A low pressure system descended from the Gulf of Alaska southward along the entire Pacific Coast and tapped into sub-tropical moisture originating from the Central Pacific to drop heavy rain throughout the state. A stalled frontal boundary over the Santa Cruz Mountains along with strong southerly winds from a coastal low-level jet, enhanced orographic rainfall over this region, leading to…
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The episode as NWS MTR wrote it up.
A low pressure system descended from the Gulf of Alaska southward along the entire Pacific Coast and tapped into sub-tropical moisture originating from the Central Pacific to drop heavy rain throughout the state. A stalled frontal boundary over the Santa Cruz Mountains along with strong southerly winds from a coastal low-level jet, enhanced orographic rainfall over this region, leading to rainfall totals over a 2 day period (Dec 12th-13th) of up to ten inches in the coastal mountains. The stalled boundary was reinvigorated by a secondary boundary bringing in a cold unstable airmass that set off convective showers and lowered snow levels below 2000 ft. A wind advisory, later upgraded to a high wind warning was issued for the southern half of the region from the SF Peninsula down to the Big Sur Coast including the inland zones due to the coastal jet bringing sustained winds up to 40 mph with gusts up to 55 mph. A high surf advisory was issued for the entire coast due to the arrival of a strong NW swell train, along with Gale Warning throughout the coastal waters. A flash flood watch was issued for the Santa Lucia Mountains and Dolan Burn Scar area, where up to 12 inches of rain fell.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS San Francisco Bay Area, CA wrote its last area forecast discussion before the first event at Dec 13, 4:08 AM PST, 1 hour 32 minutes earlier.
Area Forecast Discussion
National Weather Service San Francisco Bay Area
408 AM PST Mon Dec 13 2021
.SYNOPSIS...
A potent weather system will impact the region today bringing
strong winds, locally heavy rain, and hazardous marine
conditions. Another weaker system arrives for mid week bringing
light precipitation and much colder temperatures by Friday night.
&&
.DISCUSSION...as of 3:15 AM PST Monday...
Today through Tuesday:
An expansive mid to upper trough enveloped the northeast Pacific and
Gulf of Alaska this morning with a cold core low centered near
50N/140W. A belt of 45 kt low to mid level southwesterly flow was
located along the leading edge of the trough with a focused moisture
plume angled into the Central California Coast in the form of an
Atmospheric River. A surface cold front was analyzed just south of
the Golden gate. Light to occasionally moderate rain showers were
occurring mostly north of a Watsonville to Gilroy line with rain
rates on the order of 0.1 to 0.3 inches per hour. Heavier showers
were approaching the Big Sur and Monterey Bay coast areas as of
this writing.
Given the progressive flow pattern aloft the cold and unstable
airmass associated with the upper low should filter into the area
later today into Tuesday. This should reinvigorate the storm system
while giving extra "oomph" to the frontal zone in the form of
increased frontogenesis (or forcing for ascent -- which aids in
precipitation development). So not only will the large-scale
dynamics improve with increased differential cyclonic vorticity
advection and jet-level divergence (sources for large-scale lift),
but the low to mid level flow should increase to around 60 kt
later this morning into this afternoon. This keeps a focused
moisture feed (precipitable water anomalies ~2 to 2.5 standard
deviations) into the Central Coast through the remainder of the
day with increasing rain rates anticipated. Of particular concern
is for localized rates in the Santa Lucia range to approach or
possibly exceed 1 inch per hour, which would be problematic for
hydrologically sensitive areas such as the Dolan wildfire burn
scar. For this reason a Flash Flood Watch was issued for this area
yesterday.
In addition to hydrologic concerns (more on that below) this will be
a multi-pronged event with strong winds, hazardous marine and beach
conditions, and the potential for a few thunderstorms. Please see
below for more details on timing, additional rain amounts, and other
impacts expected from this system.
* Rain Timing: As the cold front intensifies today the focus for
moderate to locally heavy rainfall should shift south of the
Golden Gate. This should initially include the Santa Cruz
mountains this morning with a gradual shift towards the Santa
Lucias during the afternoon and evening. This isn't to say that
showers aren't possible elsewhere, but that it appears the
steadiest/highest rain rates should be concentrated along the Big
Sur coast through the afternoon and evening hours. As mentioned
rain rates in this area could approach or exceed 1 inch per hour
with the potential for debris flows in sensitive areas. This
period of higher rain rates may persist into this evening before
decreasing Tuesday morning. Scattered showers and isolated
thunderstorms remain possible into Tuesday before diminishing
during the late morning to early afternoon.
