A big summer cooldown is on the way—and the price is bumpy weather for tens of millions of people.
A mass of polar air pushing south from Canada this week will threaten record lows for this time of year in the Midwest. Temperatures could dip into the 40s in some places.
The cool air, plunging down from Canada like a fist into the nation's middle, will collide with warmer, moist air streaming up from the south and cause trouble. As many as 84 million Americans, from Arkansas to Rhode Island, are in the path of potential severe thunderstorms beginning Monday afternoon.
"We've got a lot of players that are going to make your forecast pretty crazy," said Reynolds Wolf, a meteorologist for The Weather Channel.
The biggest threat for flooding—as much as three to five inches of rain through Wednesday—should be across the Northeast, from Philadelphia through New York and into parts of Connecticut and Massachusetts.
The National Weather Service warned of "torrential downpours" for New York.
Read More NY emergency health facility opens after Sandy