Most of the northeastern United States roasted on Monday, according to the National Weather Service. About a quarter of the country faced local heat advisories the morning after a dangerous heat wave expanded from the Midwest to the East Coast, according to the National Weather Service. The agency expected many local temperatures above 90 or 100 degrees to break historical records. It forecast the weather to remain warm even overnight, offering little relief on Tuesday.

The heat wave is the first sizeable one of the year, so many people aren’t yet acclimated to the weather and could face health risks, the agency cautioned. Heat-related deaths in the United States reached highs of nearly three per million people in 2022, according to the Centers for Disease Control and Prevention.

The city of Philadelphia declared a heat health emergency beginning on Sunday to activate a heatline phone number, home visit services, and increased daytime outreach to homeless people. The city was also opening cooling sites for people without home air conditioning and would not shut off water for failure to pay until the emergency ended, according to its website.

What did the National Weather Service say people should do? The agency recommended that people stay inside and keep themselves hydrated. The agency also said people should check on relatives and be especially careful not to leave children or pets in cars. Vehicles’ interiors can reach lethal temperatures in a few minutes during a heat wave, the National Weather Service said.

This story originally appeared in WORLD. © 2025, reprinted with permission. All rights reserved.