Excessive Heat in California, September 4 to 13, 2021

Name: Excessive Heat in California, September 4 to 13, 2021
Start: 2021-09-04T00:00
End: 2021-09-13T23:59
Location: California

September 4 to 13, 2021 · NWS SGX · 4 events in 4 counties and zones

Where it happened

Every event with a place to put it; 4 filed by county or forecast zone are drawn at its center, in a lighter ring. Tap one for its entry below.

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Wind
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Winter weather
Heat and drought
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Fire, smoke and dust
Marine and coastal
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The toll

Deaths: 0
Injuries: 0
Property damage: None recorded
Crop damage: None recorded
Events: 4

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.

What happened

The episode as NWS SGX wrote it up.

A prolonged heat event occurred from Labor Day Weekend through the following weekend across Southern California from September 4th through September 13th. The first peak of the heat occurred on Sunday, September 5th; this was the hottest stretch for some mountain locations, the High Desert and the Inland Empire. A second peak occurred on Monday, September 13, when some of the highest temperatures occurred across some mountain locations and the lower deserts. At six locations, the highest temperatures of the event occurred on September 13, when all heat products had expired.

What forecasters said that day

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 Sep 3, 7:54 PM PST, 4 hours 6 minutes earlier.

Area forecast discussion, Sep 3, 7:54 PM PST, 4 hours 6 minutes before

Area Forecast Discussion
National Weather Service San Diego CA
854 PM PDT Fri Sep 3 2021

.SYNOPSIS...
High pressure will build into California this holiday weekend,
bringing hot inland weather. The marine layer will become
shallower and less extensive, with clouds staying confined mainly
to coastal areas and the western valleys. Monsoon thunderstorm
chances return midweek to the mountains and deserts. A gradual
cooling trend inland is expected next week.

&&

.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...

A strong marine inversion enabled coastal clouds to persist in San
Diego County this afternoon. Overall, high temperatures were up
several degrees from yesterday, especially in the valleys and
deserts. The warming trend will continue as we head into our
holiday weekend as high pressure aloft over the New Mexico
strengthens over California. Temperatures for inland valleys
appear to peak out Sunday, with most of the Inland Empire
achieving 100-106 degrees, then slightly cooler Monday and
Tuesday. The deserts are hottest on Monday and Tuesday: 100-105
in the high desert and 110-112 in the low desert. If you have
outdoor plans, please take extra precaution against the hot inland
weather this holiday weekend. That higher pressure will suppress
on the marine layer, diminishing the coastal cloud presence. The
time and extent of coastal clouds will become less, and probably
quite patchy during nights and mornings Sunday through Tuesday.
That being said, the strong marine inversion should gain strength,
which would mean more persistent clouds at the beaches even
during the day, something like we had today. The high pressure
aloft migrates east and weakens next week, allowing temperatures
to trend back to near normal by Thursday or Friday. Monsoon
moisture remains mostly south of the border for several days.
Guidance shows no credible chance of thunderstorms in our region
until Wednesday. That's the day we start forecasting a slight
chance for mountains and deserts during the afternoons through
Friday. The weakening high and that monsoonal moisture and higher
humidity will help trim off several degrees from the hot inland
weather, but it won't be such a dry heat anymore. Looking toward
the following weekend (Sep 11-12), a slim majority of ensemble
solutions point to a weak trough over the West Coast. That would
mean cooler and drier weather, along with a marine layer rebound.

&&

.AVIATION...
040345Z...Coast/Valleys...Areas of BKN-OVC stratus will mainly
affect coastal San Diego County tonight, spreading up to 10-15 miles
inland by 12Z Sat. Only patchy BKN stratus is expected 10Z-16Z Sat
in coastal Orange County. Cloud bases will be 900-1300 ft MSL with
tops to 1500 ft MSL. Local vis below 3 miles will occur over higher
coastal terrain 07Z-16Z Sat. Most areas will clear 15Z-17Z Sat with
only patchy coastal stratus Sat night.

Mountains/Deserts...Mostly clear skies with unrestricted vis through
Saturday night.