* Rain Amounts: Additional amounts of 1 inch or less are expected
for the North Bay Valleys (higher in the coastal ranges), with
around 1 to 1.25 inches for San Francisco proper and the
South/East Bays. The greatest additional amounts today should be
south of the Golden Gate. This includes 2 to 5 inches for the
Santa Cruz Mountains, 1.5 to 3 inches for Monterey Bay and the
Salinas Valley, and 5 to 8 inches for the Santa Lucia Range.
Isolated amounts of 10-12 inches are possible for the higher peaks
along the Big Sur Coast.
* Potential Hydrologic Impacts: While dry antecedent conditions have
lessened the potential for widespread impacts, notable local
issues remain possible. Among them: 1) minor nuisance flooding of
urban and/or poorly draining areas; 2) localized rockslides in
canyons and steep hillsides; and 3) flash flooding and debris
flows for hydrologically vulnerable areas such as wildfire burn
scars. This concern is greatest for the Dolan burn scar, where a
Flash Flood Watch is in effect. However we will also monitor the
CZU and River burn scars for potential issues today. While
mainstem river flooding isn't currently anticipated, heavy rain
will lead to rapid rises on area rivers, streams, and creeks.
Outside of water related concerns here's a bit more on other local
impacts we're anticipating from this system.
* Wind: strong southerly winds will develop today ahead of an
advancing cold front. Look for sustained speeds in the 20 to 30
mph range for many areas with gusts locally exceeding 60 mph. A
Wind Advisory is in effect for San Francisco and the South/East
Bays into this evening. A High Wind Warning is in effect for
Monterey and San Benito Counties through early tonight. These
winds will create difficult travel for high profile vehicles and
will result in downed trees and power lines.
* Thunderstorms: sufficient instability exists this afternoon into
early Tuesday for a few thunderstorms to develop. This potential
appears greatest for the local waters and North Bay, but chances
could extend south along the coast as well. Lightning, gusty
winds, and small hail would be the primary hazards.
* Mountain Snow: as cooler air filters into the region snow levels
should drop to ~2500-3000 ft late tonight into Tuesday. This
suggests precipitation may end as snow at some of the higher peaks
where light accumulations are possible.
* Marine and Beach Conditions: hazardous conditions are expected
with very steep wind driven seas and gale-force winds developing
across portions of the local waters. High surf is also anticipated
with large breaking waves capable for sweeping people off jetties
and docks and into the ocean.
Wednesday and Beyond:
An active pattern continues with another storm system impacting the
area on Wednesday into Thursday. This system, while weaker than the
previous one, may bring an inch or so of precipitation to parts of
the area. Rain and low elevation snow are possible given the colder
air with this system. Cold temperatures should prevail for late week
with frost and freezing temperatures possible for many areas. More
details to come with future updates.
&&
.AVIATION...as of 4:00 AM PST Monday...For the 12z TAFs. IFR/MVFR
will persist through most of the period as another and stronger
FROPA moves through the area today. RA will persist through most
of the period area wide with occasional breaks. Stronger gusty
southerly winds ahead of FROPA late this morning lasting through
the afternoon for most of the Bay Area terminals. Periods of +RA
are likely as the front moves through, which could temporarily
reduce visibilities down to LIFR conditions. Winds will diminish
quickly late this evening and tonight, however -RA/RA will
continue until Tuesday morning.
Vicinity of KSFO...Mostly MVFR conditions with occasional IFR as
stronger FROPA moves through the area today. Southerly to
southwesterly winds will become gusty beginning around 18Z and
lasting through 00Z. Gusts will reach up to 35kts with occasional
gusts up to 40kts. As front passes there could be brief periods
of LIFR visibilities due to +RA. Winds should diminish to AOB
15kts by 03Z tonight for all Bay Area terminals. -RA should stop
around 12Z Tuesday with skies starting to scatter out after 15Z.
KSFO Bridge Approach...Similar to KSFO.
