A large area of high pressure over the Desert Southwest led to hot and dry conditions across the region, most notably in the Lower Deserts July 9th through 11th with temperatures over 115 degrees reported.
Every event with a place to put it; 2 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.
A large area of high pressure over the Desert Southwest led to hot and dry conditions across the region, most notably in the Lower Deserts July 9th through 11th with temperatures over 115 degrees reported.
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 9, 4:22 AM PST, 3 hours 38 minutes earlier.
Area Forecast Discussion
National Weather Service San Diego CA
522 AM PDT Fri Jul 9 2021
.SYNOPSIS...
Strong high pressure over the southwest states will continue the
heat wave for inland areas into early next week with dangerous
heat for the deserts. Chances remain very small for any afternoon
showers or thunderstorms near the mountains with dry lightning and
gusty winds possible. Slow cooling will spread inland during the
middle and latter part of next week as the high weakens.
&&
.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...
Strong high pressure will remain centered across the southwest
states and Great Basin through early next week, then slowly
weaken during the middle and latter part of next week as a trough
of low pressure moves to near the West Coast from around the Bay
Area northward. Very hot weather will continue through Monday for
inland areas with hottest temperatures of near 120 in the lower
deserts, around 115 in the Apple and Lucerne Valleys, and 105 for
the inland portions of the Inland Empire and the lower elevations
of the mountains.
NBM probabilities for Victorville to exceed 110 for each day for
Saturday through Monday are around 50 percent while probabilities
for Palm Springs to exceed 120 are less than 10 percent on those
days. Dangerous heat will continue for the deserts into early
next week with the highest heat risk, very high heat risk, across
the high deserts. Nights will be especially warm in the lower
deserts with low temperatures in the 80s and lower 90s.
NKX soundings showed an increase in precipitable water from 1.33
inch on the 12Z early morning sounding on Thursday to 1.59 inch on
the 00Z sounding in the late afternoon on Thursday. With that
increase in moisture, coastal low clouds have had a difficult time
developing over the inner coastal waters and adjacent land areas
overnight. Some drying aloft is expected over the weekend which
may allow for a gradual return of low clouds for coastal areas.
The incoming 12Z NKX sounding shows that the narrow saturated
layer centered around 450 mb on the 00Z sounding is now gone with
a not overly moist layer between 700 and 600 mb. Conditions for
any thunder near the mountains will remain marginal with any
afternoon showers of thunderstorms isolated and mostly along
convergence boundaries in the San Bernardino Mountains and
possibly the Riverside County mountains. Dry lightning and gusty
winds would be the most likely hazards from any thunderstorms.
As the high weakens next week, there will be a gradual cooling
trend spreading inland for Tuesday through Saturday but with high
temperatures remaining above average. There could be a brief
return of slightly greater monsoonal moisture around the middle of
next week. Thunderstorm prospects remain marginal with no more
than 20 percent of ECMWF ensemble members having any measurable
rainfall for Big Bear and all of those very light.
The marine layer and onshore flow will continue to moderate the
heat from the coast inland into the western San Diego County
valleys will high temperatures remaining around average through
Saturday, then slowly cooling into the latter part of next week
with a greater return of night and morning coastal low clouds.
&&
.AVIATION...
091045Z...Coast/Valleys...FEW low clouds with bases 800-1200 feet
MSL could reach the coast this morning but impacts at coastal TAF
sites are doubtful. Any low clouds that reach the coast are expected
to clear by 16Z. Otherwise FEW-SCT clouds at or abv 15,000 ft MSL
and unrestricted vis through today.
Mountains/Deserts...FEW-SCT clouds at or abv 15,000 ft MSL and
unrestricted vis through today.
&&
.MARINE...
No hazardous marine conditions are expected through Tuesday.
&&
.FIRE WEATHER...
Elevated fire weather conditions for Saturday and Sunday
afternoons as lowest afternoon humidity for the deserts falls to
10 to 15 percent with gusty southwest to west sea breeze winds
during the afternoon and evening.
Chances remain very small, less than 15 percent, for any
afternoon thunderstorms in the mountains. The most likely location
for any to form if they did would be along covergence boundaries
in the San Bernardino Mountains this afternoon and Saturday
afternoon and along convergence boundaries in the Riverside County
mountains Saturday afternoon. Dry lightning and gusty winds would
be the most likely hazards from any thunderstorms.
&&
.SKYWARN...
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.
&&
.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Heat Advisory until 8 PM PDT Monday for Riverside County
Mountains-San Bernardino County Mountains-San Diego County
Mountains.
Heat Advisory until 10 PM PDT Sunday for San Bernardino and
Riverside County Valleys-The Inland Empire.
Excessive Heat Warning from 9 AM Saturday to 8 PM PDT Monday for
Coachella Valley-San Diego County Deserts-San Gorgonio Pass
Near Banning.
Excessive Heat Warning until 8 PM PDT Monday for Apple and
Lucerne Valleys.
PZ...NONE.
&&
PUBLIC/FIRE WEATHER...17
AVIATION/MARINE...PGThe national radar mosaic for the moment the first event began.
2 excessive heat. Local standard time, as filed.
Hot temperatures occurred with Palm Springs hitting 120 degrees on July 10th.
Hot temperatures occurred with Anza-Borrego reaching 117 degrees on the 10th and 11th.
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, July 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: 977089, 977090.