Gale Warning 54, NWS San Francisco Bay Area, CA

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1011 PM PST Fri Dec 30 2022

.SYNOPSIS...Periods of rain this evening will become more
widespread overnight especially across the North Bay and in the
hills. The main cold front will barrel through the Bay Area
Saturday morning and Central Coast Saturday afternoon. Heavy rain
with the frontal passage will likely lead to flooding along with
wind gusts to 50 mph. Rapid drying and cooler Saturday night
through Sunday. Next system arrives Monday with another round of
mainly light rain. A brief break Tuesday then a potentially
stronger atmospheric arrives Wednesday and Thursday with
subsequent storms possible into next weekend.

&&

.DISCUSSION...as of 09:25 PM PST Friday...Radar and rain gauges
confirm that we continue to see light to moderate rain this
evening. That will continue overnight until the heavier rain moves
through our CWA with tomorrow's cold front passage. Have been
looking at high resolution models and timing doesn't seem to have
changed too much since earlier today. Timing of this cold front
shows that it'll first show up in North Bay around 4 am (give or
take an hour) then make it's way to the rest of SF Bay area
between 9am to noon. Santa Cruz mountains should start seeing
heavier rain by late morning into afternoon followed by Monterey
and San Benito counties getting the cold front around 11 am and
continuing southward during the afternoon hours. Heaviest rain
should exit out of Central CA (Monterey & San Benito counties) by
early evening (between 4-6 pm). Rain will either be light for the
rest of the evening into night or some areas will start seeing
some dry conditions. As many folks may have plans for NYE, keep in
mind that just because weather has improved for NYE night, they'll
need to check road conditions. Earlier today, we started getting
reports of road closures due to shallow landslides. Tomorrow's
cold front could loosen soils even more and there could be
additional road closures by tomorrow afternoon that impact
travel. There may be standing water on roadways due to poor
drainage or because of rising creeks/streams along roadways.

As for Debris Flows from burn scars...we will be monitoring rain
rates closely during the cold front. The high resolution models
are suggesting that hourly rain rates could peak between 0.5"/hour
to 1"/hour which could put us near or at criteria depending on the
burn scar.

Wind Advisories for overnight tonight into tomorrow remain in
effect. Although some of our wind gusts are marginal in terms of
Wind Advisory criteria, with saturated, soft soils...even moderate
winds could help topple trees which could block roads, fall on
cars, or cause power outages.

As stated in the prior discussion below, we added mention of
coastal flooding around the SF Bay shoreline in our Flood Watch
(which also remains in effect). This could be shortlived, but
high tide will occur around 6 am tomorrow morning when the heavier
rain of the cold front moves through. If you are near low-lying
areas or flood prone areas along the shoreline, be alert during
high tide.

As a final note: we are in for an extended wet pattern where these
saturated soils won't have time to really dry out. The only break
we get from rain will happen on Jan 1, but then more rain occurs
Jan 2-5 with a third moderate atmospheric river moving into our
region around Jan 4-5. At this time, the rain totals look to be
similar or a little higher than what we saw on Dec 26-27 and what
we may see tomorrow. Expect several more inches. After Jan 5, it
seems light to moderate rain could persist into the second week of
January. If that's the case, then residents across the Bay Area
and Central CA need to be aware that additional flooding and
shallow mudslides could occur. It could be a messy start to 2023,
so hang in there and take care of each other!

&&

.PREV DISCUSSION...as of 3:29 PM PST Friday...It seems we often have
some active weather around the New Years holiday and this year is
no exception as we are about to enter into a very active week of
weather across the Bay Area, really all of NorCal and even SoCal
will get into the action. These are the type of storms your
grandma would reminisce about...

As noted this morning even though the KMUX radar looks benign we
continue to see rain across the region, most notably in the
coastal slopes. The NOAA profiler network shows moist onshore flow
squeezing out moisture along the coastal slopes. We're seeing
some gaudy 24 hour totals such as 4.73 Mt Tamalpais and 2.33 at
Ben Lomond. Even the Oakland (hills) South weather station is
reporting an impressive 2.50 inch 24 hour rainfall total. This is
all occuring in the benign pre-frontal environment.

The main AR up near the Oregon border will drift south tonight
with rain becoming more organized and widespread across the North
Bay after midnight. If anything the trend on the latest HRRR/NAM
runs has been to slow the arrival of the heaviest rain slightly,
closer to the pre-dawn hours or sunrise across the North Bay.
Expect a 6 hour window of heavy rain with wind advisories in
effect Saturday morning as the cold front passes through the
North Bay. High tidal cycles early Saturday morning will coincide
with the heaviest rain and wind so some coastal flooding will be
possible inside the Bay Saturday morning.

