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

The operations chief at the National Weather Service's prediction branch says 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.

Jon Gottschalcksaid the high pressure was going the wrong way. Normally U.S. weather systems move west to east. He said the high pressure is about to return eastward, extending the Northeastern heat wave an extra day or so until the weekend.

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

Thursday, the heat index hit 106 degrees in Washington and Philadelphia.

SETH BORENSTEIN, AP Science Writer