Plains and Midwest bracing for dangerous temperatures

Nighttime temperatures in the Plains and Mid-Atlantic will bring little relief.

Dangerous heat that has scorched the Southwest for days is now spreading to the nation's midsection.

Heat indices, which factor in humidity, will make it feel like 100 degrees in places like Kansas City, Missouri, and Des Moines, Iowa, beginning Sunday and stretching into the first week of September.

By Tuesday, it will feel like the upper 90s in Chicago, with hot and humid temperatures bringing heat index values over 100 degrees in Charlotte and Raleigh, North Carolina; Richmond, Virginia; and Washington, D.C.

Nighttime temperatures across parts of the southern Plains and Mid-Atlantic will bring little relief to millions of people. Nighttime lows will only get down to the 70s for multiple days.

While some warmer and more humid days are on the way, most of the Northeast will miss out on this round of dangerous heat.

As the hot weather continues to move east this week, wet and stormy conditions are possible for parts of the Upper Midwest to the Northeast. While it doesn't look like a complete washout, there could be a day or two this week with spotty flash flooding and severe storms.

Flash-flood threat for parts of the West

On Saturday, strong thunderstorms produced flash flooding in Arizona's Grand Canyon National Park. As of Sunday morning, 62 people were air-evacuated from the Phantom Ranch area at the bottom of the Grand Canyon, north of the Colorado River, according to the National Park Service , with additional evacuations planned.

On Sunday afternoon, the National Park Service said evacuations were continuing and asked the public for information on more than 20 individuals "who may be missing or unaccounted for" following the flash-flood event.

In Utah, Emery County Emergency Management officials reported flooding of Cottonwood Creek in Orangeville, about 150 miles southeast of Salt Lake City, prompting evacuations of several homes and apartments.

More flash flooding is possible on Sunday afternoon in parts of the West as another round of scattered thunderstorms develops, fueled by monsoonal moisture.

Flood watches were in effect Sunday across portions of Arizona, Utah and Colorado, including the cities of Tucson, Phoenix, Flagstaff, Arizona, Cedar City, Utah and Grand Junction, Colorado.

Additional strong thunderstorms are possible in parts of the region, producing strong wind gusts, frequent lightning and blowing dust.

ABC News' Tristan Maglunog contributed to this report.

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