These abnormal temperatures aren't necessarily records for heat or cold. What's unusual is the persistence of temperatures above or below average. It's a trend that goes back to the start of the year and is tied to the now infamous "polar vortex" that started with cold winter Arctic air coming in as the jet stream dipped south.
"It's indicative of how 'stuck' the pattern has been this year—consistently cool in the East and consistently warm in the West," said Nick Wiltgen, a forecaster at The Weather Channel.
What Arndt calls "bipolar" conditions even led to July temperatures that for the U.S. overall were near average. "So the very cool conditions in the Midwest and warm conditions in the West really canceled each other out," he said
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So what gives?
Cooler air from Canada is pushing against the jet stream, causing it to dip farther down, creating a U-shaped path across the U.S. "That ridge out West keeps it warm and that trough in the East keeps it cool," NBC meteorologist Dylan Dreyer explained in a recent report about the odd conditions.
Arndt agrees. "The big cold-air outbreaks in the Great Lakes and Midwest early in the year started the pattern" of cooler temperatures east of the Rockies, he said. "And those were reinforced several times during the spring, and dramatically in July."
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"The western warmth has been persistent all year," he said, and California is heading for a record warm year as it suffers in a decade-long drought.
The "bipolar" conditions have had at least one major contribution to the U.S. economy.
"In the northern plains, it's unusual for very cool conditions and very dry to exist simultaneously in the summer," said South Dakota state climatologist Dennis Todey. And that combo has helped push U.S. corn production to record levels.
"We've had more outbreaks of Canadian air," he said, "which tends to be dry and less humid."