Heat wave continues to boil the Midwest and East Coast while storms brew behind it

A major heat wave has blanketed much of the U.S. in scorching temperatures for about a week — and in many places the hot spell is far from over.

As of Saturday morning, more that 100 million people in the U.S. were under heat alerts, including large swaths of Ohio, Pennsylvania, Delaware, New Jersey, Connecticut, Indiana, Michigan, West Virginia, Virginia, New York and Maryland.

The Midwest is expected to see some relief from the heat after Saturday. But temperatures are expected to peak this weekend from Washington, D.C., to New York. In the South, the hottest weather — which is forecast to be well into the triple digits — is expected to arrive early next week.

On top of the hot spell, widespread showers and thunderstorms will threaten parts of the Northeast and Midwest. Southern Wisconsin and northeast Iowa are at risk of flash flooding Saturday, according to the National Weather Service.

The extreme temperatures are caused by a heat dome — a high pressure system that pushes hot air down and traps it — that is becoming more common and intense as a result of climate change.

Northeast heat will feel like triple digits this weekend

Much of the Northeast will be hot and humid this weekend, with some of the highest temperatures hitting New Jersey.

The heat index in Trenton — what the weather actually feels like when you account for both the temperature and humidity — is forecast to reach 107 degrees Saturday. In Newark, the heat index could go as high as 106 degrees, while New Brunswick could climb to 105 degrees.

The heat wave will also bake New York, though temperatures will likely not reach triple digits, according toNWS New York.

In Pittsburgh and the surrounding area, the heat index was expected to climb to 105 degrees Saturday. NWS advised to stay indoors and warned not to leave children or pets in unattended vehicles, where the heat could reach "lethal temperatures in a matter of minutes."

Temperatures in the region are forecast to cool down on Sunday as a cold front moves in. At the same time, intense thunderstorms will begin to form — possibly leading to tornadoes in western Pennsylvania.

"Some storms will be severe with winds being the main threat. A tornado or two can't be ruled out," NWS Pittsburg advised.

In Philadelphia, the peak of the city's heat wave will occur this weekend, with the heat index reaching 105 degrees on Sunday. The City of Brotherly Love will also could some intense showers at the end of the weekend.

Parts of Iowa and Wisconsin are under flood watch

In Wisconsin, a flood watch has been issued for the lower half of the state, from Wausau to Janesville. So far, Milwaukee is not under a flood warning. About 1 to 3 inches of rain is expected Saturday, with some areas forecast to receive 3 to 5 inches.

Parts of northern and central Iowa, including Fort Dodge and Waterloo, are already experiencing severe flooding.

In Rock Valley, about an hour north of Sioux City, some residents were ordered to evacuate on Friday due to floodwaters. On Saturday, dive teams and swift boat crews searched for any remaining residents in the town. Local officials added that the city was without running water.

On Saturday morning, Iowa Gov. Kim Reynolds issued a disaster proclamation in the northwest part of the state, including Rock Valley and Sioux County.

The heat wave is sending many to emergency rooms

Hospital data from across the country show an uptick of emergency room visits related to heat illness or heat stroke.

Heat stresses people’s hearts, which could lead to heart attacks. It also worsens air quality, triggering more cases of respiratory issues. Those heat risks are compounded by the humidity.

Dr. George Chiampas, an emergency medicine specialist at Northwestern Memorial Hospital in Chicago, said the real danger is the humidity.

"It's like putting Saran Wrap over you, and you can't basically dissipate or push the heat out into a cooler environment," Chiampas told NPR's Morning Edition on Friday. "So that humidity, quite frankly, is the most burdensome of what we're dealing with."

Over the last five years, emergency rooms in the Midwest, including those in Michigan and Wisconsin, saw on average about 350 heat patients for every 100,000 admissions, according to the Centers for Disease Control and Prevention. In recent days, that number was nearly double.

Emergency calls have also increased in New Hampshire, Vermont and Maine.

NPR's Alejandra Borunda contributed reporting.