A large area of mid-level high pressure over the Southwest United States contributed to a long duration heat wave from August 14th through the 20th. Many records were broken with a couple areas receiving impacts from monsoonal thunderstorm activity.
Every event with a place to put it; 6 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 mid-level high pressure over the Southwest United States contributed to a long duration heat wave from August 14th through the 20th. Many records were broken with a couple areas receiving impacts from monsoonal thunderstorm activity.
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 14, 8:02 AM PST, 1 hour 58 minutes earlier.
Area Forecast Discussion
National Weather Service San Diego CA
902 AM PDT Fri Aug 14 2020
.SYNOPSIS...
Strong high pressure over the southwest states will bring very hot
weather except near the coast for today and Saturday, continuing
for inland areas into next week. A coastal eddy could bring some
relief from the heat for the coast and valleys for Sunday and
Monday with the excessive heat returning for the middle of next
week. There will be clouds near the mountains most afternoons with
thunderstorms possible on a few of those afternoons.
&&
.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...
The higher clouds cleared out overnight, leaving us with a mostly
clear morning. Except smoke from the Ranch Fire eastern L.A. County
has filled much of the coastal basin. Surface heating and onshore
flow should help the smoke disperse somewhat today. There is a tiny
chance of thunderstorms this afternoon in the higher mountains. The
moisture level is a little high for best convection based on surface
heating, which is the only trigger mechanism we have. Clear skies
this morning will maximize that heating, although higher levels are
not very unstable. After speaking of these minor things, the major
thing is the heat. Today begins our official heat wave with heat
warning in place for all areas except higher mountains and coast.
This is valid today through Monday, although some of the inland-
coastal areas and western valleys should get some relief Sunday and
Monday (so warning status might change). The wild card is a coastal
eddy, depicted by guidance to weakly develop Saturday and strengthen
Sunday. That would help bring back more coastal low clouds and
deepen a marine layer (which is mostly squashed to near nothing
right now). A cooling trend at the coast should get started
Saturday, and extend a little farther inland Sunday and Monday. Far
inland areas won't see much change at all, shaving off only few
degrees from temperatures. There isn't a whole lot of change in the
pattern next week. With the center of the upper ridge north or our
latitude, the resulting southeast flow continues to track monsoon
moisture into SoCal. The moisture is still rather high for
thunderstorm production, and the air is not very unstable, so
thunderstorm chances remain rather slim, but nonzero, each
afternoon, mainly in the mountains. Hot weather appears to get
another boost later next week, with deserts going over 115 again, if
they ever drop under 115 at all.
&&
.AVIATION...
141600Z...Coast/Valleys...Mostly clear conditions with unrestricted
visibility through this evening. Patchy low clouds may form tonight
into Saturday morning, mainly in San Diego County, but confidence is
low in impacts to KSAN and KCRQ.
Mountains/Deserts...A slight chance for isolated
showers/thunderstorms across the mountains this afternoon. If
thunderstorms develop, bases AOA 10,000 feet MSL and tops to 30,000
feet MSL. Otherwise, mostly clear skies with unrestricted vis.
&&
.MARINE...
Patchy fog with visibility of 1 nautical mile or less could develop
over the coastal waters each night and morning through this weekend.
Wind gusts around 20 knots can be expected each afternoon and
evening near San Clemente Island. Otherwise, no hazardous marine
weather is expected through Wednesday.
&&
.SKYWARN...
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.
&&
.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Excessive Heat Warning from noon today to 9 PM PDT Monday for
Apple and Lucerne Valleys-Coachella Valley-Orange County
Inland Areas-Riverside County Mountains-San Bernardino
County Mountains-San Bernardino and Riverside County
Valleys-The Inland Empire-San Diego County Deserts-San
Diego County Mountains-San Diego County Valleys-San
Gorgonio Pass Near Banning-Santa Ana Mountains and
Foothills.
PZ...NONE.
&&
PUBLIC...MM
AVIATION/MARINE...Maxwell/SchenkThe national radar mosaic for the moment the first event began.
6 excessive heat, 2 heavy rain. Local standard time, as filed.
Portions of the Coachella Valley recorded temperatures of 110 to 120 degrees during this heat wave. The 14th was the hottest day, where the Palm Springs ASOS reported a 120 degree reading.
Portions of the San Diego deserts recorded temperatures of 110 to 120 degrees during this heat wave. Anza Borrego State Park recorded 118 degrees for three consecutive days from the 14th through the 16th.
Portions of the San Diego valleys recorded temperatures of 95 to 105 degrees during this heat wave. El Cajon reported a high of 102 degrees, breaking the record for the day.
Temperatures ranged from 105 to 115 degrees across the Inland Empire during this event. The Riverside Airport broke 2 daily records with this event.
Temperatures of 95 to 105 degrees were recorded during this heat event. The Laguna Hills CWOP recorded a high temperature of 107 degrees on August 14th.
Areas of the High Desert reported temperatures of 100 to 110 degrees during this heat wave.
Monsoon thunderstorms brought areas of light rainfall with some heavier spots along the Santa Ana River in the San Bernardino County Mountains, where automated gauges picked up over one inch in some spots.
CWOP in the eastern portion of Hemet reported 1.35 inches of rain from monsoon thunderstorms on the 18th.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in California in 2020Every state in 2020This day in weather historyWarnings in California, August 2020
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: 916831, 918531, 918533, 918535, 918537, 918538, 918566, 918570.