WASHINGTON: A widespread and dangerous heat wave was building across the US on Saturday, with triple-digit highs expected in the Southwest and Great Plains this weekend before spreading eastward under a dome of high pressure that meteorologists say could trap oppressive temperatures for a week or more.

Forecasters advised people to stay hydrated and find places to cool off, warning of temperatures 15 to 25 degrees Fahrenheit (8 to 14 degrees Celsius) warmer than normal in many areas, including at night — especially bad for people’s health because their bodies won’t have a chance to recover. The heat dome was expected to affect as much as two-thirds of the continental United States.

“The heat doesn’t necessarily stop when it’s dark out,” said Josh Adam, a meteorologist with the National Weather Service in Bismarck, North Dakota, where temperatures will surpass 100 F (37 C) until Tuesday, a dramatic spike for a state where summer temperatures are typically in the 80s.

Tynika Smith of Bloomington, Minnesota, handed out frozen towels and wash cloths along with battery-operated fans at encampments of homeless people in nearby St. Paul and will continue next week, when temperatures are forecast to climb into the mid- to high 90s. The residents put the ice packs around their necks and on their heads.