NWS Forecast Area Discussion - San Francisco Area

FXUS66 KMTR 240030

Area Forecast Discussion
National Weather Service San Francisco Bay Area
530 PM PDT Sat Oct 23 2021

.SYNOPSIS... Light rain will continue over the North Bay and
along the coastal ranges this afternoon ahead of the upcoming
atmospheric river. Rainfall will increase in coverage Sunday and
be accompanied by gusty winds before shifting southward across the
entire region Sunday night through early Monday morning. A slight
chance of North Bay light rain Tuesday otherwise warming and
drying trend as high pressure rebuilds.

&& of 02:37 PM PDT Saturday...Saturday afternoon
continues the calm before the storm. A few light showers have been
passing through the North Bay but the remaining rain amounts from
this aren`t adding up to much compared to Sunday`s forecasted
amounts. Expect conditions to remain calm until the late night,
when the front begins to interact with the North Bay.

After considering the updated model and ensemble outputs as well as
the inclusion of short-term, higher resolution models, the timing of
the AR event has been shifted slightly later. The afternoon forecast
update consisted mostly of tweaks in timing and some fine-tuning in
rainfall amounts as well as adjustments in peak winds, but the bulk
of the forecast remains on track.

Winds will quickly increase into the night for Sonoma Co and these
stronger winds will continue south ahead of the front. Expect the
strongest winds ahead of the band of rain some strong gusts
lingering well into the rain band itself. Max wind gust will range
from 60 to 70 mph at the highest peaks, 40 to 50 mph around the Bay
Area, and 35 to 45 mph around Monterey Bay and the Central Coast.
The heaviest rains will be focused over higher elevations along the
coast as well as higher elevations in western Sonoma Co.

The official forecast timing and rain amounts will roughly go as
North Bay: Early Sunday morning through Sunday evening
           Highest peaks 8-11"
           Foothills 6-8
           Valleys 4-6"

East Bay and SF Peninsula: Late Sunday morning through Sunday night
           Higher elevations: 4-6"
           Lower elevations: 2-4"

South Bay and the Santa Cruz Mtns: Mid Sunday afternoon though late
Sunday night.
           Highest Peaks 5-7"
           Foothills and Santa Cruz coast 2-4"
           Santa Clara Valley 1-2.50"

Monterey and San Benito Counties: Sunday evening through early
Monday morning
          Highest Peaks along the Big Sur Coast 4-6"
          Coastal Monterey Co 2-4"
          Interior Monterey and San Benito counties 1-2.50"

These times and totals are subject to change but the forecast
confidence is increasing as models update.

Ahead of the event, we recommend taking today as a day of
preparation. Today is a perfect day to make sure you are ready for
the strong winds, moderate to heavy rain, and the possibility of
power outages. Charge your phone and battery back ups, tie down
outdoor furniture and trash cans, check your emergency kit
supplies, know where and how to get updates from your local
emergency management, and clear storm drains and gutters.

&& of 5:15 PM PDT Saturday...For the 00z TAFs. Wind
and ceiling impacts for terminals with the upcoming weather
system. Light S/SW winds will begin to strengthen overnight after
06Z. MVFR/IFR conditions around KSTS/KAPC will continue to move
southward reaching the Bay Area around 06Z. Poor aviation
conditions will prevail through the day Sunday and into Sunday
night for KSTS/KAPC/KLVK/KOAK/KSFO/KSJC. S/SE/SW winds will
increase to 25G35-40kts for aforementioned terminals with 40-60KT
LLWS at 1000-2000 feet starting around 10Z lasting through the end
of the TAF period. Expect moderate to heavy rain to accompany the
gusty winds with very high rain rate intensity with the main band
of precipitation leading to slippery runways. Visibility may also
drop significantly with the strong wind and heavy rain with
lowering cigs as well. Conditions improve in the post frontal
environment but lingering showers are possible after the heaviest
rain shifts southward. KMRY/KSNS will see winds impacts starting
after 16Z, with heaviest rain beginning after the end of the TAF

