PHOENIX (NEWSnet/AP) — More than 63 million people in the U.S. are under excessive heat warnings, watches or advisories, in an area stretching from the Southwest northward up through the Great Lakes region.

During the weekend, the extreme heat was primarily in Arizona and New Mexico, along with parts of Texas, Colorado and Kansas. By Sunday, the heat wave began to move into the Plains and Great Lakes regions; by Tuesday, it will expand into the Northeast.

 

Even parts of Canada will be affected, as Environment Canada’s Ontario office is warning of a “prolonged and dangerous heat event” starting Monday for an area that includes Windsor and Ontario.

Daytime temperatures are expected in many places to be in mid- to upper-90s, in some cases 100 degrees or more, near or breaking daily temperature records in some communities. Occasional thunderstorms, excessive humidity and air quality alerts mixed in with the heat dome effect in some locations will make weather conditions even more miserable.

A heat wave is defined as abnormally hot weather lasting more than two days. While many of the affected locations do occasionally see hot temperatures in the summer, the duration of this weather situation is a concern because there will be little relief overnight.

"The longevity of dangerous heat forecasted for some locations has not been experienced in decades," the National Weather Service reported.

Those who need a place to get break from the oppressive weather conditions should look up information posted by their state or county emergency agencies, NWS said.

Cooling centers, places where residents can get a break from the heat via air conditioned common places, have been opened in cities ranging from Phoenix, Ariz., to Toledo, Ohio. And the Belle Isle State Park in Detroit filled to capacity with visitors Sunday, causing a temporary closure for entries to the island recreational area.

Tropical Disturbance

 

In the meantime, a fresh batch of tropical moisture is resulting in an increasing threat of heavy rain and flash flooding to the central Gulf Coast. Heavy rain is expected to start Monday morning, with the moisture shifting toward the Gulf Coast by Tuesday.

The intense flooding from heavy rains continued to dissipate in southern Florida, where some areas in and around Miami and Fort Lauderdale were left underwater in recent days as storms dumped up to 20 inches of rain in places.

That unnamed storm system coincided with the early start of hurricane season, which this year is forecast to be among the most active in recent memory.

But Wait, There’s Snow

 

Late season snow was forecast for the northern Rockies Monday and Tuesday. Parts of Montana and north-central Idaho were under a winter storm warning, with as much as 6 inches of heavy, wet snow expected in the mountains around Missoula, Montana.

And as much as 20 inches of new snow was predicted for higher elevations around Glacier National Park.

The Associated Press contributed to this report.

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