Why the U.S. will see starkly different, unusual temperatures this weekend

A potent weather system will drive a wedge through the heart of the Lower 48 states this weekend (and beyond).

That means daily record warm temperatures in the central and eastern U.S. but unusually chilly weather across a large swathe of the west, including early-season heavy snows in Montana. "Nearly all of the lower-48 will experience temperatures far from what is normal for this time of the year — well above in the east and well below in the west," the National Weather Service noted.

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

The weather culprit is the jet stream, a relatively narrow band of high altitude, powerful winds traveling some four to eight miles up in the atmosphere. Like a wall, this band of westerly winds separates colder northern air from warmer southern air.

But critically, this band is often liable to bend and become wavier, allowing colder air to swoop down south and warmer air to travel up north, explained Paul Roebber, an atmospheric scientist at the University of Wisconsin at Milwaukee. And weather forecasters expect a big wave to develop over the coming days.

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The jet stream will drop down from the north into Montana, Idaho, and other parts of the West. "It's going to open up a door for Arctic and Canadian air," said Weber.

Meanwhile, on the other side of the bent jet stream, a vast zone of warm air, including dry desert air from Mexico, will settle over the central and eastern U.S. "There will be dramatic heat," said Weber. Think peak summertime temperatures, nothing like fall.

A wavy jet stream, though, is not unusual, emphasized Roebber. The jet stream regularly drives weather events. But this is "an amplified weather pattern," he said, as the notably wavy jet will allow a significant, more intense weather contrast to manifest in the Lower 48.

Different processes can cause a jet stream to break down and grow wavy. In this case, the powerful winds hit a zone of higher pressure in the Atlantic, which acts somewhat like a roadblock and will cause the jet stream to "buckle" as if it were running into a wall, explained Weber.

Original image has been replaced. Credit: Mashable

The consequences of this early fall weather event will likely mean hundreds of new daily temperature records — both highs as lows. But these days — as Earth's climate has warmed significantly over the last century — daily high records occur significantly more often than low records. In other words, temperatures have been given a boost. Over the last decade, twice as many daily high records have been set as low temperature records in the U.S.

Weather extremes will always transpire, but now they're transpiring on a warming, changing globe.