&&

.MARINE...
No hazardous marine conditions are expected through Wednesday.

&&

.BEACHES...
A south-southwest swell (from 200 degrees) of 2-3 feet with a period
of 16-18 seconds will arrive at Southern California beaches late
Saturday afternoon/evening. This swell will generate surf of 3 to 5
feet with sets to 6 ft at Orange County beaches Sunday and Monday,
with mostly 3 to 4 feet with local sets to 5 feet in San Diego
County. There will be a high rip current risk Sunday and Monday.
See the Beach Hazards Statement for more information. Swell and surf
will subside Tuesday.

&&

.SKYWARN...
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Heat Advisory from 11 AM Saturday to 8 PM PDT Monday for San
     Bernardino and Riverside County Valleys-The Inland Empire.

     Excessive Heat Watch from Monday morning through Tuesday evening
     for Apple and Lucerne Valleys.

     Beach Hazards Statement from late Saturday night through Monday
     evening for Orange County Coastal Areas-San Diego County
     Coastal Areas.

PZ...NONE.

&&

PUBLIC...MM
AVIATION/MARINE/BEACHES...Maxwell

The radar

The national radar mosaic for the moment the first event began.

Open the radar archive at this moment

Event by event

4 excessive heat. Local standard time, as filed.

Excessive Heat, San Bernardino and Riverside County Valleys - the Inland Empire

When: September 4, 12:00 AM PST to September 5, 11:59 PM PST
Where: San Bernardino and Riverside County Valleys - the Inland Empire forecast zone
Reported by: ASOS
Coordinates: 34.293, -116.086 (the zone’s center; the record gives no point)

High temperatures reached the lower-100s on Saturday, September 4th. The heat peaked across the Inland Empire on Sunday, September 5th, when highs reached 108 degrees in Ontario, and 107 degrees in Riverside. The heat ramped back up the following weekend, with high temperatures reaching the low-100s September 11th through September 13th.

Excessive Heat, Coachella Valley

When: September 4, 12:00 AM PST to September 13, 11:59 PM PST
Where: Coachella Valley forecast zone
Reported by: ASOS
Coordinates: 33.744, -115.994 (the zone’s center; the record gives no point)

Much of the Coachella Valley saw excessive heat Labor Day Weekend through the following weekend. The first peak occurred on Sunday, September 5th, when highs ranged from the upper-100s to the mid-110s. The heat persisted through the following weekend, when the highest temperatures of event occurred. Several locations saw their highest temperatures of the heat event on Monday, September 13, after all heat products had expired, including Ocotillo Wells (116/91), Thermal (111) and Indio (112/87).

Several sites saw very warm overnight low temperatures, exacerbating the heat risk through the event. Indio set a record for the highest minimum temperature on September 13, with the overnight low falling to 87, breaking the previous record of 86 set in 2015.

Excessive Heat, San Diego County Valleys

When: September 4, 12:00 AM PST to September 13, 11:59 PM PST
Where: San Diego County Valleys forecast zone
Reported by: ASOS
Coordinates: 33.028, -116.770 (the zone’s center; the record gives no point)

High temperatures across the San Diego County Mountains and valleys consistently reaching the mid-90s from Saturday, September 4th through September 13th. Eight Preliminary Prehospital Heat Related Calls occurred from September 5 through September 10, seven of which were mild cases, and one that was a moderate case of heat-related illness.

Excessive Heat, Apple and Lucerne Valleys

When: September 6, 12:00 AM PST to September 8, 11:59 PM PST
Where: Apple and Lucerne Valleys forecast zone
Reported by: ASOS
Coordinates: 34.841, -116.178 (the zone’s center; the record gives no point)

The High Desert saw excessive heat the first full week of September, with heat peaking on Wednesday, September 8th, when highs reached the upper-100s.

What these are

Excessive Heat: Heat at or above the local excessive heat warning criteria, or heat that caused deaths.

After NWS Directive 10-1605, the rules offices file Storm Data by.

The same day elsewhere

Every storm in California in 2021 Every state in 2021 This day in weather history Warnings in California, September 2021

Sources

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: 984163, 984166, 984173, 984175.