Leading up to the heatwave, a cooler pattern set up over Southern California, where we saw an upper level trough bring wet and windy weather on May 18th and 19th. Rain totals of up to one inch were observed in parts of the mountains and wind gusts 45-55 miles per hour in areas of the mountains and deserts.
Every event with a place to put it; 3 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.
Leading up to the heatwave, a cooler pattern set up over Southern California, where we saw an upper level trough bring wet and windy weather on May 18th and 19th. Rain totals of up to one inch were observed in parts of the mountains and wind gusts 45-55 miles per hour in areas of the mountains and deserts. By May 26th through 28th, an upper level high pressure system set up over the western U.S. and brought the region some intense heat. Temperatures were especially hot in the deserts with readings about 10 to 15 degrees above average, where parts of the Lower Deserts reached up to 111 degrees at Palm Springs.
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 May 26, 8:15 AM PST, 1 hour 45 minutes earlier.
Area Forecast Discussion
National Weather Service San Diego CA
915 AM PDT Tue May 26 2020
.SYNOPSIS...
High pressure over the area will continue to strengthen today,
keeping temperatures well above average for the deserts and inland
areas. A cooling trend will start Friday west of the mountains, with
high temperatures dropping back down to near seasonal normals.
Cooling will be slower in the deserts with temperatures expected to
remain above normal through Saturday. The marine layer and weak
onshore flow will keep coastal areas cooler with night and morning
low clouds and fog expected. The marine layer will deepen and
onshore flow will strengthen, spreading low clouds farther inland
over the weekend into early next week.
&&
.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...
Satellite imagery from 9 AM today showed a coastal eddy circulating
over the coastal waters and low clouds retreating out of the valleys
and towards the coast. Generally, visibility this morning was
reduced to 3 to 5 miles in the western valleys and on coastal mesas.
An eleven degree inversion was recorded on the morning sounding.
This strong marine inversion will make clearing slow along the
coast, with only partial clearing likely at the beaches. The
marine layer will be a little shallower tonight into Wednesday
morning, with low clouds and fog staying more confined to coastal
locations.
High pressure will continue to strengthen over Southern California
today. Temperatures in the deserts will reach their peak Wednesday
and Thursday with highs in the High Desert reaching 100-105, and 108-
112 in the low desert. An Excessive Heat Warning remains in effect
through Friday evening for the deserts. As the high pressure
system begins to move east on Friday temperatures will begin to
slowly decrease. Temperatures in the deserts will fall to seasonal
normals by Sunday.
West of the mountains, the marine layer will keep temperatures near
the coast in the 70s, with temperatures in the mid-80s to mid-90s
inland. The Inland Empire will see the warmest conditions, with
temperatures locally reaching 15 degrees above normal. Highs in the
IE are expected to reach the upper 90s. These warm conditions are
expected through Thursday, before a cooling trend begins Friday
through the weekend.
An upper level low will move into northern and central California
late Friday, which will strengthen onshore flow and deepen the
marine layer across Southern California. While there are still
some differences in the details, global ensembles keep a weak
trough along the California coast through early next week. This
will keep temperatures around seasonal normals and spread low
clouds and fog into the western valleys.
&&
.AVIATION...
261600Z...Coast/Western Valleys...Stratus is quickly scattering,
with skies clearing by 18Z. Skies could remain BKN into the
afternoon for some beach locations. Low clouds will begin to move
into the coastal areas this evening after 04Z with bases near 500-
700 feet MSL and lower vis of 3-5SM expected at the coastal
terminals.
Inland Valleys/Mountains/Deserts...Mostly clear with unrestricted
vis today and tonight.
&&
.MARINE...
Patchy dense fog will impact the coastal waters through Thursday.
Otherwise, no hazardous marine weather is expected through Saturday.
&&
.BEACHES...
Southerly swell arrives Friday, peaks Saturday and gradually lowers
Sunday. This will generate elevated surf of 3 to 5 feet with locally
higher sets of 6 to 7 feet, along with a high risk of strong rip and
longshore currents beginning late Friday.
&&
.SKYWARN...
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.
&&
.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Excessive Heat Warning until 7 PM PDT Friday for Apple and
Lucerne Valleys-Coachella Valley-San Diego County Deserts-
San Gorgonio Pass Near Banning.
PZ...NONE.
&&
PUBLIC...CO
AVIATION/MARINE...MorelandThe national radar mosaic for the moment the first event began.
3 excessive heat. Local standard time, as filed.
Temperatures ranged from near 100 in Victorville up to 107 degrees in Lucerne Valley on the 26th to the 28th.
Temperatures generally ranged from 105 to 110 degrees across the Coachella Valley each day from the 26th to the 28th, though Palm Springs did hit 111 degrees on the 26th.
Temperatures ranged from 105 to 110 degrees in parts of the San Diego County deserts.
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, May 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: 895476, 895477, 895478.