A strong upper level ridge of high pressure over the Great Basin and weak offshore flow brought a heat wave to the region during the end of August and early September. The hottest temperatures west of the mountains occurred on the 1st-3rd September.
Every event with a place to put it; 7 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 strong upper level ridge of high pressure over the Great Basin and weak offshore flow brought a heat wave to the region during the end of August and early September. The hottest temperatures west of the mountains occurred on the 1st-3rd September. San Diego Unified School District closed more than 70 schools early on the 1st due to the excessive heat. A flex alert (requesting electricity conservation) issued in late August continued through September 3rd.
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 1, 8:26 AM PST, 2 hours 34 minutes earlier.
Area Forecast Discussion
National Weather Service San Diego CA
926 AM PDT Fri Sep 1 2017
.SYNOPSIS...
Very hot weather will continue through Saturday with a slight
chance of thunderstorms over the mountains and deserts. Tropical
moisture from the south will bring more widespread showers on
Sunday. Hot weather and small chances of thunderstorms mainly for
the mountains will follow Monday and beyond.
&&
.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...
Except for patchy dense fog along parts of the coast, skies were
mostly clear this morning. The atmosphere is starting out a lot
drier than it was yesterday at this time. Some offshore breezes
are helping the drying too. Will it be dry enough for no showers
or thunderstorms today? There might be an isolated shower or
thunderstorm, most likely in the San Bernardino and San Gabriel
mountains, but it looks mostly dry today. Although the air mass is
still somewhat humid, it remains hot, and temps should skyrocket
pretty well today with unencumbered sunshine. You know the story:
strong high pressure aloft will continue to bring excessively hot
weather today through Saturday. You should be extra cautious about
strenuous activity outside. A few daily max temp records will fall
today and Saturday, which is when the heat hazards will finish up
their long life. The monsoon moisture increases Saturday, but only
slightly, so there should be a few more mountain thunderstorms,
but not widespread. A change comes Sunday. The remnants of
tropical storm Lidia, which on Sunday becomes a tropical
depression, will bring a deep swath of moisture and cloudiness
that will flow through Southern California. The greater depth of
the moisture will stay offshore, but enough over land to produce
widespread showers. The air will be more stable and cloudy, but
there is still a chance of thunderstorms. Best chances will be
mountains westward and any time early Sunday morning through
afternoon. Some winds from the east and south could accompany the
depression: you might say a glancing blow. The details are fraught
with uncertainty and models are all over the map as far as where,
when and if showers and winds happen. Hence, confidence is high
for some precip west of the mountains, but low on the spatial and
temporal details. On Labor Day the deep moisture moves out and we
transition to a more regularly scheduled monsoon pattern that will
continue for the rest of the week. High pressure aloft continues
strong, but not like this past week, and temperatures should be
above average, but not excessive. Expect small chances of
thunderstorms mainly in mountains and deserts each afternoon.
&&
.AVIATION...
011545Z...Coast...Patchy low clouds and dense fog will continue
along the immediate coast through mid morning, with cloud bases 200-
600 ft MSL and tops to 800 ft MSL. Local vis restrictions down to
1/2 SM will be possible. This could impact KSAN and KCRQ through 17Z
but confidence is low as the stratus/fog retreats west away from the
coast. Otherwise, FEW-SCT clouds at/above 10000 ft MSL will prevail
through tonight.
Valleys/Mountains/Deserts...Scattered clouds at/above 10000 ft MSL
continuing today with mostly unrestricted vis. There is a slight
chance of TSRA this afternoon in the San Bernardino County Mountains.
&&
.MARINE...
A shallow marine layer will again bring patchy dense fog early
Saturday morning.
The remnant low and moisture from Tropical Storm Lidia will bring
scattered showers and thunderstorms on Sunday to the coastal waters.
In addition to the precipitation, there is a chance that winds will
be strong enough to produce rough seas up to 7 feet with steep
waves. Conditions will gradually improve Sunday night into Monday.
&&
.SKYWARN...
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.
&&
.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Heat Advisory until 10 PM PDT Saturday for Orange County Coastal
Areas-Riverside County Mountains-San Bernardino County
Mountains-San Diego County Coastal Areas-San Diego County
Mountains.
Excessive Heat Warning until 10 PM PDT Saturday for Apple and
Lucerne Valleys-Coachella Valley-Orange County Inland Areas-
San Bernardino and Riverside County Valleys-The Inland
Empire-San Diego County Deserts-San Diego County Valleys-
San Gorgonio Pass Near Banning-Santa Ana Mountains and
Foothills.
PZ...NONE.
&&
PUBLIC...MM
AVIATION/MARINE...PGThe national radar mosaic for the moment the first event began.
7 excessive heat. Local standard time, as filed.
More than 70 schools in the San Diego Unified School District closed early due to the excessive heat. Temperatures ranged from the upper 80 at the immediate coast to 105 several miles inland.
Afternoon highs in Inland Orange County ranged from 100-106 degrees.
Ramona reached 109 degrees, the hottest September temperature on record. Other temperatures in the region included 108 in Valley Center and 106 in El Cajon.
Temperatures in the Inland Empire exceeded 110 degrees. Some sample high temperatures values include Ontario and Chino at 114 degrees and Corona at 110 degrees. Riverside reached 112 degrees, tying for the 5th highest September temperature on record.
Afternoon high temperatures in the valleys ranged from 100-106 degrees.
Afternoon highs just inland from the coast ranged from 95-106 degrees.
Riverside was 109 degrees and Chino reached 108 degrees.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in California in 2017Every state in 2017This day in weather historyWarnings in California, September 2017
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: 722966, 722967, 722970, 722971, 722965, 722968, 722969.