Major heat wave to hit Midwest and East Coast, bringing heat alerts to 66 million

The summer is coming in hot: As Thursday's solstice approaches, a heat wave is hitting the East Coast and the Midwest and is expected to last through at least Friday.

Almost 66 million people across the U.S. were under some level of heat alert Monday afternoon. About 150 million people will experience temperatures above 90 degrees Fahrenheit on Monday, and readings could rise over 100 degrees for around 9 million people.

Heat warnings, watches or advisories were in effect from Iowa to Maine, affecting Detroit, Cleveland, Cincinnati, Pittsburgh, Philadelphia, Boston, New York and other cities.

“The duration of this heat wave is notable and potentially the longest experienced in decades for some locations,” the Weather Prediction Center said Sunday.

A strong heat dome is causing the extreme conditions. Temperatures could reach as high as 25 degrees above normal in many areas. Records could be set in 200 cities from the Ohio Valley and the lower Great Lakes into the northern mid-Atlantic and the Northeast, according to the National Weather Service.

The NWS' Phoenix branch said temperatures reached 112 degrees there Sunday — 7 degrees above average and just under the record of 115.

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The Nevada Division of Emergency Management warned residents not to rely on fans to stay cool and urged them instead to seek air-conditioned spaces such as libraries, shopping malls or the cooling centers that have been opened across the Southwest in recent weeks.

New York Gov. Kathy Hochul activated 50 members of the National Guard to help boost the state's response. New York City’s more than 500 cooling centers are set to open Tuesday.

"We are going to be seeing temperatures at levels we have not seen in our lifetimes," Hochul said in a statement Monday, adding: "This could be a deadly event. Extreme heat is the leading cause of weather-related deaths."

Indeed, deaths from extreme heat have increased in recent years. About 2,300 people in the U.S. died from extreme heat last year.

Public school systems in at least two states have adjusted their hours. Schools in Worcester, Massachusetts, will end the school year a few days early. And schools in Buffalo and Rochester, New York, implemented half-days for the rest of week to allow pre-K through eighth graders to leave after lunch.

Some parts of the U.S. are also expected to get heavy rain this week. In the Dakotas and Minnesota, the NWS said, there is a chance of flash flooding and severe thunderstorms. Rain is pushing in from the Gulf of Mexico, as well, and it is expected to hit parts of Texas and Louisiana, possibly through Wednesday.

Research predicts more frequent and extreme weather events as climate change’s effects continue to intensify. Each of the last 12 months set a record for high temperatures, according to the European Union’s Copernicus Climate Change Service.