WASHINGTON – The oppressively hot weather in the Northeast has surprised meteorologists: It’s moving backward across America, something that rarely happens.

Normally U.S. weather systems move west to east. The western Atlantic high pressure system behind the hot dry weather started moving east to west last week and by Tuesday was centered over lower Michigan, said Jon Gottschalck, operations chief at the National Weather Service’s prediction branch.

“It’s definitely unusual and going the wrong way,” Gottschalck said Thursday. “This is pretty rare.”

He said the high pressure is about to return eastward, extending the Northeastern heat wave an extra day or so until the weekend.

And just before the high pressure moved east to west, a rainy and cooler low pressure system moved from the Mid-Atlantic to Texas, he said. That storm system broke off the jet stream, which is parked up in Canada, and made the U-turn first.

The unusual movement wasn’t seen in computer models until four or five days in advance, which is relatively late for these models, so meteorologists were surprised, he said.

Gottschalck said there’s no evidence pointing at man-made climate change, but this is likely just natural chaos in the atmosphere.

He couldn’t say how often these backward weather flows occur, but they are less frequent than once a year.

On Thursday, the heat index, which factors in humidity, hit 106 degrees in Washington, Baltimore and Philadelphia and was in triple digits along much of the Northeast. The air temperature hit a record 100 at New York City’s John F. Kennedy International Airport on Thursday, according to the weather service.

The weather service issued heat advisories and warnings Thursday for parts of 23 states. More than 141 million people live in those areas.