Flash Flood and Flood in California, July 30 to 31, 2021

Name: Flash Flood and Flood in California, July 30 to 31, 2021
Start: 2021-07-30T12:35
End: 2021-07-31T13:35
Location: California

July 30 to 31, 2021 · NWS SGX · 7 events in 4 counties and zones

Enhanced monsoonal moisture surged into Southern California, producing heavy thunderstorms across the San Bernardino County Mountains, the Riverside County Mountains and the San Diego County Mountains. A significant debris flow occurred on July 30 across northern portions of the El Dorado Burn Scar, with debris covering Highway 38.

Where it happened

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.

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: $12,000
Crop damage: $12,000
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 SGX wrote it up.

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 San Diego, CA wrote its last area forecast discussion before the first event at Jul 30, 8:04 AM PST, 4 hours 31 minutes earlier.

Area forecast discussion, Jul 30, 8:04 AM PST, 4 hours 31 minutes before

Area Forecast Discussion
National Weather Service San Diego CA
904 AM PDT Fri Jul 30 2021

.SYNOPSIS...
Monsoonal moisture will bring a chance of mainly afternoon and
early evening showers and thunderstorms, mainly for the mountains
and deserts, through Saturday. Then drier and warmer for Sunday
through Thursday. Night and morning low clouds and fog will
continue for coastal areas.

&&

.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...

A greater surge of clouds materialized on the both the desert
side and the coastal side of our region this morning. High clouds,
debris from Arizona thunderstorms last night, have moved into our
region. Coastal low clouds extended farther inland into western
valleys of San Diego County. The deeper, more copious monsoon
moisture today will lead to more thunderstorms over the mountains,
deserts and far inland valleys this afternoon. Arizona and the
Colorado River Valley should get really active late today and
tonight. Some of those storms will move westward while dying
overnight, and showers could actually hit our mountains and
deserts tonight into Saturday morning. A little less moisture, but
better instability will fuel thunderstorms in the mountains and
deserts Saturday afternoon. High resolution models are not too
bullish with storm coverage or rainfall amounts, but of course any
thunderstorm is capable of intense rain producing a localized
flash flood. Inland temperatures will be dampened by the added
humidity and cloudiness today and Saturday. But skies should be
mostly sunny west of the mountains Saturday as the west-moving
moisture of today takes a U-turn. A low pressure trough off the
coast will amplify and push out the monsoon moisture late Saturday
and Sunday, effectively ending our chances for any precipitation
for at least several days. The marine layer will be fine, despite
today's monsoon push, but will continue a little disrupted, which
is the seasonal pattern for early August. A temporary high
pressure ridge builds over our region Monday and Tuesday, along
with drier air will help temperatures quickly rebound up to
several degrees above normal. That will bring back temperatures
around 102 degrees in the Inland Empire and high desert, and
around 112 for the lower desert. Ensembles show increasing
monsoonal moisture moving back into northern Sonora and Baja
Mexico around Wednesday through Friday next week. If it comes more
north, it could bring back chances of rain to SoCal. But SoCal
remains firmly on the drier side of the border for now, so our
forecast remains confidently dry for at least most of next week.

&&

.AVIATION...
301600Z...Coast/Valleys...Areas of low clouds with bases 800-1000 ft
MSL will clear back to near the beaches by 18Z. Local vis 2-4 miles
in haze will also continue through 18Z. Local BKN conditions with
bases around 1000 ft MSL will continue along the beaches this
afternoon, but otherwise SCT-BKN clouds at/above 15000 ft MSL will
prevail. Patchy stratus with bases 600-1000 ft MSL within 10 miles
of the coast and local vis below 3 miles in fog over higher coastal
terrain will occur 03Z-17Z Sat.

Mountains/Deserts...SCT-BKN clouds at/above 15000 ft MSL and mostly
unrestricted vis will continue through Sat morning. Scattered
SHRA/TSRA will occur starting around 20Z, continuing through early
evening. CB bases will be 9000-10000 ft MSL with tops to 40000 ft
MSL, along with locally gusty winds. There is a slight chance of
TSRA reaching KPSP/KTRM this afternoon/early evening.

&&

.MARINE...
Wind gusts approaching 20 knots are possible in the outer waters
each afternoon through Monday. Otherwise, no hazardous marine
conditions are expected through Tuesday.

&&

.SKYWARN...
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...NONE.

&&

PUBLIC...MM
AVIATION/MARINE...Maxwell

The radar

The Santa Ana Mts radar (KSOX), 79 km away, was scanning. The volume nearest the first event began at 20:35 UTC.

Open the radar archive at this moment

Event by event

2 heavy rain, 1 flash flood, 1 flood, 1 dust storm, 1 lightning, 1 hail. Local standard time, as filed.