Monterey Bay...VFR ceilings will fall to MVFR later this morning
as RA will persist for most of the period. Gusty southerly winds
ahead of FROPA this afternoon and evening with winds diminishing
around 06Z. There will be chances for +RA and LIFR visibilities
as the front moves through this afternoon.
&&
.MARINE...as of 03:40 AM PST Monday...Gusty southerly winds across
the waters will continue through tomorrow. Expect strong and
gusty winds especially across the Monterey Bay and Big Sur coast
today lasting through tonight. Southerly winds will also
strengthen across the San Francisco Bay Monday afternoon and
early evening. Very steep southerly wind waves are expected to
continue today creating hazardous conditions for vessels. A large
longer period WNW swell train will move into the waters tonight
through Tuesday and bring hazardous conditions through harbor
entrances and the surf zone. The Coast Guard advises all
mariners to ensure their boats are tethered to port prior to the
arrival of this potent storm system.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
.Tday...High Wind Warning...CAZ516>518-528-530
Flash Flood Watch...CAZ517
Wind Advisory...CAZ006-508>513-529
High Surf Advisory...CAZ006-505-509-529-530
GLW...Pigeon Pt to Pt Pinos 0-10 nm until 9 AM
GLW...Pigeon Pt to Pt Piedras Blancas 10-60 nm until 9 AM
GLW...Pt Pinos to Pt Piedras Blancas 0-10 nm
SCA...Pt Arena to Pt Reyes 0-10 nm
SCA...Pt Reyes to Pigeon Pt 0-10 nm
SCA...Pt Arena to Pigeon Pt 10-60 nm
GLW...Mry Bay
SCA...SF Bay from 12 PM
&&
PUBLIC FORECAST: SPM
AVIATION: Dhuyvetter
MARINE: Dhuyvetter
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www.youtube.com/nwsbayareaPRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
426 AM PST MON DEC 13 2021
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0329 AM NON-TSTM WND DMG 3 ENE SANTA CLARA 37.38N 121.91W
12/13/2021 SANTA CLARA CA LAW ENFORCEMENT
TREE DOWN IN ROAD WAY, ALSO MINOR STREET
FLOODING.
&&
EDPRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
431 AM PST MON DEC 13 2021
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0217 AM FLOOD 2 W PIEDMONT 37.83N 122.27W
12/13/2021 ALAMEDA CA LAW ENFORCEMENT
6 INCHES OF WATER ON ROADWAY.
&&
EDPRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
534 AM PST MON DEC 13 2021
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
0511 AM FLOOD 2 NW OAKLAND 37.79N 122.25W
12/13/2021 ALAMEDA CA LAW ENFORCEMENT
12 INCHES OF WATER ON ROADWAY CAUSING 3
LANES TO BE BLOCKED.
&&
EDThe San Francisco radar (KMUX), 79 km away, was scanning. The volume nearest the first event began at 13:41 UTC.
7 flood, 2 strong wind, 1 debris flow, 1 lightning. Local standard time, as filed.
Heavy rain led to 1 ft of standing water in the roadway.
Heavy rain led to a rockslide on SR1 and partially blocked the road.
Heavy rain caused flooding on US101 at S Airport Blvd.
Heavy rain caused flooding and closure of SR 12 between Route 116 and 8th St.
Roadway flooding caused hard closure of SR 92 between SR 35 AND SR1.
Video, https://www.youtube.com/watch?v=4LFZJqrqidw, of rapid debris flow in Boulder Creek area, 200 ft from where Foreman Creek crosses under SR236. Creek runs through part of the CZU burn scar area.
Strong winds knocked down a power pole at 14680 Shannon Rd. Wires were in the roadway.
A tree fell into the roadway and smashed a vehicle at Blackstone Dr. and Heatherstone Dr.
Rocks slid onto SR1 and blocked the northbound lane near Bixby Bridge. https://kion546.com/news/2021/12/14/a-look-at-the-damage-from-the-rain-storm/.
Heavy rain caused roadway flooding on Labath Ave.
A lightning strike blew out a transformer along Del Monte Blvd. Eight power poles fell down but no injuries were reported. https://kion546.com/news/2021/12/14/a-look-at-the-damage-from-the-rain-storm/.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in California in 2021Every state in 2021This day in weather historyWarnings in California, December 2021
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 1000075, 1000122, 1000076, 1000074, 1000077, 1000078, 1000120, 1000121, 1000119, 1000044, 1000118.