The heat wave gripping a wide swath of the United States will soon end, but not before severe thunderstorms and flooding downpours threaten communities.

More comfortable air is pouring into the Upper Midwest, replacing the severe thunderstorms that left more than 560,000 people without power late Friday through Saturday across the Upper Midwest.

Cooler and less humid conditions will continue to advance southward across the eastern two-thirds of the nation through early this week. However, the much-welcome heat relief will come at the expense of severe weather.

"When an unusually strong cold front moves into an unusually hot and humid air mass, severe weather can be expected," according to AccuWeather Senior Meteorologist Adam Douty, "and that will be the case through Monday."

"The strongest thunderstorms will produce flooding downpours and damaging wind gusts," he added.

One such band of slow-moving heavy thunderstorms raised concerns for flooding over Chicago's southern suburbs on Sunday.

Through Sunday night, thunderstorms will slice into the sweltering heat over parts of the interior Northeast and from the Lower Midwest states and back to the central Plains.

While any thunderstorm within that corridor can unleash strong wind gusts, the area at greatest risk lies over Missouri and eastern Kansas. This includes the cities of Wichita and Topeka, Kansas; and Kansas City and Springfield, Missouri.

The strongest thunderstorms in this region can also produce hail and an isolated tornado.

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Drenching rain and thunderstorms will then focus on areas from the Ohio and Tennessee valleys to the Northeast on Monday.

Localized flash flooding will be a concern throughout this area, but the risk for damaging winds will be greatest in the mid-Atlantic.

The strongest thunderstorms can down trees and cause power outages from New York City to Philadelphia and Harrisburg, Pennsylvania; Baltimore and Washington, D.C., during the afternoon and evening hours. The threat zone will also stretch down to Charlottesville and Richmond, Virginia.

An isolated tornado cannot be ruled out.

Flight operations may be halted for a time at the airports in these cities. It may also be a slow ride home during the evening rush hour as motorists can face reduced visibility and a heightened risk of vehicles hydroplaning.

Even in the absence of severe weather, residents in the path of the thunderstorms through Monday will face disruptions to outdoor plans, travel delays and lightning dangers.

"In this type of situation, these thunderstorms will produce a significant amount of lightning," AccuWeather Senior Meteorologist Brett Anderson said. "With people picnicking and doing other activities outdoors, they have to remember that trees and pavilions are not safe places to be during a thunderstorm."

The first clap of thunder is nature's warning that you can be struck by lightning and to seek shelter immediately.

A dramatic and much-welcome reduction in the sweltering heat and humidity will follow the slow-advancing line of downpours and thunderstorms. Temperatures first will be slashed, followed by the high humidity the next day.

Comfortable air over the Great Lakes and northern Plains on Monday will press over the Ohio and Tennessee valleys and Northeast Tuesday into Wednesday.

Less humid air may also make a rare July appearance southward to the Interstate-20 corridor at midweek.

Download the free AccuWeather app to stay alert to severe weather watches and warnings. Keep checking back for updates on AccuWeather.com and stay tuned to the AccuWeather Network on DirecTV, Frontier and Verizon Fios.

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