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Summer's passed, but temperatures in the nation's capital are expected to reach between 20 and 25 degrees above normal on Wednesday, meaning highs of around 96 degrees Fahrenheit.

And the heat trend is widespread. Well over 100 daily high temperature records will likely fall across a vast portion of the central and eastern U.S. over the coming days.

What's gives?

A pattern of unusual heat in the east and chilly temperatures out West (including a major blizzard) is a well-predicted consequence of a powerful weather system, called the jet stream, that has sliced the nation in half, temperature-wise.

"It will be a tale of two nations," Jeff Weber, a meteorologist at the University Corporation for Atmospheric Research told Mashable last week. "It will be a big contrast." 

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Indeed there is. The jet stream — a relatively narrow band of high altitude, westerly, powerful winds traveling some four to eight miles up in the atmosphere — acts as a powerful barrier separating colder northern temperatures from warmer southern temperatures.

But, as atmospheric scientist Paul Roebber told Mashable, this band of wind can bend and become wavier. This has happened in a significant way, as the jet stream has bent down into the western U.S., allowing colder air to come down, somewhat like a freezer door being left open.

The jet stream on Oct. 2 ,2019.Credit: WXMAPS.ORG / GEORGE MASON UNIVERSITY

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Meanwhile, the bent jet stream has allowed warmer southern air to travel up and then settle over the other half of the U.S. "There will be dramatic heat," said UCAR's Weber. He was right.

Record heat, reaching into the upper 90s, hit the Midwest and South on Tuesday. More daily high temperatures will fall on Wednesday and Thursday.

These days, with overall global temperatures boosted by a relentlessly warming climate, high temperature records are much more likely to break than low temperature records. Every weather event is happening on a heated planet. That's why over the last decade, twice as many daily high records have been set as low temperature records in the U.S.

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What's more, there's growing evidence that the jet stream is becoming more prone to wavier behavior, which produces more extreme weather and temperature events, particularly heat waves. This is an intensive, ongoing area of scientific research.

For now, the jet stream has bent over the U.S., and a large part of the nation is experiencing a potent bout of "summertime" heat.

(责任编辑：娛樂)