Very warm temperatures occurred inland on September 20-22, with highs reaching 95 to 105 degrees. Weak Santa Ana winds brought breezy conditions to the mountains and Inland Empire.
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.
Very warm temperatures occurred inland on September 20-22, with highs reaching 95 to 105 degrees. Weak Santa Ana winds brought breezy conditions to the mountains and Inland Empire. Multiple wildfire starts in the Inland Empire, with fires exacerbated by hot conditions and dry vegetation.
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 20, 7:50 PM PST, 4 hours 10 minutes earlier.
Area Forecast Discussion
National Weather Service San Diego CA
850 PM PDT Mon Sep 20 2021
.SYNOPSIS...
Building high pressure aloft, and weakening Santa Ana winds will set
us up for a couple of very warm days. Any low clouds that try and
develop, will likely remain patchy through Wednesday as well. For
the latter half of the week, a developing low pressure system from
the north will bring a return of marine clouds and fog, and cooler
conditions. There will be a chance of showers and thunderstorms as
well, but timing and amounts are uncertain.
&&
.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...
Satellite imagery this evening shows the coastal eddy drifting
farther offshore, and carrying most of the low clouds with it. For
the remainder of SoCal, skies are clear. The sfc pressure gradients
across SoCal remained weakly onshore to the lower deserts, but were
solidly offshore from the north. The stronger E-NE wind reports at 8
PM PDT were mostly 10-15 MPH, with peak gusts 20-25 MPH. Our 00Z
Miramar sounding still had a weak marine inversion based near 1650
FT, and showed some recovery in humidity aloft as winds turned
easterly between 4K and 10K FT. No forecast updates are planned.
From previous discussion...
Key points:
*Weak Santa Ana winds expected through Tuesday afternoon.
*Above average temperatures expected through Wednesday, with
Wednesday expected to the be warmest day of the week.
*Potential for showers and thunderstorms Thursday through Saturday.
A lot of uncertainty surrounding the details of the forecast.
Periods of weak Santa Ana winds will continue into Tuesday, with
wind gusts peaking at 20 to 30 mph. Tuesday morning the weak Santa
Anas will become easterly and more focused on the Riverside and San
Diego County foothills and far eastern valleys. Minimum relative
humidity values will fall to 10 to 15% today and Tuesday, lowest in
the deserts, Inland Empire, and the eastern portions of the San
Diego County valleys. Temperatures will be on the rise for the first
half of the week, peaking at 6 to 12 degrees above average on
Wednesday. Coastal low clouds will be very patchy to potentially non-
existent Tuesday and Wednesday mornings.
The high pressure over California begins to break down and move east
Wednesday, allowing a low pressure trough to drop off the California
coast and cut off Thursday and Friday. There is still a lot of
uncertainty in timing, location, and how much moisture will be drawn
into Southern California. There is high confidence a lot more mid
and high clouds will move into SoCal, along with more coastal clouds
as the marine layer rebounds. There is less confidence surrounding
the chances of showers and thunderstorms. Enough moisture seems to
be present Thursday through Saturday for precipitation, with
instability for potential thunderstorms. The GEFS continues to be the
wetter solution, bringing scattered but widespread showers and
thunderstorms across the entire region. The ECMWF ensemble solutions
are drier, with the main chances of showers and thunderstorms
confined to the mountains. For now the forecast has chances of
showers and thunderstorms in the mountains each afternoon Thursday
through Saturday, with slight chances of thunderstorms moving west
into the valleys Friday and Saturday afternoons. A lot of
uncertainty still surrounds the forecast for the latter half of the
week and it is subject to change as model solutions settle on an
outcome.
The cutoff low is expected to slowly move east over the weekend,
resulting in drier conditions late in the weekend and early next
week. Night and morning low clouds and patchy fog are expected to
continue. Temperatures are expected to remain below average west of
the mountains through Monday, with near average temperatures in the
deserts.
&&
.AVIATION...
210345Z...Coast/Valleys...Mostly clear skies will prevail most of
the night, though patchy BKN stratus will be possible within 10
miles of the coast 09Z-16Z Tue. Bases will be 800-1200 ft MSL with
tops to 1300 ft MSL, with local vis 2-4 miles in BR over higher
coastal terrain. Confidence in timing and even occurrence at coastal
airports is low, but KSAN, KCRQ and KSNA could have a few hours of
cigs. Stratus will likely only be patchy again late Tue night.
Mountains/Deserts...Mostly clear with unrestricted vis through
Tuesday night.
&&
.MARINE...
No hazardous marine weather is expected through Saturday.
&&
.FIRE WEATHER...
Weak Santa Ana winds, a fairly dry airmass over the region, and
above average temperatures will create elevated fire weather
conditions at times through midweek inland. Northeast winds 10-15
mph, with local gusts 20-25 mph through the passes and along the
coastal foothills on Tuesday, minimum relative humidity values of 10-
15%, and temperatures some 6-12 degrees above average will elevate
fire weather conditions. The heat will peak on Wednesday, followed
by cooling and increasing humidity for the remainder of the week as
a weak low pressure system develops. Some showers are possible as
well.
&&
.SKYWARN...
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.
&&
.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Heat Advisory from 10 AM Tuesday to 8 PM PDT Wednesday for San
Bernardino and Riverside County Valleys-The Inland Empire-
San Diego County Valleys.
PZ...NONE.
&&
PUBLIC/FIRE WEATHER...10/CO
AVIATION/MARINE...MaxwellThe national radar mosaic for the moment the first event began.
4 excessive heat, 2 wildfire. Local standard time, as filed.
Anaheim broke the daily record for maximum high temperature on September 21. Anaheim recorded a high of 104 degrees, breaking the record of 102 set in 2015.
Alpine broke the highest maximum temperature record for September 21 with an observed high of 98 degrees, breaking the record of 97 degrees set in 2009. Ramona tied their high temperature record with a high of 101 degrees, which tied for the same date in 1975.
Vista tied the maximum high temperature record for September 21 reaching a high of 95 degrees, tying for the same date set in 1992.
CAL FIRE/Riverside County Fire Department reported a vegetation fire in Winchester at 2:20 PM. Three acres burned at a moderate rate. Forward spread was stopped around 3 PM.
Campo tied for the highest maximum temperature for September 22, reaching a high of 101 degrees, tying for this same date in 1982.
The University Fire started at 11:27 am on September 22 at I-215 and University Parkway in San Bernardino. San Bernardino County Fire issued Evacuation Orders for residences east of N State St, Morgan Road, Pepperdine Drive and Grossmont Road. 100 homes were threatened. The fire burned 71 acres and was 100% contained by 7 PM on September 23.
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, September 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: 984127, 984129, 984131, 984124, 984132, 984116.