Extreme heat this week will blanket a majority of American states through the July 4 weekend, according to forecasters.
The National Weather Service on Sunday said "dangerous to record setting heat will expand across the eastern two-thirds" of the country.
In areas including Ohio, parts of North Carolina and Washington, D.C., the extreme temperatures and humidity will be especially threatening for people with respiratory issues and the elderly.
"With the combination of high humidity, heat indices may reach 100-110 Degrees," said the NWS. "Much of the central and eastern U.S. is under a Moderate to Major HeatRisk, which can pose health impacts on those without hydration or cooling."
Parts of Iowa, Missouri and Kansas are under extreme heat warnings.
A heat dome is driving the heatwave. It occurs when a very hot air mass parks itself over a region and gets trapped under a "lid" above the Earth's surface.
But the high temperatures are not the only concern, said NWS forecaster Bryan Putnam.
"You get temperatures in the 90s to low 100s, that's obviously pretty hot. But you combine that with the humidity, those heat indices will go well into the 100s and that's the temperature that it's going to feel like," Putnam told NPR on Sunday.
Risks for extreme heat are also expected to continue after July 4 and in the West. Daytime temperatures could feel like 100 to 105 degrees and the heat could limit overnight relief, the NWS said.
Putnam said people gathering outdoors for the July 4 weekend, including at night to see fireworks, should be vigilant.
"Your temperatures might stay in the 80s and the 90s in the heat in the evening, as well as the fact is with the humidity, that's going to keep those heat indices high as well," he said. "Just because the sun goes down doesn't mean it still isn't going to be hot."
Prepare and limit time outdoors, officials warn
Those in the extreme heat's path should take precautions now and prepare for days of high temperatures, officials said.
The Missouri State Emergency Management Agency on Sunday said everyone should "plan accordingly and limit prolonged outdoor exposure if possible." The agency added that people should "take immediate action" if someone shows any sign of a heat-related illness.
Heat exhaustion, heat rash and heat stroke are a few types of heat-related illnesses, according to the Centers for Disease Control and Prevention. Nausea, headache, confusion and dizziness are each possible symptoms.
New York City Health Commissioner Alister Martin on Saturday said heat strokes are "fast, deadly, and almost always preventable."
"As an ER doctor, I watched heat stroke come through the doors on days exactly like the ones ahead," Martin wrote in a social media post.
The CDC notes that heat stroke can be fatal or cause permanent disability if someone does not receive emergency treatment. Heat stroke symptoms include confusion, altered mental status, slurred speech and loss of consciousness.
Martin also advised people to use air conditioning, hydrate and check on their neighbors during the heatwave.
Copyright 2026 NPR