Heat wave to grip central and eastern U.S. through July Fourth

A broad, dangerous heat wave is expected to build across the central and eastern United States this week, raising health risks as millions prepare for July Fourth events. The National Weather Service said humidity will make already high temperatures feel more severe from Texas and the Great Plains to the East Coast.

On Sunday, National Weather Service maps showed more than 130 million people in southern and Great Plains states under moderate to severe heat risk. Forecasters said the area under threat is expected to grow as the week continues, with temperatures rising in several major population centers.

Phoenix, central Texas and much of the Southwest were already near 100 degrees Fahrenheit on Sunday, according to the weather service. The agency also warned that dry, windy conditions were creating severe wildfire danger across much of the West.

Major cities face their hottest weather of the year

The Weather Service said several days of high heat will settle over the lower Great Lakes, the mid-Atlantic, the Mississippi Valley and the Ohio Valley. Some locations from the lower Great Lakes to the mid-Atlantic and New England could set daily records later in the week, meteorologist Bryan Putnam said.

New York, Philadelphia, Washington and Baltimore are among the East Coast cities expected to feel the heat. The forecast also includes Chicago, Indianapolis, St. Louis and Detroit, along with Dallas, Little Rock and Memphis.

The timing could complicate large outdoor gatherings, including World Cup matches and events tied to America’s 250th anniversary. The Weather Service said heat is likely to last into next weekend across the Great Plains, the Southeast and the mid-Atlantic.

Highs are forecast to reach the upper 90s and low 100s in Fahrenheit, according to the Weather Service. With humidity, heat indexes could run from 100 to 110 degrees and reach as high as 115 degrees in some areas.

Putnam said the conditions can affect anyone outdoors, not only children, older adults or people doing strenuous activity. The heat index, used in many forecasts, estimates how hot conditions feel when humidity is included and helps gauge risk during prolonged exposure or hard exertion.

Warm nights could add to health risks

AccuWeather meteorologist Tyler Roys said temperatures in some cities will run about 10 or 11 degrees above typical early July levels. He cited Washington, where the average high around July Fourth is about 89 degrees, and Indianapolis, where it is about 85.

Roys said a strong high-pressure system, often called a heat dome, will keep storms away from much of the East and limit rainfall. Overnight temperatures in the 70s or even upper 80s will offer little cooling, meteorologists said.

That lack of nighttime relief raises the risk of heat illness, Roys said, because bodies have less chance to recover. He said people without air conditioning, including residents in Eastern Seaboard cities such as New York where lows may stay above 80 degrees, could have difficult nights.

The Weather Service advised people to reduce outdoor activity during extreme heat, drink water and make sure they can reach air conditioning or other cooling spaces.

Western wildfire threat grows

In the West, the Weather Service said extremely dry and windy conditions on Sunday could help fires spread quickly across the Great Basin and the Southwest. The Associated Press reported that wildfire activity has increased in Utah, Colorado, Arizona and other states as heat, wind and dry air fueled flames.

Near the Colorado-Utah border, three firefighters working for the U.S. Wildland Fire Service and U.S. Forest Service were killed when fast-moving flames overtook them, the Associated Press reported. Two others suffered burn injuries.

This story draws on original reporting from Fortune.