A very large long period northwest swell train arrived in early December to the coastline with buoys reporting swells in excess of 20 feet. Mavericks saw breaking waves in the surf zone of approximately 40 feet.
Every event with a place to put it; 5 filed by county or forecast zone are drawn at its center, in a lighter ring. Tap one for its entry below.
Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.
The episode as NWS MTR wrote it up.
A very large long period northwest swell train arrived in early December to the coastline with buoys reporting swells in excess of 20 feet. Mavericks saw breaking waves in the surf zone of approximately 40 feet. Large powerful waves knocked boats loose and damaged docks near Santa Cruz. Multiple water rescues were reported up and down the coast with one assumed fatality.
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 7, 10:00 PM PST, 2 hours earlier.
Area Forecast Discussion
National Weather Service San Francisco Bay Area
1000 PM PST Mon Dec 7 2020
.SYNOPSIS...Offshore flow will persist through the afternoon but
winds will continue to subside. Dry conditions keep fire weather
concerns over the region through this afternoon. This flow will
also produce unseasonably mild temperatures through Tuesday. Beach
hazards and high surf are also expected through Tuesday.
Otherwise, dry weather conditions persist through late week with
mild days and cool nights.
&&
.DISCUSSION...as of 08:51 PM PST Monday...Moderate to strong
offshore flow over the region brought increased fire weather
concerns, gusty winds, and record breaking warmth this afternoon.
First and foremost, the Red Flag Warning was allowed to expire.
Winds are decreasing, albeit slowly. There are a handful of our
typically windy spots like Diablo and Mt St Helena that are still
gusting. Near term hi-res models do show the weakening of the
winds will continue overnight. Despite the dropping winds relative
humidity recoveries will be poor. Even though there is no Red
Flag, conditions will be near critical, especially given the
record dry fuel moisture. The downslope flow brought well above
normal temperatures this afternoon all the way to the coast. A few
records were set (Santa Rosa 75, SFO 73, Half Moon Bay 72) and a
few tied (Kentfield 72 and Napa 74).
For tonight, as mentioned previously winds will weaken overnight,
but remain breezy in the hills. Given the dry airmass and clear
skies if winds go calm in any protected valley temperatures will
be cooler than forecast. Definitely a lower confidence part of the
forecast tonight.
Tomorrow will feature some cooler temperatures (still above normal
however) for areas north of Monterey Bay. Monterey Bay region
southward could actually see temperatures a few degrees warmer due
to lingering offshore flow.
Further cooling is expected on Wednesday, but once again despite
the cooling, temperatures will be in the 60s to lower 70s or
still above normal. All focus in the forecast once again turns
toward fire weather. Models show another burst of offshore flow
across much of the Bay Area. The SFO-WMC is only projected to be
be around -11 to -12mb. Last night's offshore gradient peaked
close to -14mb. One difference is leading up to the last event a
weak boundary brought much higher RH before the offshore flow
kicked in. This time, no such boundary and RH looks to be lower.
Did a cross section across the North Bay and downward momentum
looks less, which means less wind reaching the valley floor.
However, above 1,000 feet still looks pretty windy. Local WRF has
some 50 kt flags at just above some of the highest peaks (Mt St
Helena) Wednesday night. One thing that has not changed are the
record low fuel moistures. Therefore, a Fire Weather Watch may be
needed if the trend continues. Night shift can coordinate with the
surrounding offices.
Previous Discussion....Beyond Thursday morning, an upper-level
trough will rapidly amplify and center itself over the
intermountain west before moving east of the Rockies through the
weekend. As a result, can expect a more concrete NW-return flow
that will trigger a cooling trend through at least Friday. Should
see a return to more widespread morning lows in the mid-to-upper-
30s F across the interior and highs that are closer to seasonal.
ECMWF ENS do hint at another trough developing just off the PAC NW
coast during the latter half of the weekend and through early
next week. Should that play out, could see the development of
another moisture tongue over Washington, Oregon, and far Northern
California. How much of that moisture makes it down into our CWA
is still open for debate, but it should be noted that some of the
GEFS members take the plume on a more robust southward track. For
now, any chances of precip remain extremely unlikely given the
temporal distance between now and then (210+ hours) and the
ensemble uncertainties in the actual trajectory of the plume. But
will keep an eye on it as we get closer to the second half of the
week.
