As millions of Americans return to work June 23 for the first full week of summer, they will face scorching temperatures that could reach as high as 100 degrees, particularly in the Mid-Atlantic region, weather experts said.

“The toughest days are going to be Monday through Wednesday to get through,’’ said Brian Hurley, a meteorologist with the Weather Prediction Center, part of the National Weather Service.

A heat wave is expected to continue across the Midwest, Mid-Atlantic and the Northeast putting more than 150 million Americans under heat advisories, heat warnings or extreme heat watches, according to the National Weather Service. A heat warning is more extreme with temperature possibly reaching as high as 110 degrees, but all the conditions can be dangerous, said Hurley.

“It’s a concerning issue just because of the fact that we’re not really getting much relief, if any, at night," he said. “We’re not cooling off so it’s going to feel pretty extreme.’’

Some temperatures could reach 15 degrees above normal for June. It's early to see triple-digit temperatures, which usually happen in July, Hurley said.