Punishing heat wave peaks as East Coast readies for sweltering July 4 weekend

(CNN) — A dangerous heat wave is tightening its grip on the East Coast, with the worst conditions arriving as millions prepare to celebrate the Fourth of July outdoors.

While record-breaking heat scorched parts of the Midwest on Wednesday, the most intense heat shifted into the Interstate 95 corridor Thursday and will persist through Friday. Temperatures both days are forecast to climb into the upper 90s and low 100s from Washington, DC, to Boston, with humidity pushing the heat index — what heat actually “feels like” to the body — above 110 degrees in spots.

Heat indices had topped 100 degrees by 10 a.m. Thursday in every major metro from Washington, DC, to New York City. It will only soar higher through the afternoon.

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The timing couldn’t be worse. The heat is peaking during one of the busiest travel and outdoor celebration periods of the year, prompting cities to expand cooling centers, shorten parade routes and activate emergency response plans ahead of America’s 250th birthday celebrations.

More than 160 million people are under a Level 3 of 4 “major” or Level 4 of 4 “extreme” heat risk through the end of the week, according to the National Weather Service. These categories are associated with a sharp increase in heat-related illnesses, especially during prolonged heat waves with little overnight relief.

The combination of temperatures and humidity increases the health risks: High humidity keeps temperatures elevated and prevents sweat from evaporating efficiently, making it harder for the body to cool itself.

Heat is the deadliest weather in the United States, with a higher annual average death toll than tornadoes, hurricanes and lightning combined, according to the National Weather Service.

This heat wave is the result of an intense heat dome — a stubborn area of high pressure that locks hot air in place. It’s a phenomenon experts say is clearly supercharged by global warming.

Cities prepare for dangerous holiday heat

Many cities, including Boston, New York, Philadelphia, Washington, DC, and Raleigh, North Carolina, are opening cooling centers and expanding public resources as the dangerous heat settles in.

The federal government is also preparing for the strain. The Department of Energy has issued emergency orders aimed at reducing the risk of power outages by increasing electricity generation within the PJM Interconnection grid, which serves much of the Mid-Atlantic.

Philadelphia has declared a Heat Health Emergency through Saturday evening and shortened the route for its July 4 parade. A ceremony featuring a live virtual address from the pope has been moved indoors, and officials have reduced hours for the city’s World Cup Fan Festival ahead of Saturday’s match between Paraguay and France.

Washington, DC, has activated an Extreme Heat Alert through July 5 and is expanding cooling centers, hydration stations and emergency medical resources as hundreds of thousands of visitors descend on the nation’s capital for holiday events. Organizers of National Mall celebrations are adding water stations, cooling tents and air-conditioned buses for attendees.

New York City is opening hundreds of cooling centers and deploying mobile medical vans to provide water, sunscreen and wellness checks. More than 2,200 LinkNYC kiosks will display directions to the nearest cooling center.

The searing temperatures could also impact air travel: Delta Air Lines has issued a high heat advisory for New York’s LaGuardia Airport through July 4 “due to hot weather compounding with operational constraints.”

Rewriting the record books

Dozens of long-standing temperature records are set to fall Thursday and Friday.

New York City could experience one of its most oppressive days in decades. If Thursday’s heat index reaches 110 degrees, as forecast, it would join only three other days since records began in 1943 with a heat index that high. The city is also forecast to reach consecutive triple digits Thursday and Friday for the first time since 2011, according to the National Weather Service.

Washington, DC, is forecast to reach around 103 degrees Thursday and Friday, just shy of its all-time record high of 106 degrees. July Fourth in the nation’s capital is forecast to be the hottest on record at 102 degrees, beating the old record of 100 set in 1919. Thursday to Saturday may be the hottest three-day stretch in DC since 1930.

Philadelphia could climb to near 103 degrees Thursday and 104 degrees on Friday, while Boston is forecast to top 100 degrees Thursday and hover close to that mark Friday.

In eastern Virginia, the weather service says this could be the region’s most significant heat wave since July 2012, which resulted in a dozen deaths in the state. That heat wave caused more than 30 heat-related fatalities across four states.

The biggest concern isn’t just the daytime heat: Nights are warming faster than days in our changing climate. Warm, humid nights will provide little relief, with temperatures only falling into the 70s in many locations and struggling to drop below 80 degrees in urban areas. Without cooler nights, the body has little opportunity to recover before another day of dangerous heat.

Some relief is expected to begin arriving across parts of the Midwest later in the holiday weekend before gradually spreading into portions of the Northeast. Much of the South, however, is expected to remain hotter than normal into next week.

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CNN’s Kate S. Peterson, Andrew Freedman and CNN Meteorologists Mary Gilbert and Chris Dolce contributed to this report.