The stifling weather that has been scorching much of the Midwest is intensifying in the Northeast, pummeling a huge section of the United States on Thursday with hazardous levels of heat and humidity. Several states and cities are taking precautions to limit the risks of oppressive conditions that are expected to last into the long holiday weekend.

Some 238 million people — from the Dakotas to Boston and south to Miami — live in areas that are likely to experience some level of dangerous heat Thursday, according to the National Weather Service.

Heat indexes — a measure of how hot it feels based on temperature and humidity — could soar above 110, meaning heat cramps or heat exhaustion are likely, and heat stroke is possible with prolonged exposure and/or physical activity. The latest “heat risk” forecast puts several major cities — including Boston, New York, Washington, Philadelphia, Pittsburgh and Atlanta — into the most extreme risk category through Saturday.

Temperatures in those cities were hovering around 100 by noon Thursday, with several daily records at risk of falling. In much of the Northeast, the temperature is not expected to dip much below 80 on Thursday and Friday night. Many people will be facing the third or fourth consecutive day of stern warnings to avoid being outside in the warmest parts of the day.

Parts of the Great Lakes region — such as Ohio and Illinois, where heat indexes have hovered in triple digits — have been baking all week. Temperature-related conditions disrupted rail travel along parts of the Northeast Corridor. Local officials urged people to stay indoors, in air conditioning and to check on their neighbors.

Here’s What Else to Know:

— Why it’s hot: The meteorological phenomenon causing the extreme heat is a sprawling high pressure system, also known as a heat dome. Like a lid on a boiling pot, heat domes trap the air beneath them, pushing warm air toward the ground that, as it sinks, compresses and becomes much hotter.

— Global warming: While tying a single heat wave to climate change requires extensive analysis, scientists say that heat waves have grown longer, hotter and more frequent. The past 11 years have been the hottest on record.

— Cooling off: Cities and towns in many parts of the country have opened libraries, senior centers and other air-conditioned spaces for those seeking refuge from sweltering homes. Experts urged Americans to stay hydrated, plan their days around the heat and to know the warning signs of heat illness.

— When will it end?: For many, the heat wave will start to ebb Sunday, as the heat dome responsible starts to weaken. Many of the most extreme heat warnings will have passed, but dangerous heat is likely to remain across parts of the Washington metro area, Virginia and the Carolinas.

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This article originally appeared in The New York Times.

By Nazaneen Ghaffar, Erin McCann and Christine Hauser/Madeleine Hordinski
c. 2026 The New York Times Company