U.S. summer trend: West getting hotter days, East hot nights

As outlandish as the killer heat wave that struck the Pacific Northwest was, it fits into a decadeslong pattern of uneven summer warming across the United States.

The West is getting roasted by hotter summer days while the East Coast is getting swamped by hotter and stickier summer nights, an analysis of decades of U.S. summer weather data by The Associated Press shows.

State-by-state average temperature trends from 1990 to 2020 show America’s summer swelter is increasing more in some of the places that just got baked with extreme heat over the past week: California, Nevada, New Mexico, Arizona, Utah, Oregon and Colorado.

The West is the fastest-warming region in the country during June, July and August, up 3 degrees on average since 1990. The Northwest has warmed nearly twice as much in the past 30 years as it has in the Southeast.

That includes Portland, Oregon which set a record 116-degree high that was 3 degrees warmer than temperatures ever recorded in Oklahoma City or DallasFort Worth.

Although much of the primary cause of the past week’s extreme heat was an unusual but natural weather condition, scientists see the fingerprint of humancaused climate change, citing altered weather patterns that park heat in different places for longer periods.

“The ridiculous temperatures in the Pacific Northwest may on one hand be considered a black swan (ultra-rare) event, but on the other hand are totally consistent” with long-term trends, said meteorologist Judah Cohen of the private firm Atmospheric and Environmental Research. “So I am not going to predict when is the next time Portland will hit 116, but I believe hotter summers for the broader region are here to stay.”

Climate change is altering and weakening the jet stream, narrow bands of wind that circle the Earth flowing west to east. Those changes allow key weatherproducing patterns of high and low pressure to stall in place. High pressure is stalling more often in the West in summer, said Pennsylvania State University climate scientist Michael Mann. High pressure brings hot and dry weather that, when stalled, can create what are known as heat domes. Low pressure brings wet weather.

Another factor is higher water temperatures in the Pacific Ocean that also generate more so-called high-pressure ridges the West, said Gerald Meehl, a National Center for Atmospheric Research scientist who studies heat waves.

These patterns are showing up so often that their effects can be seen in long-term data. The U.S. Northwest, western Canada and Siberia, which also just saw a stunning heat wave, are among Earth’s fastest warming land areas during summer since 1990, Cohen said.

The Midwest is warming slower during the summer than either coast. That’s because stalled low pressure areas often drive cooler air into the Great Lakes region, said North Illinois University climate scientist Victor Gensini.

Water explains the big difference between western and eastern heat trends, scientists said.