Millions of Americans gathering to celebrate the country’s 250th birthday are expected to be under what meteorologists…

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Millions of Americans gathering to celebrate the country’s 250th birthday are expected to be under what meteorologists call a heat dome, ensuring that this Fourth of July weekend will be a dangerously hot one.

This heat dome is expected to stretch across most of the East Coast and across the South and part of the Midwest, with dangerous conditions affecting more than two dozen states, according to AccuWeather.

The National Integrated Heat Health Information System (NIHHIS) describes a heat dome as an exceptionally hot air mass that develops when high-pressure systems remain over a certain area for a period and trap warm air over that area.

One notable U.S. heat dome was in the Pacific Northwest in 2021, which caused more than 1,000 heat-related emergency department visits. Oregon and Washington saw record-breaking heat, with Portland reaching a high of 116 degrees, 42 degrees hotter than the average daily June high temperature, according to the Centers for Disease Control and Prevention (CDC).

Through the weekend, cities along the Eastern Seaboard are expected to see temperatures reach 100 degrees, with high humidity making conditions feel hotter.

A jet stream is expected to push the heat dome back toward the Plains and ease the heat on Sunday, giving New England and the interior Northeast relief. More heat relief should expand to the rest of the mid-Atlantic on Monday and Tuesday, according to the National Weather Service (NWS).

Washington and much of the DMV area will be under an extreme heat warning until 8 p.m. EDT Thursday and for most of Friday and Saturday. The NWS anticipates heat index values up to 111 degrees on Thursday, 112 degrees on Friday and 110 on Saturday.

The capital forecast calls for a high of 103 degrees, with temperatures slowly lowering into the 90s and high 80s until midnight Sunday, according to the NWS. Heat indices could be between 105 and 108 degrees, and relative humidity should reach 48 percent at 11 a.m. Saturday, decrease through the afternoon and rise again to 70 percent by 2 a.m. Sunday.

New York City and Philadelphia could reach 100 degrees, and Newark, N.J., is likely to reach or exceed 100 degrees, AccuWeather reported. Forecasters said cities in New England could see temperatures reach the low to high 90s, with Boston possibly seeing a high near 97 degrees, but heat indices could come close to or exceed 100 degrees through the weekend.

AccuWeather recommends that anyone who chooses to be outside limit their risk of heat exposure and watch for symptoms of heat-related illness, including intense thirst, dizziness, nausea and a rapid heartbeat. Wear light-colored, loose-fitting clothes that are lightweight, and plan any outdoor activities for the early morning or evening.

The heat dome has also led some cities in the Eastern U.S. to cut back or cancel Fourth of July events. But Washington is set to go ahead with its 250th anniversary celebration on the National Mall.

President Trump is slated to speak on the National Mall on Saturday night, having vowed to deliver “a really long speech just to show that I can do anything” before the massive fireworks show later in the evening.

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