EASTERN U.S. — A heat wave is affecting an estimated 90 million people across the United States, stretching from the Midwest to the East Coast. Temperatures in major East Coast cities are expected to exceed 100 degrees Fahrenheit on July 2, 2026.

High temperatures in the Baltimore-Washington, D.C. region and southeastern Pennsylvania, including Philadelphia, are expected to reach the low to mid-100s on Thursday, according to the National Weather Service forecast. Heat indices in these areas are projected to be around 110 degrees Fahrenheit.

Overnight low temperatures are expected to remain in the low 80s for many areas within the Baltimore-D.C. region and southeastern Pennsylvania. Ashley Ward, director of the Heat Policy Innovation Hub at Duke University's Nicholas Institute, stated, "We're seeing temperatures well into the evening that are staying shockingly high and, in some cases, never dropping below 80 or 75 degrees. And this is really concerning."

Ward added, "We're not getting the opportunity for recovery overnight, as we've historically seen. Our bodies need it, of course, but plants and animals … also our energy infrastructure. We need to be able to cool off overnight." Meteorologist Alex DaSilva with Accuweather noted, "The trends have been for those overnight lows to be warmer and warmer and warmer, especially when we're dealing with these big heat wave events."

The atmosphere is retaining more heat at night. The Fifth National Climate Assessment, published in 2023, indicates that nighttime temperatures across the U.S. are rising faster than daytime temperatures. The assessment also states that the number of nights where the temperature never falls below 70 degrees Fahrenheit is increasing everywhere in the U.S., with the exception of the Northern Great Plains.

Kenney stated, "It's really a combination of temperature and relative humidity. The sweating response is really only valuable if that sweat can evaporate." High humidity conditions reduce the body's ability to evaporate sweat from the skin. Kenney advised, "Make sure they drink before their heat exposure and especially afterward to try to catch up."

As of 2020, 88% of U.S. households had air conditioning installed, according to the U.S. Energy Information Administration. A study by KFF indicated that lower-income households and Asian, Black, and Hispanic households were more likely to lack air conditioning compared to those with higher incomes or White households.