Millions of Americans are facing dangerous, potentially life-threatening heat as a powerful heat dome continues to bake much of the central and eastern United States through Friday, with extreme conditions lingering along the East Coast throughout the Independence Day weekend.

The National Weather Service warns that temperatures and humidity will combine to produce heat index values exceeding 100 degrees in many locations, with some areas experiencing “feels-like” temperatures approaching 115 degrees.

The prolonged stretch of oppressive heat is expected to challenge daily temperature records in numerous cities from the Midwest to the Northeast. Overnight temperatures are also expected to remain unusually warm, providing little relief and increasing the risk of heat-related illnesses, particularly among older adults, young children, outdoor workers, and people without access to air conditioning.

Officials are urging residents to limit outdoor activity during the hottest parts of the day, stay hydrated, check on vulnerable neighbors and relatives, and never leave children or pets inside parked vehicles.

The heat arrives as millions prepare for Fourth of July celebrations, outdoor festivals, travel, and sporting events, raising concerns about public safety and increased demand on electrical grids as air conditioning use reaches peak levels. Several communities have modified holiday events because of the dangerous weather.

Forecasters expect the most intense heat to gradually ease early next week, although scattered thunderstorms may accompany the transition to cooler temperatures in some regions.