Tornadoes and Flash Flood in California, November 17 to 18, 2020

Name: Tornadoes and Flash Flood in California, November 17 to 18, 2020
Start: 2020-11-17T10:00
End: 2020-11-18T18:00
Location: California

November 17 to 18, 2020 · NWS STO · 7 events in 5 counties and zones

Where it happened

Every event with a place to put it. Tap one for its entry below.

Tornadoes
Tropical storms and surge
Floods
Wind
Hail
Winter weather
Heat and drought
Cold
Fire, smoke and dust
Marine and coastal
Other

Open the live radar over this ground

The toll

Deaths: 0
Injuries: 0
Property damage: $15,000
Crop damage: $1,000
Events: 7

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.

What happened

The episode as NWS STO wrote it up.

Low pressure formed off the coast of California and tapped into tropical moisture which lead to widespread Valley rain, mountain snow, and eventually even thunderstorm development over interior northern California. Valley rain amounts generally ranged from 0.25 to 1.0 inch of rain while foothill locations received 1.0 to 4.0 inches of rain. High elevation mountain snow was bountiful too with most sites reporting 1 to 2 feet of snow. The next day, behind the main front, widespread showers and thunderstorms developed on Nov 19.

What forecasters said that day

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 Sacramento, CA wrote its last area forecast discussion before the first event at Nov 17, 4:00 AM PST, 6 hours earlier.

Area forecast discussion, Nov 17, 4:00 AM PST, 6 hours before

Area Forecast Discussion
National Weather Service Sacramento CA
400 AM PST Tue Nov 17 2020

.SYNOPSIS...
Rain, high elevation snow, and gusty winds thru Wednesday with
drier weather returning towards the end of the week.

&&

.DISCUSSION...
Mid and high clouds are increasing across NorCal early this
morning as the trough approaches. Southerly winds are also
beginning to increase across portions of the region, and local
downslope from the Sierra is keeping temperatures in the upper 40s
and 50s along the eastern edge of the Sacramento Valley and the
northern San Joaquin Valley.

Radar is beginning to pickup precipitation associated with the
frontal band off the northwest coast of California. Hi-res models
time the north-south oriented band of precipitation with the front
into Butte/western Plumas Counties around noon (where the flash
flood watch for the North Complex burn scar is in effect), and
south to around the I-80 corridor late this afternoon (just in
time for the evening commute in the Sacramento area).

HREF is indicating high probability (over 80 percent) of hourly
rainfall rates exceeding 1/2 inch across the North Complex burn
scar during the mid to late afternoon hours as the N-S orientation
will prolong heavier rainfall potential. Additionally, 24-36 hour
QPF for that area is around 2.5 to 4.5 inches, so ash flows from
the recent wildfires are a distinct possibility. The heaviest
precipitation is expected to taper off in this area this evening,
but several more incoming short-waves will continue showers, some
heavy at times, through Wednesday.

Significant snowfall is forecast across the higher mountain
elevations later today into Wednesday with several feet of
accumulation expected near the higher northern Sierra passes and
across the peaks. Strong southwest winds will also be possible
into tonight. As this is a milder, wetter storm, snow levels will
be higher around 6500 to 7500 feet, lowering to 5500 to 6500 feet
on Wednesday. Heavy snowfall will likely hold off across Donner
Pass on I-80 until late this afternoon.

Unstable post-frontal airmass will support continuing showers over
the mountains on Wednesday. A few lines of showers/thunderstorms
may organize over the Central Valley on Wednesday as a couple
stronger short-waves move across the region, but QPF likely won't
be as high as today given the faster movement of storms along with
less deep available moisture. Shower chances expected to taper off
Wednesday night and Thursday as the trough begins to weaken and
lift out.

&&

.EXTENDED DISCUSSION (Saturday THROUGH Tuesday)...
Dry Saturday with highs slightly below normal as upper ridging
moves through. Pacific frontal system progged to drop into
northern portions of the CWA Sunday and weaken. Best chances for
precipitation will be over the coastal and northern mountains.
Threat of overrunning light precipitation will continue in the
northern portions of the forecast area Monday into Tuesday.

&&

.AVIATION...
Dtrg conds fm NW to SE ovr intr NorCal tda aft 15z as Pac fntl
sys movs inld. Areas MVFR with LCL IFR and LIFR omtns in pcpn nxt
24 hrs. Sn lvls 060-070 amsl. Stg SWly flo alf with areas Sly sfc
wnd gsts to 25 kts in Cntrl Vly and S-SW sfc wnd gsts 30-50 kts
ovr hyr mtn trrn.

&&

.STO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from 10 AM PST this morning through late
tonight for Northeast Foothills/Sacramento Valley-Western Plumas
County/Lassen Park.