&&
.AVIATION...as of 10:00 PM PST Monday...For the 06z TAFs. VFR
conditions with mainly clear skies through the period as the area
remains under a dry air mass. Winds light and variable overnight.
Tuesday, light to moderate winds will prevail, with continued
offshore flow keeping skies mostly clear.
Vicinity of KSFO...VFR through the taf period. Winds SSE at 6 kt,
becoming light and variable overnight into Tuesday morning.
Nothing more than a few high clouds anticipated overnight.
Continued light winds and clear skies Tuesday afternoon.
SFO Bridge Approach...Similar to KSFO.
Monterey Bay Terminals...VFR with mainly clear skies through the
period. Generally light winds around the Bay. Steady SE drainage
winds expected to develop in the Salinas Valley overnight. Light
to locally moderate onshore winds forecast for Tuesday afternoon.
&&
.BEACHES...as of 03:45 PM PST Monday...The expected very large,
very long period northwest swell train has arrived to the coastal
waters and will peak from tonight through Tuesday along the
coast. For today, the primary hazard will be infrequent yet
dangerous sneaker waves and stronger rip currents. By tonight, the
swell heights are forecast to increase rapidly and the primary
hazard will shift towards large breaking waves of roughly 25 feet
at west and northwest facing beaches. A Beach Hazard Statement is
in effect during the day today for the sneaker wave threat while a
High Surf Warning is in effect for tonight through the day
Tuesday for the large breaking waves. A Beach Hazard Statement was
also added to the Northern Monterey Bay (including Santa Cruz)
through tomorrow afternoon. Though typically more sheltered from
northwest swell trains, the large nature of the upcoming swell
will still increase the risk of dangerous sneaker waves and rip
currents throughout the event. Cold water drownings occur each
year with these type of events but are completely avoidable by
remaining a safe distance from the coastline. If you must visit
the coastline, avoid venturing out on coastal rocks, outcroppings,
jetties, etc, and remain extremely vigilant of your surroundings
at all times.
&&
.MARINE...as of 08:54 PM PST Monday...Winds have diminished across
the coastal waters and will remain light to locally moderate and
northerly through Tuesday afternoon. Late Tuesday evening, winds
will gradually increase over the waters and become gusty through
late week. A long period northwest swell has reached the coastal
waters with building seas peaking overnight into Tuesday. This
building swell will result in hazardous seas conditions,
particularly for smaller vessels. Potential hazards are also
expected along the immediate coast in the surf zone. Swell
heights and periods will gradually subside mid- week.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
.Tngt...High Surf Warning...
SCA...Mry Bay
SCA...Pt Arena to Pt Reyes 0-10 nm
SCA...Pt Reyes to Pigeon Pt 0-10 nm
SCA...Pigeon Pt to Pt Pinos 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: MM/Diaz
AVIATION: Lorber
MARINE: Lorber
FIRE WEATHER: RGass/MM/RWW
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www.youtube.com/nwsbayareaThe national radar mosaic for the moment the first event began.
4 high surf, 1 sneakerwave. Local standard time, as filed.
The buoy at Half Moon Bay reported swells of 15 to 20 feet at periods of 17 to 19 seconds.
Surfers at Mavericks near Half Moon Bay reported waves in excess of 40 feet https://www.surfline.com/surf-news/mavericks-day-days/106147.
A man and women were swept into the ocean in Pacifica during dangerous high surf. The woman was rescued but crews were unable to save the man https://fox40.com/news/california-connection/dangerous-surf-sweeps-3-people-out-to-ocean-woman-rescued/.
A woman walking her dog on a beach was knocked over by a sneaker wave and nearly drowned https://fox40.com/news/california-connection/dangerous-surf-sweeps-3-people-out-to-ocean-woman-rescued/. Exact timing and location unknown.
High Surf near Santa Cruz caused boats to become loose and damage docks. The Capitola Wharf was ultimately closed to be inspected for damage. One surfer needed to be rescued https://www.ksbw.com/article/high-surf-causes-damage-and-major-water-rescues/34911169. Exact timing of damage unknown.
As the offices recorded each death: direct deaths were caused by the weather itself, indirect ones by something it set in motion.
| Event | Date | Age | Sex | Where | Kind |
|---|---|---|---|---|---|
| High Surf, San Francisco Peninsula Coast | 2020-12-08 | In Water | Direct |
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in California in 2020Every state in 2020This day in weather historyWarnings in California, December 2020
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: 927851, 927850, 927849, 927848, 927852.