No September on record in the West has seen a heat wave like this

Dried sunflowers can be seen in a field near Cottonwood Slough, Calif. on Wednesday, Sept. 7, 2022. Western states are struggling through one of the hottest and longest September heat waves on record. Bronte Wittpenn/San Francisco Chronicle via AP

The heat wave that’s been gripping California and other parts of the West for 10 days and counting is the most severe ever recorded in September, weather experts have said – confirming what California’s governor is calling the “hottest & longest on record” for the month.

The data supporting the assertion is overwhelming. Records began falling on Aug. 30 when Seattle and Portland set calendar day records of 90 and 100. And it’s not yet over – while the region’s heat wave peaked on Tuesday, among California’s hottest days ever observed, it’s expected to continue until Saturday, ending after a total of 12 days.

In just the past week, nearly 1,000 heat records have been broken, including more than 270 monthly records. Some places, like Salt Lake City, Sacramento and Reno, Nev., have broken their September records multiple times and by large margins.

Sacramento and San Jose, where it reached 116 and 109 degrees on Tuesday, clinched all-time records – meaning their temperatures exceeded levels observed on any previous day or month.

It’s the “greatest September heat wave ever west of the Rockies hands down,” tweeted weather historian Maximiliano Herrera on Wednesday.

In addition to its magnitude and duration, the heat wave has also been exceptional for its scope. Record-shattering temperatures have stretched from Arizona to Washington state and as far east as North Dakota.

At least two states have posted their highest temperatures ever observed during September:

• Utah: St. George, in the southwest corner of the state, hit 112 degrees Tuesday.

• Montana: Big Horn hit 108 Wednesday.

• When Death Valley soared to 125 degrees Tuesday, it fell just one degree shy of the September California and world records.

Of all the states, California has seen the most extreme temperatures for the longest period of time. Sacramento, Death Valley, Stockton and Fresno all saw their warmest 7-day periods in September “by far” according to an analysis by weather enthusiast Don Sutherland.

At the heat wave’s peak on Tuesday, California Gov. Gavin Newsom (D) called it “unprecedented,” tweeting that it “will be the hottest & longest on record for September.”

Experts concurred.

“This will be essentially the worst September heat wave on record, certainly in Northern California and arguably for the state overall,” said Daniel Swain, a climate scientist at UCLA, in a live discussion on Twitter.

Christopher Burt, author of a book on weather extremes, called Tuesday “one of the, if not the, hottest single day in California’s observed weather history.” That day, California’s power demand spiked to an all-time high.

Although the heat wave is slowly easing, it is still setting records. The National Weather Service projects dozens more will be broken or tied through Saturday. Excessive heat warnings and heat advisories are still in place for 45 million people in California, southwest Oregon, southern Nevada and parts of Colorado.

Denver could hit 100 degrees on Thursday, which would be the latest such instance on record. In June, it matched its earliest 100-degree reading. On Wednesday, it set a calendar day record of 99.

This is exactly the kind of heat wave that is expected to become more common because of human-caused climate change, which has already made such events more intense, frequent and prolonged.

Here are some of the most significant heat records set since the heat wave began:

• Salt Lake City tied its all-time high of 107 on Wednesday. It broke its previous September record high five times in seven days. Before hitting 107 on Wednesday, it hit 105 on Tuesday, 104 on Monday, 103 Saturday and 102 Thursday. The previous monthly record was 100. It has hit at least 100 degrees seven times this month. Before 2022, it had only done so three times during September dating to 1928.

• Several locations in the Mountain West set September record highs on Wednesday: Glasgow, Mont. (106); Sheridan, Wyo. (104); Boise, Idaho (104); Bowman, N.D. (102, tie); Helena, Mont. (102); and Cheyenne, Wyo. (97).

• Downtown Sacramento set an all-time high of 116 degrees on Tuesday, surpassing the previous mark of 114 from July 17, 1925. Records there date back to 1877. Stockton, Calif., tied its all-time high of 115.

• Death Valley hit 125 degrees Tuesday, tying its September record high and missing the September world record by just 1 degree. On Saturday, the low temperature in Death Valley of 102 degrees tied the highest minimum temperature ever recorded worldwide during September. It topped 120 degrees on nine straight days, setting calendar day records of 123 on Aug. 31, 124 on Thursday, 124 on Friday, 122 on Saturday, 124 on Sunday, 125 on Tuesday and 122 on Wednesday. It has established a new September record for most days reaching at least 120.

San Jose set an all-time record of 109 degrees Tuesday. King City (116), Santa Rosa (115), Napa (114) and Redwood City (110, tie) also set all-time highs, according to the Weather Service office serving the Bay Area.

Ukiah, Calif., about 100 miles north of San Francisco, tied an all-time high of 117 degrees Tuesday.

Bakersfield, Calif., hit 115 Tuesday, tying its September record high. Other September highs set Tuesday include Vacaville (115, tying Monday), Marysville (115), Hanford (114), Fresno (114) and Modesto (111).

Reno, Nev., set a September record high of 106 on Tuesday, beating the previous record of 104 set Sunday, after tying the previous monthly mark of 102 on Friday.

Grand Junction, Colo., set a September record high of 101 Tuesday.

Fairfield, Calif., about 40 miles northeast of San Francisco, set an all-time high of 117 degrees Monday – three degrees above the previous record. Livermore set an all-time high of 116 on Monday which it matched on Tuesday.

San Diego tied its all-time warmest low temperature on record Sunday, falling to just 78 degrees.

Burbank, Calif., matched its all-time warmest low temperature on Sunday, only dropping to 82 degrees. On Wednesday, it set an August record high temperature of 112.

Las Vegas set a September record high minimum temperature of 87 on Sunday.

Casper, Wyo., set a September record high of 100 Sunday.

On Saturday, many locations in the Mountain West set September record highs, including Bozeman (10o) and Butte (96) in Montana and Pocatello (102) and Idaho Falls (99) in Idaho.

The heat even surged into western Canada on Friday, where the village of Lytton set a September record for British Columbia of 103.3 degrees (39.6 Celsius).

Redmond, Ore., set a September record high of 106 on Friday.