Flash Flood, San Bernardino

When: July 30, 12:35 PM PST to 6:30 PM PST
Where: San Bernardino County, 2.67 mi ENE of Seven Oaks to 2.22 mi NE of Seven Oaks
Cause: Heavy Rain / Burn Area
Property damage: $10,000
Crop damage: $10,000
Reported by: Department of Highways
Coordinates: 34.170, -116.890

Significant debris flow occurred 1:30 to 2:00 pm on July 30 on Highway 38 along northern portions of the El Dorado burn scar. CalTrans reports road closure both directions on Highway 38 from Glass Road to Seven Oaks due to rock and mudslides on roadway. Restriction from Forest Falls to Lake Williams Drive. Radar has large thunderstorms over Highway 38 and Upper Santa Ana watershed estimated 2 to 2.5 inches of rain on radar in Frog Creek, Barton Creek and South Fork Santa Ana River.

Most significant flows occurred between PM 24.1 and 25.2, where boulders up to 1 m in diameter (potentially up to 2 m) moved. Net anchors initially held and restrained the initial surge of debris, but the volume eventually overtopped the nets and filled catchment basins. Debris overtopped a catchment basin at VAR EF0094 (west fork of the creek) and flowed onto the highway, resulting in a debris flow 150 feet wide and 5 feet deep.

|Vehicles reported stuck and road is impassable at 3 locations. Highway 38 was closed for 7 hours due to several areas of debris flows. Large rocks, logs, mud and boulders being removed. Some damage reported to road. 4300 cubic yards or 400 truckloads were removed from Highway 38.

Heavy Rain, Riverside

When: July 30, 1:30 PM PST to 3:00 PM PST
Where: Riverside County, 4.84 mi N of Anza
Reported by: Mesonet
Coordinates: 33.620, -116.680

Thomas Mountain hads gauge reports 2.56 inches of rainfall. Appears to be in about 60 to 90 minutes duration. This is located just south of Lake Hemet along Highway 74.

Flood, San Bernardino

When: July 30, 2:15 PM PST
Where: San Bernardino County, 8.67 mi W of Yucca Vly Gnt Rck Ar to 8.45 mi W of Yucca Vly Gnt Rck Ar
Cause: Heavy Rain
Property damage: $1,000
Crop damage: $1,000
Reported by: Trained Spotter
Coordinates: 34.350, -116.530

Spotter reported water flowing on Highway 247 Old Woman Springs Road. Radar estimates up to 1 inch of rain from thunderstorms.

Heavy Rain, Riverside

When: July 30, 2:45 PM PST
Where: Riverside County, 0.57 mi E of Idyllwild
Reported by: Mesonet
Coordinates: 33.750, -116.710

Automated weather station at fire station reports 1.5 inches of rain in 60 minutes. 0.66 inches in 15 minutes.

Dust Storm, San Diego County Deserts

When: July 30, 8:00 PM PST to 8:30 PM PST
Where: San Diego County Deserts forecast zone
Reported by: Public
Coordinates: 33.028, -116.770 (the zone’s center; the record gives no point)

Twitter report with video of heavy blowing dust in Borrego Springs. Corresponding peak wind was 30 mph from the south-southeast at approximately 7 pm PDT. Minimum visibility unknown. Duration estimated.

Lightning, San Bernardino

When: July 31, 11:30 AM PST to 12:30 PM PST
Where: San Bernardino County, Big Bear City
Crop damage: $1,000
Reported by: Park/Forest Service
Coordinates: 34.270, -116.850

A 200 year old Jeffery pine tree was struck by lightning. It burned later in the evening until extinguished. The large tree split in half while burning. The thunderstorm on radar appeared to be during several cloud to ground strikes near Big Bear airport around 1230 pm.

Hail, San Diego

When: July 31, 1:35 PM PST
Where: San Diego County, 1 mi SW of (Czz)campo
Measured: 0.5 in hail
Property damage: $1,000
Reported by: Public
Coordinates: 32.590, -116.482

A thunderstorm produced a layer of hail on the ground just north of Campo. Photos were shared. Some hail was around 1/2 inch diameter.

What these are

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.
Heavy Rain: Rainfall totals unusual for the place and season that did not produce a flood.
Flood: Flooding that develops more slowly than a flash flood: rivers leaving their banks, or water standing over land for an extended time.
Dust Storm: Blowing dust that reduced visibility to a quarter mile or less.
Lightning: A lightning strike that killed, injured or caused damage.
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 2021 Every state in 2021 This day in weather history Warnings in California, July 2021

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: 976749, 976811, 976809, 976806, 976818, 976813, 976821.