The heat index hit 102 in St. Louis; and in Denver and Dallas, the mercury climbed into the 90s.
National Weather Service meteorologist Norman Junker says that given the season, people should expect to swelter.
"It is the first major heat spell and it's going to be very humid, which is going to make the heat index very high," he says. "And that's the critical thing of how you feel."
In Boston, it feels so terrible the mayor has issued a heat alert. And overheated New Yorkers should plan for things to get even worse. Forecasters say that with the humidity, it will seem like 110 for the next two days there.
For the Diamond Ice Company, that prediction translates into record-breaking back-breaking work. Les Hendler says they delivered almost two tons of cubes Saturday.
"Restaurants need ice," he explains. "In this humidity, the ice machines can't keep up."
The culprit is a weather system known as the Bermuda High, high pressure out over the Atlantic that circulates tropical air in from the Gulf.
"The jet stream, which is usually what delivers fronts for the U.S., is now moved back north into Canada," Junker says. "There are no fronts moving down into the U.S. bringing cooler weather, so the heat's starting to build."
For the elderly, weekend exercisers and people with high blood pressure, the heat is more than a nuisance. Emergency rooms are gearing up to treat victims of heat prostration, and doctors have special warnings for Fourth of July party-goers.
"If you're going to be drinking a lot of alcohol and during a long period of time and out in the heat, it is very possible that you could become dehydrated," warns Dr. Richard Stumacher.
Fortunately, the cure is relatively simple: drink lots of fluids, especially water. And if you can, step into some air conditioning and wait for midweek, when temperatures are expected to drop.