Heat continues to build in the Northern and Central Plains of the United States, with more than 15 million people there under heat alerts on Sunday. Temperatures in Minnesota and Nebraska were expected to climb to the triple digits.
The National Weather Service said that heat indexes could top 100 degrees on Sunday, from the Gulf Coast to near the Twin Cities in Minnesota, where it was expected to be “dangerously hot and humid.”
A heat dome that scorched the Southwest last week and brought record-high temperatures to more than a dozen cities has lingered, moving steadily eastward and settling in the Plains on Sunday, said Marc Chenard, a meteorologist at the National Weather Service.
The heat wave is expected to continue into the week.
“The heat is certainly related to the general pattern we’ve been seeing over most of the U.S. going back a week,” Mr. Chenard said. “And it’s actually going to continue for another week or so as well.”
A few records were set on Saturday along the central Gulf Coast, where the temperature reached 98 degrees in Tampa, Fla., and 101 degrees in Mobile, Ala., according to Weather Service data.
Starting on Monday, the heat dome is expected to move across the Mississippi Valley and the mid-South, the Weather Service said. By Tuesday, it will shift to the Great Lakes and the Ohio and Tennessee Valleys, and then into the Southeast by Wednesday, Mr. Chenard said.
Temperatures across a large portion of the country will hover above average through the week, Mr. Chenard said, adding that “quite a few potential record highs” could be set across Texas and into much of the Southeast.
The heat wave has since broken in the Southwest, where “early-season monsoonal moisture” was expected to cause flash floods and scattered thunderstorms on Sunday, the Weather Service said.
Temperatures in California and Nevada were significantly lower on Sunday, with potential record lows dipping to 40 degrees just a week after both areas baked under the heat wave.
On Sunday, red flag warnings covered large parts of Arizona and Utah. Six wildfires, encompassing a total of more than 53,000 acres, remained active in Arizona, according to the National Interagency Fire Center. Fire conditions were expected to improve on Monday, the Weather Service said.
Heat waves are more frequent and hotter and last longer than in previous decades, and are part of an overall warming trend worldwide, scientists say.
The average number of heat waves in the United States has tripled, from two per year in the 1960s to six in the 2010s, according to the federal National Climate Assessment. The heat wave season is also 45 days longer than it was in the 1960s.