The front will move through the heart of the Bay Area Saturday
morning and then reach the Central Coast late morning through
mid-afternoon. One item in our favor is the fairly fast movement
of the system should preclude widespread significant flooding.
However we do expect several of the flashier rivers and creeks to
respond and rise quickly as the soils are becoming saturated and
we will receive copious amounts of rain over a a relatively short
period of time (1-4 inches over 6-12 hours).

Any winds will quickly ease on the backside of the front by
Saturday afternoon with rain quickly ending. At least NYE
activities will be dry but cooler in a post-storm environment.

Sunday remains a day of reprieve and (storm) recovery with dry and
mostly sunny weather. The break in the weather looks short lived.

Next shortwave arrives Monday with another shot of light rain.
Early qpf estimates of 0.25-0.50 valleys with 1 inch in the hills.
Enough rain to keep the soils saturated and water filling the
streams and reservoirs but no flooding concerns with the Monday
system.

Another break day Tuesday. Next atmospheric river looks to arrive
later Weds into Thursday. Early indications show the potential to
bring even more rain than the current system with another 2-4
inches at least. Incoming trajectory would allow the system to
potentially move a little slower and bring copious precip to many
of the watersheds across the state.

All indications are that after that system ejects eastward the
pattern stays active with more storms likely into next weekend and
through mid-month. If anything this is feeling like the winter of
2017 when we had numerous AR events across NorCal.

&&

.AVIATION...As of 9:40 PM Friday... For the 06Z TAFs. Currently,
terminals are experiencing a mix of VFR, MVFR and IFR conditions but
any terminals that are experiencing VFR conditions will decrease to
MVFR or IFR through tomorrow afternoon. As the cold front passes
through the region tonight expect strong gusty winds through
tomorrow evening with winds starting to calm late tomorrow night.
Gusty winds can get up to 30 -33 knots along with moderate chance of
LLWS tomorrow morning from strong winds aloft from a low level jet.
High confidence that VFR conditions slowly make their way back over
terminals, starting with North Bay, as the rain event moves out of
the region.

Vicinity of SFO...MVFR conditions until early tomorrow morning where
condition decline to IFR with moderate chance of LLWS. High
confidence that winds will be gusty, but lower confidence on wind
speed. High confidence that as the rain clears out of the area
tomorrow night, VFR conditions will return.

SFO Bridge Approach... Similar to SFO.

Monterey Bay...VFR with a chance of MVFR conditions until late
tonight where conditions lower to IFR. Conditions are expected to
improve to MVFR or VFR tomorrow evening. Moderate confidence of
gusty winds through the TAF period, but at least expect breezy winds
through the TAF period.

&&

.HYDROLOGY...As of 1:34 PM PST Friday...A Flood Watch remains in
effect this evening through Saturday Evening, as flooding from
excessive rainfall is expected.

Moderate to heavy rains will develop late tonight and continue
through Saturday, causing rapid rises along area rivers, streams,
and creeks. Current forecasts for several streams and creeks
across the region to exceed monitor stage if not flood stage.
These sites include, but are not limited to...

In North Bay...
-Laguna De Santa Rosa at Stony Point Road near Cotati
-Mark West Creek near Mirabel Heights
-Willow Brook at Penngrove Park
-Maacama Creek near Kellogg

In East Bay...
-Alameda Creek above Div Dam near Sunol

South Bay
-Coyote Creek above Hwy 237 at Milpitas
-Pacheco Creek near Dunneville

While no mainstem river flooding is forecast at this time,
mainstem rivers will see rapid rises as well. The Russian River
at Geyserville is forecast to exceed monitor levels, and will be
monitored closely.

Localized flooding is expected, especially in low-lying or poorly
drained areas, such as freeway on and off ramps. There is concern
for localized coastal flooding along the San Francisco Bay
shorelines Saturday morning since high tide will occur while the
heaviest rain is moving through the region. Localized flooding is
also expected in areas where storm drains have not been cleared
of debris. There is also a concern for mud, rock, and debris
flows, especially near the Glass Burn Scar in Sonoma County as
well as the Colorado Burn Scar, the Dolan Burn Scar, and the River
Burn Scars in Monterey County.

&&

.MARINE...as of 09:25 PM PST Friday...A cold front moves through
from the north overnight into Saturday morning with intense rain,
gusty winds, and chances of thunderstorms. Winds turn to the
northwest and remain strong behind the cold front. Dry conditions
return for New Year's Day, then periods of rain throughout the
next week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...Flood Watch...CAZ006-508>510-512>518-528>530
             Wind Advisory...CAZ006-508>510-512>515-529
             Flood Watch...CAZ502>506
             Wind Advisory...CAZ502>506
             Wind Advisory...CAZ516>518-528-530
             SCA...SF Bay
             SCA...Mry Bay from 3 AM
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             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

&&

PUBLIC FORECAST: Bingaman
AVIATION: SO
MARINE: BKM

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