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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 3 to 5 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.

Image: Numerous severe thunderstorms are expected from New England into the Southeast
Numerous severe thunderstorms are expected from New England into the Southeast, as well in the Front Range of the Rockies. Wind damage is the primary threat in the East and South, along with local flash flooding. Large hail, damaging wind, and perhaps a few tornadoes are possible in the Front Range of the Rockies, along with local flash flooding.The Weather Channel

Along the tail end of the cold front, hail, strong wind and flash floods are possible through Tuesday in Colorado, New Mexico, Kansas, Texas and Oklahoma. Some highways were closed because of flooding over the weekend in Colorado.

As for the cooler air, don’t call it the polar vortex. But it will be plenty unusual for the middle of July.

The afternoon high Monday in Minneapolis is expected to be 64, a full 18 degrees lower than normal for this time of year and four degrees lower than the coolest high on record there for July 14. That record was set in 1884.

The forecast morning low for Tuesday is 53 in Minneapolis and 54, which would also be a record, in Chicago. In the northern reaches of North Dakota and the Upper Peninsula of Michigan, the high 40s aren’t out of the question.

And the baseball All-Star Game on Tuesday night in Minneapolis won’t feel much like a midsummer classic.

Matt Lanza, a meteorologist and blogger, crunched weather data and determined that it could be the coolest All-Star Game ever played. To set the mark and beat the 1984 game in San Francisco, the temperature will have to fall to 57 at Target Field.

“We’ll have plenty of coffee and hot chocolate,” said Chris Iles, a spokesman for the Minnesota Twins, who moved into an outdoor ballpark in 2010. The ballpark will turn on radiant heating in the concourses, he said.

Still, he said: “It’s Minnesota. We see worse than this in April and September.”