Summer heat trends: warm days in the west, warm nights in the east

As outlandish as the killer heat wave hitting the Pacific Northwest was, it fits into a decades-long pattern of uneven heat across the United States.

An analysis of decades of U.S. summer weather data from The Associated Press shows that the West is roasted by hot summer days, while the East Coast is marred by hot and sticky summer nights.

State-by-state average temperature trends from 1990 to 2020 show that U.S. summer heat is increasing in some places that have been baked by 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, with an average of 3 degrees above that since 1990. The North-West has warmed almost twice as much as the South-East in the past 30 years.

That includes Portland, Oregon, which set a record 116-degree high that was 3 degrees warmer than the temperature recorded in Oklahoma City or Dallas-Fort Worth.

Although the primary cause of last week’s extreme heat was an unusual but natural weather condition, scientists looked at the fingerprint of human-caused climate change, citing changing weather patterns that park the heat in different locations over long periods of time. .

“The ridiculous temperatures in the Pacific Northwest can be considered a black swan (ultra-rare) event on the one hand, but are completely consistent with long-term trends on the other,” said Judah Cohen, a meteorologist at the private firm Atmospheric and Environmental. research. “So I’m not going to predict when Portland hits 116 next time, but I believe a hot summer for the wider region is here to stay.”

Climate change is changing and weakening the jet stream, the narrow bands of air that circle the Earth, flowing from west to east. Those changes allow the major weather-producing patterns of high and low pressure to remain in place. Michael Mann, a climate scientist at Pennsylvania State University, said high pressure in the west is stopping more often in the summer. The high pressure brings hot and dry weather which, when stopped, are known as heat domes. Low pressure brings wet weather.

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

These patterns are visible so frequently that their effects can be seen in long-term data. Cohen said the US Northwest, western Canada and Siberia, which have just seen a surprise heat wave, are among Earth’s fastest warming regions during the summer since 1990.

The Midwest is warming at a slower rate than any other coast during the summer. That’s because areas of stagnant low pressure often drive cold air into the Great Lakes region, said Victor Gensini, a climate scientist at Northern Illinois University.

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

“In the western states where droughts have been expanding and accelerating during the past decade, soil moisture has declined. Dry soils compare to daytime moist soils,” said Jennifer Francis, a climate scientist at the Woodwell Climate Research Center. heats up faster because all the solar energy heats up instead of the moisture evaporating.” “Even dry soil cools faster at night.”

That’s partly why the West, which has been drier for decades and plunged into a major 20-year drought, is seeing those crazy triple-digit daytime temperatures.

NOAA records show the East has been getting wetter for decades, and the East Coast is seeing its biggest warming increase at night. Overnight minimum temperatures in New Jersey and Delaware have warmed by 3 degrees since 1990, the biggest increase in the nation.

Water vapor is a greenhouse gas, Francis explained, “so it traps more heat at night.”

Cathy Dello, a North Carolina state climatologist, attributes the trends to human-caused warming. “There is no other explanation,” she said.

He said that while extreme daytime highs can be eye-popping, hot nights can also be dangerous. “Hot nights may not seem like a problem, but they are a public health risk for people who lack adequate cooling,” she said.

And it’s getting harder and harder to hide from the heat: “All my places to go for a quick break were absurdly hot — Oregon, North Carolina, even over New York? Where’s the hell to go? Even Canada is not safe.”

———— . Read more stories on climate issues by The Associated Press at


Follow Seth Borenstein on Twitter at @borenbears.


The Associated Press Department of Health and Science receives support from the Howard Hughes Medical Institute’s Department of Science Education. AP is solely responsible for all content.


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