A mesoscale convective system (organized thunderstorms) producing a dust storm (haboob) in the Imperial Valley, Salton Sea and Borrego deserts on the evening of August 10th. Visibility was reported as low as 1/2 mile to 1 mile observed.
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The episode as NWS SGX wrote it up.
A mesoscale convective system (organized thunderstorms) producing a dust storm (haboob) in the Imperial Valley, Salton Sea and Borrego deserts on the evening of August 10th. Visibility was reported as low as 1/2 mile to 1 mile observed. This was followed by hours of lightning and heavy rain in the deserts, mountains and even reaching the coast at the city of San Diego. The system brought wind gusts 40 to 50 mph, heavy rainfall up to 2 inches in the San Diego mountains and desert slopes, and copious amounts of lightning with parts of the desert with localized flash flooding. The southern coastal San Diego County received near one quarter of an inch.
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 Aug 10, 7:21 AM PST, 3 hours 54 minutes earlier.
Area Forecast Discussion
National Weather Service San Diego CA
821 AM PDT Tue Aug 10 2021
.SYNOPSIS...
Increasing monsoonal moisture will bring isolated to scattered
thunderstorms over the mountains, and to a lesser extent, the
deserts and far inland valleys each afternoon and early evening
through Wednesday. Locally heavy rainfall and isolated flash
flooding will be possible near thunderstorms. Drying for Thursday
and Friday with decreasing chances for thunderstorms. Monsoonal
moisture may increase some again for Saturday through Monday with
chances for afternoon and early evening thunderstorms in the
mountains and deserts.
&&
.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...
We have clouds from both worlds in our region this morning, the
coastal low clouds and the higher thunderstorm debris clouds from
the east. The presence of the coastal clouds will be diminishing
over the next few days while the monsoon clouds increase. Today
and Wednesday we have increasing moisture and instability leading
to a greater chance of thunderstorms during the afternoons and
evenings. These are most likely in the mountains, with gradually
lesser chances through the high desert, low desert and inland
valleys. Some models indicate some very powerful storms in Arizona
or the far southeast CA desert late at night could migrate
westward into our region as they collapse and weaken. So leftover
showers are possible later tonight and early Wednesday in these
same areas. Wednesday afternoon and evening looks very similar for
the coverage, timing and intensity of rain. Rates either afternoon
in thunderstorms could reach 1.25+ inches per hour, which would
be sufficient to trigger flash flooding, but this rate would be
very isolated. Much less (0.50 in/hr) would do the trick in burn
scars like the Apple and El Dorado. Obviously, flash flooding and
debris flows are our biggest hazard concern today through
Wednesday, as indicated by the flash flood watch. Near the coast,
the marine layer will take a step or two backward while the
monsoon push takes center stage. On Thursday and Friday the
moisture decreases gradually, although there's still a slight
chance of afternoon thunderstorms in the mountains and high
desert Thursday afternoon. Right now it appears too dry for any
chance Friday. The coastal clouds will be patchy at most during
nights and mornings these days. Inland temperatures will rise
Thursday into Saturday with the drier and sunnier skies. This
weekend we've been watching various forms of another increase in
monsoonal moisture, maybe in the form of an easterly wave. Recent
model runs have backed off a bit and ensembles show less
organization and lower confidence. The chance of afternoon
thunderstorms mainly in mountains should return Saturday, possibly
peaking Sunday and Monday afternoons. Ensembles are showing
greater confidence in a West Coast trough gradually developing
next week that would sweep away any monsoon moisture to the east.
But will the drying and cooling trend start Monday, Tuesday or
Wednesday? Or not at all? We're moderately confident that things
will gradually trend toward cooler and drier next week around
Monday or Tuesday, and the trough would also help rebuild the
marine layer and coastal cloud regime.
&&
.AVIATION...
101515Z...Coast/Valleys...BKN-OVC low clouds with bases 500-1000 ft
MSL with tops to 1400 ft MSL will continue to clear through 17Z.
Patchy low clouds redeveloping at the beaches after 06Z Wed with
bases 500-1000 ft MSL. Cigs expected to form at coastal TAF sites 07-
11Z Wed. Local vis restrictions of 3-5SM where low clouds meet
higher coastal terrain.
There is a slight chance of TSRA in the valleys from 21Z-03Z Wed. CB
bases would be around 10,000 ft MSL and tops to 25,000 ft MSL along
with strong up/downdrafts and local strong gusty surface winds.
Otherwise, mostly unrestricted vis will prevail with occasional
SCT/BKN clouds at/above 10,000 ft MSL through Tue evening.
Mountains/Deserts...SCT TSRA will occur over the mountains and
deserts after 21Z this afternoon with CB bases around 9,000 ft MSL
and tops to 45,000 ft MSL. FEW-SCT clouds at/above 10,000 ft MSL in
places where t-storms do not develop. Strong up/downdrafts and local
strong gusty surface winds expected near TSRA, with local vis below
3 miles possible.
&&
.MARINE...
No hazardous marine conditions are expected through Saturday.
&&
.SKYWARN...
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.
&&
.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Flash Flood Watch from noon PDT today through Wednesday evening
for Apple and Lucerne Valleys-Riverside County Mountains-
San Bernardino County Mountains-San Diego County Mountains-
San Gorgonio Pass Near Banning.
PZ...NONE.
&&
PUBLIC...MM
AVIATION/MARINE...COThe San Diego radar (KNKX), 51 km away, was scanning. The volume nearest the first event began at 19:16 UTC.
9 heavy rain, 3 flash flood, 2 thunderstorm wind, 1 lightning, 1 dust storm. Local standard time, as filed.
A thunderstorm produced heavy rainfall at Palomar Mountain RAWS. 1.09 inches of rain fell from 12:15 pm-1:15 pm.
Lightning caused 6 wildfires starts despite heavy rain. The wildfires were contained and kept to small acreage.
A dust storm (haboob) moved across the San Diego Deserts from east to west and originated in the Imperial Valley and southeast California ahead of organized thunderstorms. Visibility went to 1/2 mile in the deserts before heavy rain occurred.
Julian RAWS measures 0.70 inches of rain.
CNRFC's San Felipe station reports a total of 2.49 inches of rain.
The Ranchita RAWS measures 2.27 inches of rain, most which fell in two hours.
Lake Cuyamaca measures 0.82 inches of rain.
Ocotillo Wells reports 0.84 inches of rain.
Volcan Mountain measures 1.26 inches of rain.
CNRFC site Coyote Canyon Creek measures 1.66 inches of rain.
CHP reports flash flooding at Borrego Salton Seaway road at Fonts wash, where a car was stuck in the low water crossing.
Mason Valley station reports 1.84 inches of rain.
Reports of water and mud covered Borrego Valley Airport after the storm's heavy rainfall. Pilots were not able to land on the runway. The airport also reported blowing dust with two miles of visibility and a wind gust to 40 mph.
Large tree downed on Tamarisk Row by strong winds. Reported by KESQ TV.
Flash flooding reported by Desert Research Institute officials morning after heavy rains went through the area. Monsoon storms led to flash flooding of Coyote Canyon and adjacent citrus tree fields.
Big Black Mountain measures thunderstorm wind gust of 57 MPH.
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, August 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: 981110, 983708, 983705, 980579, 980577, 980578, 980580, 980581, 981104, 981106, 981109, 981105, 981107, 981112, 981804, 981111.