Severe Thunderstorm Warning 1, NWS San Francisco Bay Area, CA

Area Forecast Discussion
National Weather Service San Francisco Bay Area
359 PM PST Wed Jan 4 2023

.SYNOPSIS...Warm front passed through this morning with much
stronger cold front now fast approaching. Areas of flooding and
possibly damaging winds continue to be expected late this
afternoon and tonight as it moves through. It's then quickly
followed later tonight and Thursday by locally heavy showers and
possible thunderstorms in association with the trailing upper
level trough. Friday looks to be a bit of a break day, but then
subsequent weather systems are projected to bring additional
precipitation and periods of stronger winds this weekend and next
week.

&&

.DISCUSSION...As of 03:20 PM PST Wednesday...Very active weather
situation with a lot going on. For context, this is the 3rd
significant storm system in a remarkable sequence that looks to
continue on this weekend and next week. The first was December
26-27 and the second this past New Year's Weekend. Precipitation
totals from these two systems have exceeded 1/2 foot even at lower
elevation locations (6.76 inches at SFO Dec 26 - Jan 2 for
example), while wetter portions of the coastal slopes received
more than 1 foot (16.7 inches at the Ben Lomond RAWS site for
example for the same 8 day period). So needless to say, the soils
are saturated enhancing runoff and increasing the risk of shallow
landslides, especially during intervals of heavier precipitation.

Latest satellite imagery shows a complex structure to the inner
portion of the cyclone itself, now centered about 625 miles WNW
of San Francisco. The associated warm front and precip band out
ahead moved through our area this morning. Rainfall amounts ranged
from as much as about 1.4 inches wettest spots in the North Bay
coastal slopes and Santa Cruz Mountains, to generally a few tenths
of an inch around SF Bay and just a couple of hundredths of an
inch at KSJC, KMRY and other drier locations. Winds got quite
strong out of the SE over the peaks and ridges as well as in the
favorably aligned coastal stretches and valleys. KSNS for example
reported a peak gust of 58 mph right around noon.

Just with the present system, we have two significant rounds of
rain and wind to come. The first, with the cold front, will be
moving through late this afternoon and evening. Expect a several
hour long period of accompanying moderate to heavy rain and strong
and gusty winds. This is when risks of flooding and possible damaging
gusts resulting in downed trees and power outages will really
start to ramp up. Will also then need to closely monitor radar as
frontal structure looks to be in the form of a NCFR and thus
can't rule out the possibility of some local spin-ups. And then
the trailing upper level trough will quickly approach tonight and
move in late tonight and Thursday along with more rainfall,
locally gusty winds and also a chance of thunderstorms. Both a
High Wind Warning and Flood Watch are in effect for the entire
CWA, the first through 10 am Thursday morning and the latter
through Thursday afternoon.

A number of additional near-term concerns of note: Dangerously
high surf is expected along the coast Thursday and Thursday night
and a High Surf Warning is in effect. The combination of the high
tide Thursday plus an additional 1 to 1.5 foot increase in water
levels from the storm system may result in minor coastal flooding
along the lines of what typically happens during King Tide events,
primarily the bayshore of San Francisco, Marin and Sonoma
counties. Therefore have a Coastal Flood Advisory in effect from 6
am to 1 pm Thursday. Will have to monitor for possibility of rain
rates reaching debris flow triggering thresholds in our burn scar
areas. And given widespread saturation of the soil column that
now exists, shallow landslides are a possibility during periods
of heavier rain anywhere more prone to such occurrences.

Thursday night and Friday look to be a bit of a break, with just a
chance of some lingering showers. But but by later in the day
Friday, the next significant weather system looks to move in to
the North Bay, with rain districtwide over the weekend.

In the extended, ECMWF, GFS and Canadian model ensembles are all
in good agreement in the storm track remaining active and focused
on our area with additional Pacific weather systems moving
through and bringing more rounds of rain and wind.

&&

.AVIATION...As of 3:59 PM Wednesday...For the 00Z TAFs. A frontal
band with heavy wind and rain along the coastal North Bay to the
south along the immediate coastline is advancing eastward, with
strong surface to lower level winds coupled. Damaging winds with
frontal band as it moves through early this evening. Strong low
level wind shear through late afternoon and evening. Primary deep
low center approx 500 miles to the northwest of the Bay Area
however after a strong frontal passage this evening additional
upper level troughing late tonight and Thursday will keep showers
going across the forecast area. Relatively less wind late tonight
and Thursday morning with gradually decreasing lower level wind
shear beginning after midnight tonight.

Vicinity of SFO...Very strong E-SE winds at the surface and aloft
leading up to and around the time of frontal passage during mid
evening. Not a classic cold frontal passage with wind shift to the
west; thermal cooling behind the front is not very strong, thus
southeast winds continuing at least tonight and Thursday morning.
IFR developing in heavy rain and wind especially 00z-04z this
evening. Otherwise MVFR during the period.

SFO Bridge Approach... Similar to SFO.

Monterey Bay...Strong to very strong E-SE winds at the surface and
aloft leading up to and around the time of frontal passage during
mid to late evening. Not a classic cold frontal passage rather
winds will remain primarily from the SE overnight and Thursday.
IFR developing in heavy rain and wind especially 01z-05z this
evening, otherwise VFR-MVFR during the period.

&&

.MARINE...As of 02:37 PM PST Wednesday...An cold front coming
into the outer waters, and progressing east towards the coast,
brings gales, intense rain, and a chance of thunderstorms late
this afternoon and tonight. Hazardous westerly swell builds behind
the cold front and peaks at 20 to 25 feet, continuing through the
end of the week with large westerly swells and another storm
system this weekend.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...Flood Watch...CAZ006-502>506-508>510-512>518-528>530
             High Wind Warning...CAZ006-502>506-508>510-512>518-528>530
             GLW...SF Bay until 3 AM
             GLW...Mry Bay until 3 AM
             GLW...Pt Arena to Pt Reyes 0-10 nm
             GLW...Pt Reyes to Pigeon Pt 0-10 nm
             GLW...Pigeon Pt to Pt Pinos 0-10 nm
             GLW...Pt Pinos to Pt Piedras Blancas 0-10 nm
             GLW...Pt Arena to Pigeon Pt 10-60 nm
             GLW...Pigeon Pt to Pt Piedras Blancas 10-60 nm

&&

PUBLIC FORECAST: Blier
AVIATION: Canepa
MARINE: Dial

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PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
558 PM PST WED JAN 4 2023

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0552 PM     FLOOD            3 W UNION CITY          37.61N 122.06W
01/04/2023                   ALAMEDA            CA   LAW ENFORCEMENT

            ROADWAY FLOODING.

&&

BKM

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
600 PM PST WED JAN 4 2023

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0555 PM     NON-TSTM WND DMG SCOTTS VALLEY           37.06N 122.00W
01/04/2023                   SANTA CRUZ         CA   LAW ENFORCEMENT

            DOWNED TREE.

&&

BKM

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
632 PM PST WED JAN 4 2023

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0600 PM     NON-TSTM WND DMG SAN FRANCISCO           37.78N 122.42W
01/04/2023                   SAN FRANCISCO      CA   PUBLIC

            TREE FALLEN ON VEHICLES, FAMILY HAD TO BE
            RESCUED.

&&

BKM

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
646 PM PST WED JAN 4 2023

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0556 PM     NON-TSTM WND DMG 1 WNW SAN BRUNO         37.63N 122.45W
01/04/2023                   SAN MATEO          CA   LAW ENFORCEMENT

            80 FOOT TREE BLOCKING ROAD.

&&

BKM

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