A cold upper low moved through the region in early March bringing widespread showers and isolated thunderstorms to the Greater Bay Area. This system caused roadway flooding, debris flows, lightning, and small hail.
Every event with a place to put it; 1 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 cold upper low moved through the region in early March bringing widespread showers and isolated thunderstorms to the Greater Bay Area. This system caused roadway flooding, debris flows, lightning, and small hail. Snow was also reported on some of the areas peaks throughout the region as snow levels dropped down to 2,000 ft. A mudslide occurred along the River Fire burn scar in Monterey County sending mud and debris into nearby homes.
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 Mar 10, 4:09 AM PST, 39 minutes earlier.
Area Forecast Discussion
National Weather Service San Francisco Bay Area
409 AM PST Wed Mar 10 2021
.SYNOPSIS...Numerous showers and isolated thunderstorms will
continue today into this evening producing brief heavy downpours
and small hail. Showers will end by early Thursday morning for
the Bay Area but linger across the Central Coast into Thursday.
High pressure builds Friday and Saturday with some dry northerly
winds bringing a warming trend with mostly clear skies. Next
chance of rain arrives Sunday into early Monday as the next trough
approaches Northern California.
&&
.DISCUSSION...as of 3:44 AM PST Wednesday...Showers and isolated
thunderstorms have continued overnight and this trend will
continue through today into this evening. Have already received
reports of small hail overnight with convective cells as they move
onshore. The coldest air aloft is still offshore but will
approach the Bay Area today. Over the last hour or so we've seen
several lightning strikes over the ocean waters from the Monterey
peninsula southward to Big Sur. As this band moves onshore expect
more small hail, brief heavy downpours and some lightning
flashes.
The core of the upper low is west of Cape Mendocino and will
slowly wobble towards the Golden Gate today but is progged to
remain just offshore and over the ocean. 700 mb temps are down to
around -14 celsius with good ice growth in the clouds along with
steep lapse rates aloft given the dynamic upper low. All this to
say is we can expect numerous showers and isolated t-storms
through the day and into this evening. Daytime heating is almost
inconsequential but of course can give a little added lift.
Nonetheless given the slow movement of the upper low expect
showers to last through around midnight for the Bay Area with
showers continuing for the Central Coast right into Thursday
morning. Some of the noted weather impacts we will be monitoring
the next 24 hours include: hail that can quickly cover roadways,
heavy downpours over 2020 burn areas and low elevation snows.
The 12z sounding that is still lifting off showed the freezing
level around 875 mb but were seeing snow levels down to around
2000-2500 feet today in heavier showers as the coldest air aloft
passes overhead. In general this should remain above most Bay Area
roadways but we could get some reports along Highway 9 and Skyline
Blvd as well as Highway 29 heading out of Calistoga. More
significant snowfall accumulations will be possible over the
higher terrain of Monterey County as well, especially with showers
lingering right into Thursday. Well most of the region will dry
out on Thursday the instability with the upper low will allow
plenty of afternoon cumulus with some showers Thursday afternoon
for the higher terrain from Mt Hamilton southward into San Benito
county as well as the Santa Cruz mtns and Santa Lucia range.
As the low drops south and east by Friday high pressure will nose
over the region. Circulation around the low will generate some
drying northeast winds across the Bay Area. This will bring a
noted warming trend Friday afternoon, especially for the North Bay
as temps warm into the upper 60s (it'll feel warm after low to mid
50s today). Dry and mild weather Saturday.
Models coming into better agreement for some more light rain
chances Sunday into early Monday with NBM PoPs in the 40-50% range
and nearly all mid range models converging on the idea of a light
to moderate strength wx system.
No clear signals for next week with pattern remaining active
enough to keep some rain chances in the forecast later next week.
No signal for any strong systems but no strong ridge forecast
either.
&&
.AVIATION...as of 4:09 AM PST Wednesday...For the 12Z TAFs.
Scattered showers continue across the Bay Area and the Central
Coast, with isolated thunderstorms popping over the South Bay and
around the Monterey Peninsula at this hour. Cigs remain mostly
SCT above 2500 and BKN above 3500 with reduced visibility only
during pockets of heavier rain. Scattered storms will continue
through the day with slight clearing in some areas by midday, but
cigs/vis should remain VFR/MVFR throughout the period. Winds are
mostly out southerly and breezy. A few stronger gusts during
heavier rain. Winds will remain breezy through the day and slowly
shift from south to southwest, before becoming lighter Wednesday
evening. As the low tracks southward down the California
coastline, chances for continued showers start to diminish after
06Z from North to South, with showers remaining in the vicinity
of the South Bay down the Central Coast.
Vicinity of KSFO...Scattered showers continue with the chance of
an isolated stronger period of showers and thunderstorms. While
observations have most of the lightning down the Big Sur coast,
the risk for lightning near the terminal still exists, and will be
monitored through the morning. Winds are breezy and out of the
south, shifting to the southwest later today with a few stronger
gusts around 20 kts possible. Scattered showers will prevail
through Wednesday. As the driving system moves southward, showers
will become more infrequent after 6Z. Winds will remain breezy and
out of the south Wednesday night. MVFR/VFR conditions through the
TAF period, with IFR briefly possible for reduced vis if a strong
cell passes directly over the terminal.
SFO Bridge Approach...Similar to KSFO.
Monterey Bay Terminals... Thunderstorms just passed over the
terminal to make for an exciting morning. Strongest cells now pass
farther inland, but scattered shower continue to move onshore from
the ocean. MVFR/VFR cigs and vis expected through the TAF period
with intermittent rain through Wednesday. Winds will be out of the
south-southwest through the day and breezy. Scattered showers
expected Wednesday evening as winds weaken.
&&
.MARINE...as of 02:36 AM PST Wednesday...Continuous scattered
showers persist this Wednesday morning with occasional
thunderstorms producing lightning and small hail. Storms are
expected through Thursday morning. The strongest storms can
generate erratic gusty winds and steep fresh seas, reduced
visibility, and other hazards. Winds remain out of the west-
southwest and breezy, expecting to shift to the northwest by
Wednesday afternoon. Seas remain hazardous for smaller craft
vessels through Thursday as the large northwest swell prevails.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
.Tday...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...Pigeon Pt to Pt Piedras Blancas 10-60 nm
SCA...Pt Arena to Pigeon Pt 10-60 nm
&&
PUBLIC FORECAST: RWW
AVIATION: DK
MARINE: DK
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www.youtube.com/nwsbayareaThe San Francisco radar (KMUX), 68 km away, was scanning. The volume nearest the first event began at 12:45 UTC.
6 hail, 4 flood, 1 winter weather. Local standard time, as filed.
Mudslide reported on River Road between Chualar and Chualar River Road. Over a foot of mud into several homes. Homes were condemned in January so were not occupied at the time.
Public reported accumulating hail in Scotts Valley at around 1500 ft.
This roadway flooding occurred at 3601 Telegraph Ave.
Image on social media showing about an inch of snow in the Santa Cruz Mountains at 3000 ft.
Image on social media shows small hail in Aptos.
Image on social media shows small hail in Marin.
Roadway flooding northbound 280 & 87 due to heavy rain.
Roadway flooding due to heavy rain at the southbound 280 offramp to Alum Rock offramp.
Santa Rosa Fire Department reports multiple vehicle accidents on hwy 101 due to accumulating small hail.
Spotter reported small hail in San Francisco.
Image on social media showing small hail near the San Jose Airport.
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, March 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: 947397, 947424, 947402, 947422, 947423, 947418, 947404, 947406, 947417, 947421, 947419.