A supremely wavy, loopy jet stream pattern is responsible for the hot, dry weather that’s returned to the western half of North America, and for the shock of cold headed for the middle and eastern parts of the U.S.

“The weather pattern across the United States is about to get wild,” wrote meteorologist Guy Walton last week. In his more than 30 years of weather forecasting, he’s never seen such an extreme pattern set up at this time of year. His predictions have started to become reality.

“We’re looking at a whole week of record heat in the West, from Sunday all the way through the following Saturday,” he says, along with a “great blue trough” of cold air coming down