Heat dome drives dangerous U.S. heat into July 4 weekend

A sprawling heat dome is pushing dangerous temperatures across much of the United States, with forecasters warning that high humidity could make it feel as hot as 110 degrees or more in parts of the central and eastern U.S. heading into the July 4 weekend.

The National Weather Service said Sunday that “dangerous to record setting heat will expand across the eastern two-thirds” of the country. More than 130 million Americans across southern and Great Plains states were under moderate to severe heat risk conditions Sunday, The Guardian reported, citing NWS maps, and the area was expected to grow as the week continued.

Forecasters said several days of temperatures in the 90s and low 100s will settle over the lower Great Lakes, the mid-Atlantic and the Mississippi and Ohio River valleys. Heat indexes, which account for humidity, may reach 100 to 110 degrees in many areas and as high as 115 degrees in some places, according to the weather service. Parts of Iowa, Missouri and Kansas were under extreme heat warnings.

“That’s heat that’s impactful to anyone,” NWS meteorologist Bryan Putnam told The Guardian. “It’s not just older adults or younger children or people who are spending a ton of time outdoors, maybe straining themselves a little more than normal. This is heat that really could impact everyone, especially with people outdoors going into the holiday weekend.”

The heat will affect cities across the East, Midwest and South, including New York City, Philadelphia, Washington, Baltimore, Chicago, Indianapolis, St. Louis, Detroit, Dallas, Little Rock and Memphis. Areas including Ohio, parts of North Carolina and Washington, D.C., face especially dangerous conditions for older adults and people with respiratory problems, NPR reported.

A heat dome forms when a very hot air mass settles over a region and becomes trapped beneath a lid of high pressure. Tyler Roys, an AccuWeather meteorologist, told The Guardian that the high-pressure system will act like a “rock,” forcing storms around it and limiting rainfall across the East. He said temperatures around the holiday could run 10 or 11 degrees above average in Washington and Indianapolis.

Putnam told NPR that humidity will make the heat more dangerous. “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,” he said.

Forecasters also warned that nighttime may bring little relief. Lows in the 70s or even upper 80s could keep homes and streets hot after sunset, especially for people without air conditioning. “Just because the sun goes down doesn’t mean it still isn’t going to be hot,” Putnam told NPR, urging caution for people gathering outdoors for fireworks and other holiday events.

Officials urged residents in affected areas to prepare now, limit prolonged outdoor exposure, stay hydrated and use air conditioning or other cooling spaces. The Missouri State Emergency Management Agency said people should “plan accordingly and limit prolonged outdoor exposure if possible,” and should “take immediate action” if someone shows signs of heat-related illness.

The Centers for Disease Control and Prevention lists heat exhaustion, heat rash and heat stroke among heat-related illnesses. Symptoms can include nausea, headache, confusion and dizziness. Heat stroke can be fatal or cause permanent disability without emergency treatment, and symptoms include confusion, altered mental status, slurred speech and loss of consciousness.

New York City Health Commissioner Alister Martin warned Saturday that heat strokes are “fast, deadly, and almost always preventable.” He added: “As an ER doctor, I watched heat stroke come through the doors on days exactly like the ones ahead.”

The heat is also contributing to wildfire danger in the West. The weather service warned Sunday of extremely dry and windy conditions that could promote rapid fire spread across the Great Basin and the Southwest. The Guardian reported that three firefighters working for the U.S. Wildland Fire Service and U.S. Forest Service were killed near the Colorado-Utah border and two others sustained burn injuries after they were overcome by flames from fast-moving wildfires.