An area of high pressure over the Southwest led to warm conditions over SoCal, especially across the deserts on August 15th and 16th.
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 SGX wrote it up.
An area of high pressure over the Southwest led to warm conditions over SoCal, especially across the deserts on August 15th and 16th.
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 15, 1:30 AM PST, 6 hours 30 minutes earlier.
Area Forecast Discussion
National Weather Service San Diego CA
230 AM PDT Sun Aug 15 2021
.SYNOPSIS...
A strong ridge, centered over northern California, will team up with
a west bound trough over Mexico, to keep a brisk easterly flow aloft
over Southern California through Monday. Monsoonal moisture embedded
in that flow will bring periods of clouds and even a chance for a
stray shower or thunderstorm at times. The best chance for any heavy
rainfall will be near the mountains and over the high deserts.
Occasional clouds and increasing low-level moisture will help to
moderate daytime temperatures, except over the high deserts where
excessive heat is expected through Monday. Later this week, a trough
along the West Coast will bring a drier southwest flow aloft, and
cooler weather.
&&
.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...
Scattered showers and thunderstorms have been forming overnight over
western AZ, and drifting across the lower desert areas of Southern
CA. One shower developed over northern San Diego County earlier
and left some very light rainfall in spots there. Other showers
were moving into the deserts at 2 AM PDT. Based on radar tops
and lightning activity, they were weakening. Still, some scattered,
light showers seem likely to continue as this activity drifts
westward. Lower, marine clouds have been masked by the higher
clouds, but were mostly offshore. The sfc pressure gradients have
weakened, and wind reports were mostly light early this morning.
Clouds and a few showers have been added to the forecast for San
Diego County this morning, otherwise the forecast continues to
highlight any significant thunderstorm activity near the mountains
in the afternoons. Winds aloft are too strong to favor strong or
persistent shwrs/tstms, but slight chance wording remains in the
forecast. The best chance will be over the northern mountain areas
where mid-level winds are forecast weaker.
The strong ridge centered over northern California and the Great
Basin will peak in strength today, then slowly weaken through
Tuesday as a west bound easterly wave passes to the south. The flow
aloft over SoCal between these two systems will be rather strong
from the E-NE through Monday, and then weaken on Tuesday, before
switching to SW on Wednesday as a trough in the Westerlies deepens
over over the West. Under the strong ridge, and weaker onshore
flow, heat will increase over our high deserts today and Monday,
where an Excessive Heat warning remains in effect. Elsewhere,
seasonally warm to hot summer weather will continue.
The brisk E-NE flow aloft will continue to steer weak disturbances
and debris from Lower Colorado River Valley convection, over us
through Monday. This will mean periods of mid-high clouds, and
possibly a few showers or isolated thunderstorms. The best chance
for any measurable, or brief, heavy rainfall will be near the
mountains though. On Tuesday, the winds aloft will weaken as the
ridge to the north collapses and the trough deepens from the north.
Precipitable water values will still be quite high, so given the
weak winds forecast, it would seem thunderstorms would have a better
chance to develop over the mountains then.
By Wednesday, the deepening trough from the north should be in full
control, with southwest flow aloft, cooling, and decreasing chances
for convection as atmospheric moisture dries up over the mts/
deserts. This will not be the case over the coastal basin though,
because low pressure will be slowly developing in the mid-lower
atmosphere just off the CA Coast. This cyclonic flow will favor a
deep marine layer and more low clouds into next weekend. The trough
retrogrades westward early next week as a ridge rebuilds along 30N
Latitude. This should keep the weather dry with a warming trend.
&&
.AVIATION...
150830Z...Patchy low clouds with bases 700-1000 ft MSL and tops to
1200 ft MSL continuing along the coast and up to 5 mi inland through
the morning. Coastal airports may have brief periods of CIGS through
about 15Z, mainly in San Diego county. Low clouds clearing to the
coast by 16Z. Patchy low clouds may redevelop after 16/07Z with
bases 800-1100 ft MSL.
Isolated SHRA/TSRA will move west across San Diego County this
morning. CB bases near 10,000 ft MSL with tops to 30,000 ft MSL. Low
confidence in any impacts at TAF sites. There is a slight chance of
TSRA over the mountains this afternoon with bases 9,000 ft MSL and
tops to 40,000 ft MSL and strong up/downdrafts this afternoon,
dissipating by 01Z. Otherwise SCT-BKN clouds at/above 10,000 ft MSL
through Monday morning.
&&
.MARINE...
No hazardous marine conditions are expected through Thursday.
&&
.SKYWARN...
Skywarn activation is not requested.
&&
.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Excessive Heat Warning until 8 PM Monday for Apple and Lucerne
Valleys.
PZ...NONE.
&&
PUBLIC...10
AVIATION/MARINE...SSThe national radar mosaic for the moment the first event began.
1 excessive heat. Local standard time, as filed.
Areas of the High Desert reported temperatures between 100 and 105 degrees.
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: 979454.