Heat dome locks in scorching temperatures from Texas to Maine through July Fourth weekend

A stalled high-pressure lid

A massive, stagnant high-pressure system is parked over the eastern half of the country, trapping heat and moisture near the surface. The National Weather Service's Alex Lamers described the phenomenon as acting “like a lid on a pot,” allowing heat to build day after day while suppressing clouds and rain.

If you’ve made grilled cheese in a pan and you put a lid on there, it melts the cheese faster because the lid helps trap the heat. It’s a similar concept here: You get a big high-pressure system in the upper parts of the atmosphere and it allows that heat to build underneath over multiple days.

The high is centered over Tennessee and is influencing weather from Texas to Maine, with forecasters calling it the hottest stretch so far this season.

Soaring heat indices and dangerous nights

By Monday, nearly 40 million people were under extreme heat warnings. Daytime highs are forecast in the 90s and low 100s, but humidity will make it feel far hotter. Heat index values are expected to reach 105–115° across the southern Plains, the Mid-Mississippi Valley, and the Mid-Atlantic. In Nashville, the NWS warned that conditions could feel like 115°F; in Iowa, the heat index was projected to hit 110°F.

Overnight temperatures will only drop into the mid-70s or even low 80s in some major cities, offering little relief. “The combination of prolonged daytime heating and limited nighttime relief will increase the risk of heat-related illnesses, especially for vulnerable populations and those without adequate cooling,” the NWS said.

These are the hottest temperatures so far this season. The danger is further heightened because of the number of people expected to spend time outdoors in the lead-up to the long holiday weekend.

Cities race to prepare

Chicago, still shaped by the deadly 1995 heat wave that killed 739 people, has activated its comprehensive Heat Emergency Plan. Cooling centers have opened, park district programs have moved indoors, and the city is using a Heat Vulnerability Index to direct resources to the most at-risk neighborhoods. Illinois Emergency Management Agency’s Theodore Berger urged residents to check on neighbors and know where local cooling centers are.

Preparedness is key. Limit time outdoors during peak heat, check in on neighbors and those who may need extra assistance, and know where your local cooling centers are.

In Nashville, emergency management is deploying heat patrols to check on unhoused residents. Cleveland has extended recreation center hours to serve as cooling centers, and Richmond is bracing for what the NWS says could be the region’s worst heat wave in years.

‘Corn sweat’ adds to the misery

Across the Midwest, an extra source of humidity is kicking in: evapotranspiration from cornfields, colloquially called “corn sweat.” As corn absorbs water and releases vapor, it pumps additional moisture into already oppressive air, pushing heat indices even higher. NWS meteorologist Andrew Ansorge noted that while the heat is not unusual for summer, this is the first prolonged wave of its magnitude this year.

How the heat wave unfolds

Richmond’s week of scorching numbers

A day-by-day outlook for Virginia’s capital shows the intensity building. The NWS Wakefield office forecasts a dangerous combination of high temperatures and humidity well into the holiday.

Health risks and how to cope

Heat stress is the top weather-related cause of death in the U.S., and officials warn that this event could produce illness in anyone without adequate hydration or cooling. Advice from experts and agencies is consistent: stay indoors in air conditioning when possible, drink plenty of fluids, limit strenuous activity, and use cool compresses or tepid baths to lower body temperature safely. As a precaution for the holiday weekend, officials are reminding people to double-check back seats for children and pets.