Winter Storm Warning from 10 AM this morning to 6 PM PST
Wednesday for West Slope Northern Sierra Nevada-Western Plumas
County/Lassen Park.

&&

The radar

The Beale AFB radar (KBBX), 119 km away, was scanning. The volume nearest the first event began at 18:01 UTC.

Open the radar archive at this moment

Event by event

2 winter storm, 2 tornado, 1 flood, 1 flash flood, 1 hail. Local standard time, as filed.

Winter Storm, West Slope Northern Sierra Nevada

When: November 17, 10:00 AM PST to November 18, 6:00 PM PST
Where: West Slope Northern Sierra Nevada forecast zone
Reported by: Public

Multiple ski resorts across the Sierra reported roughly 1 to 2 feet of snow for the event. Squaw-Alpine Ski resort reported 23 inches of storm total snow. Boreal and Soda Springs ski resorts reported 21 inches of storm total snow. Sugar Bowl reported 16 inches and Bear Valley reported 10 inches of storm total snow accumulation.

Winter Storm, West Slope Northern Sierra Nevada

When: November 17, 10:00 AM PST to November 18, 6:00 PM PST
Where: West Slope Northern Sierra Nevada forecast zone
Reported by: Department of Highways

Caltrans reported roughly 2 to 2.5 feet of snow for the event along Interstate 80 for the higher elevations of the Sierra. Kingvale reported 17 inches of storm total snow. Donner Summit reported 27 inches of storm total snow and Soda Springs reported 26 inches of storm total snow.

Flood, Shasta

When: November 18, 10:00 AM PST to 12:00 PM PST
Where: Shasta County, 0.8 mi SSE of Anderson to 1.05 mi SE of Anderson
Cause: Heavy Rain
Reported by: Law Enforcement
Coordinates: 40.440, -122.292

California Highway Patrol reported minor flooding along Interstate 5 near Anderson, California. Time of flooding is estimated by radar. Flooding location is also estimated.

Flash Flood, Butte

When: November 18, 12:31 PM PST to 3:15 PM PST
Where: Butte County, 1.74 mi SE of Brush Creek to 0.6 mi ESE of Berry Creek
Cause: Heavy Rain / Burn Area
Property damage: $15,000
Crop damage: $1,000
Reported by: County Official
Coordinates: 39.660, -121.310

A number of trees were reported down in roadways on Bald Rock Road. Significant amounts of ash and sediment were reported in roadway ditches that also overtopped the roadway in multiple locations. Timing is based on radar estimate. Location is also estimated. Damage is also estimated for debris and sediment removal, as we are unaware of any crop damage.

Tornado, Butte

When: November 18, 1:55 PM PST to 2:15 PM PST
Where: Butte County, 3.48 mi WNW of Bloomer to 3.89 mi WNW of Bloomer
Rating: EF0
Path length: 0.47 mi
Path width: 3 yd
Reported by: Public
Coordinates: 39.650, -121.560

A tornado was reported by multiple public sources. A chain link fence was knocked over and a few tree branches fell. No other damage has been reported. Tornado width and path is estimated.

Tornado, El Dorado

When: November 18, 3:30 PM PST to 3:50 PM PST
Where: El Dorado County, 3.4 mi NW of Dugan to 0.34 mi S of Frenchtown
Rating: EF0
Path length: 4.39 mi
Path width: 2 yd
Reported by: Public
Coordinates: 38.630, -121.000

A funnel cloud briefly touched down near this location with no damage reported. Location is estimated based on radar. Tornado width is also estimated.

Hail, Calaveras

When: November 18, 4:15 PM PST to 4:20 PM PST
Where: Calaveras County, 1.38 mi S of Valley Spgs
Measured: 0.88 in hail
Reported by: Trained Spotter
Coordinates: 38.180, -120.830

Nickel size hail reported near New Hogan Dam. Duration of hail is estimated.

What these are

Winter Storm: A storm that brought snow, sleet or freezing rain at warning levels, usually over several hours or days.
Flood: Flooding that develops more slowly than a flash flood: rivers leaving their banks, or water standing over land for an extended time.
Flash Flood: Flooding that begins within six hours of the rain that causes it, or of a dam or levee failure or ice jam release, often with deadly speed.
Tornado: A violently rotating column of air in contact with the ground and extending from a thunderstorm, filed with its path, width and Fujita or Enhanced Fujita rating from the damage survey.
Hail: Hail three-quarters of an inch or larger, or smaller hail that caused damage, filed with the largest stone measured.

After NWS Directive 10-1605, the rules offices file Storm Data by.

The same day elsewhere

Every storm in California in 2020 Every state in 2020 This day in weather history The tornado archive for California, 2020 Warnings in California, November 2020

Sources

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: 930957, 930959, 930952, 930949, 930947, 930948, 930953.