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

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 and 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.

Sacramento and San Jose, where it reached 116˚F (47˚C) and 109˚F (43˚C) on Tuesday, clinched all-time records — meaning their temperatures exceeded levels observed on any previous day or month.

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˚F (44˚C) Tuesday.
  • Montana: Big Horn hit 108˚F (42˚C) Wednesday.

Of all the states, California has seen the most extreme temperatures for the longest period of time.

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