More than half the US experiencing dangerous heat wave

In 1995, a heat wave in Chicago killed more than 700 people; emergency rooms are on alert, bracing for an influx of heat-related emergencies.
2:44 | 07/17/19

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Transcript for More than half the US experiencing dangerous heat wave
Much of this country, at least 28 states, bracing for this dangerous heat. And tonight, as we come on the air, severe thunderstorm watches right here in the northeast from D.C. Up through philly and new York. And look at the map tonight. More than two dozen states under heat alert straight through the the heat index there, it will feel like 110 in Philadelphia, 106 in New York City, 107 in Detroit. And here in New York, that concern after the blackout already in recent days. What happens with this kind of heat and the power grid? ABC's Alex Perez leading us off from Chicago tonight. Reporter: Tonight, from the east coast to the plains, more than half the country in the midst of what forecasters are calling the most dangerous heat wave in years. Oppressive humidity gripping the Delaware valley today. It is hot, hot, hot out there and it is only going to get hotter. Reporter: In Richmond, Virginia, the heat turning deadly Tuesday. A 10-month-old left in a hot car dying at the hospital. The heat index was 103. In 1995, a heat wave here in Chicago killed more than 700 people. Tonight, emergency rooms are on alert, bracing for an influx of heat-related emergencies. So, if someone comes in and we think it's heat stroke, we'll put them into this ice bucket to get their body temperature down as fast as possible. Reporter: And in New York City, after that weekend blackout plunged a sectional of Manhattan into darkness, fears that triple-digit temperatures this weekend could bring more outages. We'll continually remind people to only use the energy they need. Reporter: David, here in Chicago, we're bracing for temperatures that will feel like 105 degrees the next few days. The emergency room doctor I talked to says even if you're staying hydrated, you can still be susceptible to heat stroke. He says the key is to get out of the heat and somewhere cool. David? Alex Perez leading us off tonight. Great advice, Alex. Let's get to ginger zee, tracking this all. And you were telling us this will last for days? Reporter: Days. And cumulative heat is when it becomes dangerous, David. So, New York City hasn't seen a 100-degree temperature since 2012. I really think we'll get close this weekend, if not get right to that century mark. Let's look at the map. Tonight, we've got an imminent threat. That's severe thunderstorms. Damaging winds anywhere from Wilmington, Delaware, up there you, say, Hartford, Connecticut. See damaging winds and flash flooding, one to two-inch rainfall rates per hour. Very heavy rains going into tonight. Let's focus on that heat. Really the core of it tomorrow, St. Louis, Kansas City. Saturday, I think we could break records from Wilmington, Delaware, up to Worcester, Massachusetts. The overnight temperatures will stay in the 70s or 80s. Ginger zee, thank you.

This transcript has been automatically generated and may not be 100% accurate.

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