As the clock ticks down the final days of July, thermometers will climb across much of the eastern half of the United States, creating a dangerous and long-lasting heat wave that will put millions at risk of heat stroke and other illnesses.

Those in the path of the heat wave should plan to take extra measures to stay cool through at least July 30. Keep things chill by staying in air-conditioned spaces, visit friends with pools, pile up the popsicles and lemonade and pull out the recipe for your favorite cold summer salad.

“This will be a long duration heat wave, with little to no overnight relief and high humidity levels, leading to an increased danger,” the National Weather Service Weather Prediction Center said on July 24.

Daily highs in the upper-90s to near 100 degrees are forecast to combine with humidity to create conditions where the “feels like” temperatures could exceed 110-115 degrees, according to a July 25 forecast by Jennifer Tate at the prediction center.

A dome of high pressure sitting over the eastern part of the country will help trap the heat while abundant moisture in the air will make actual temperatures feel several degrees warmer. The unusual heat and humidity could linger for several days, keeping heat risk in the major to extreme categories through the end of the month across much of the eastern United States.

The dangerous heat will be longest lasting across the Tennessee Valley/mid-South region and the Southeast into the Carolinas, said a Friday forecast by Jennifer Tate at the Weather Prediction Center. Morning lows in the 70s "will not provide much relief." Farther north, into the Midwest, Ohio Valley and mid-Atlantic can expect hotter-than-normal temperatures into the 90s.

Forecast maps from the weather service show the United States blanketed in warm shades of purple, red and orange, signifying intense heat. By July 30 or 31, a cold front is expected to help cool things in the Ohio valley region....