the Monterey Bay, holding off on VFR until the late morning
and early afternoon for Monday.

&&

.MARINE...As of 02:54 PM PDT Sunday...Sunday offers an increase
in northwest winds over the coastal waters, expect hazardous seas
due to fresh wind waves. Locally gusty west to southwest winds
north of the Bay Bridge this afternoon and evening. Northwest
swell arrives later today mixed with a lighter, longer period
southerly swell.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Reyes to Pigeon Pt 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

&&

PUBLIC FORECAST: DRP
AVIATION: Palmer
MARINE: Murdock

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