Vicinity of KSFO...VFR. Skies are filling in from the north,
becoming OVC020-030 after 03Z. Intermittent -RA starting after
03Z, with RA/+RA after 16Z Sunday. Winds aloft strengthening
tonight and Sunday resulting in SW 35-55 kt low level wind shear.
Gusty southerly surface winds are then expected to arrive by
sunrise tomorrow before peaking through midday. Wind gusts of 30
to 50 mph are possible and an airport weather warning for these
winds is likely. RA starts to become intermittent after 04Z
Monday, with winds starting to slowly drop to 20ktG25.

KSFO Bridge Approach...Similar to KSFO.

Monterey Bay...VFR . MVFR cigs possible with VCSH/-RA tonight.
Marginal SE LLWS of 35-45kt then expected through tomorrow morning
with increasing southerly surface winds and gusts through
tomorrow. +RA starts to impact terminals at end of TAF period.

&& of 02:37 PM PDT Saturday...A strong storm system will
transit southward through the waters  from late tonight through
tomorrow night. Winds will back towards the south, increase, and
become gusty ahead of and with the front. Peak gusts will push
into upper gale force range and could occasionally gusty into
lower storm force range, ie 40 to 50 kts. These winds will
generate very steep fresh southerly swell that  is hazardous for
smaller craft vessels. In addition, this storm system will
generate a large WNW swell train that will arrive shortly after
the storm passes from Sunday night into early Tuesday. Swell of 16
to 24 feet at 16 to 18 seconds will then wash ashore, peaking
Monday, and bring hazardous conditions in the surf zone and near
harbor entrances with breaking waves of 20 to 30 feet.

&& of 12:30 PM PDT Saturday...A strong, early season
storm system arrives tomorrow and will result in significantly
increased wave energy from late Sunday into early Tuesday as it
arrives and after it passes. This wave energy will transition
through the coastal waters from late Sunday into Tuesday, and peak
on Monday. Swell of 16 to 24 feet at 16 to 18 seconds is forecast to
arrive with the swell train as it peaks on Monday and result in a
number of coastal hazards. These hazards include large breaking
waves of 20 to 30 feet (higher at favored break points), increased
risk of strong longshore and rip currents, increased risk of coastal
erosion and minor coastal flooding, and enhanced coastal run up
concerns due to the summer beach profiles in place.
Due to the early arrival of these large waves, many beaches are
still transitioning from their summer beach profiles and lack the
features and steepness to resist larger wave run up on coasts. This
means that more of this wave energy will have a chance to move onto
the beach and overtake individuals, potentially injuring them, or
pulling them into the cold ocean. Each year, people die at the coast
due to these or similar ocean conditions. A high surf warning for
the entire coast has been issued for this threat and is in effect
from 11PM Sunday to 11AM Tuesday, with the highest risks once again
at west to northwest facing beaches.


     .Tngt...Wind Advisory...CAZ006-508>513-529
             Wind Advisory...CAZ505>507
             SCA...Mry Bay from 3 AM
             SCA...Pigeon Pt to Pt Pinos 0-10 nm from 3 AM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm from 3 AM
             SCA...Pt Arena to Pt Reyes 0-10 nm until 3 AM
             SCA...Pt Arena to Pigeon Pt 10-60 nm until 3 AM
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm until 3 AM
             GLW...Pt Arena to Pt Reyes 0-10 nm from 3 AM
             GLW...Pt Arena to Pigeon Pt 10-60 nm from 3 AM
             GLW...Pt Reyes to Pigeon Pt 0-10 nm from 3 AM
             GLW...Pigeon Pt to Pt Piedras Blancas 10-60 nm from 3 AM
             GLW...SF Bay from 5 AM



AVIATION: Dhuyvetter

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NWS MTR Office Area Forecast Discussion