Southwest heat wave simmering since spring will expand to cover much of U.S.

A heat wave simmering in parts of the Southwest since spring was expected to expand into the central and eastern parts of the country for the last week of July, forecasters said Sunday.

"For much of July hot dangerous conditions have been the normal in parts of the West, Texas and Florida," the National Weather Service said in a forecast discussion. "These summer conditions will build and expand across the Eastern two-thirds of the country this week, starting in the north-central states and Plains."

Federal forecasters have issued excessive heat warnings and heat advisories for a wide swath of the U.S., including parts of California, Utah, Nevada, Colorado, Oregon, Idaho, Wyoming, Nebraska, the Dakotas, the desert Southwest and Texas, as well as the southern tip of Florida.

Three high pressure systems of the type associated with hot weather were expected to move over the interior West, the Midwest and the Northeast, and Florida, according to the weather service.

The heat wave's geographic expansion through at least Wednesday could be dampened by expected thunderstorms in the mid-Atlantic and the South and along the Gulf Coast, the weather service said.

NASA reported July 13 that last month marked the hottest June on record for the planet. Last week, the director of the agency's Goddard Institute for Space Studies, Gavin A. Schmidt, said at a news conference that July is likely to be the hottest month ever.

Schmidt also said 2023 could go down as the fourth-hottest year on record, with the possibility it could ultimately rank higher as a new El Niño releases greenhouse gases and provides a warming influence for the North American West.

The National Park Service has reported that at least four visitors in the Southwest are suspected to have succumbed to heat-related deaths since the beginning of June.

The most recent was reported Tuesday after Steve Curry, 71, of Los Angeles, collapsed in 121-degree heat at Death Valley National Park, according to the park service.

The weather service in Phoenix said Sunday that the city's record-breaking streak of high temperatures above 110 degrees continued into its 24th day. The high was 114.

Maricopa County has counted at least 12 heat-related deaths since the warm months began in April. Additional deaths were under investigation to determine whether they were heat-related, according to a county report.

Also Sunday, Salt Lake City recorded a temperature of 104, tying its record for the date, federal forecasters said.

Miami posted a high of 98, beating its record of 95 for the date, the weather service said. Monday will be the 44th consecutive day in Miami with a heat index temperature, a "feels like" figure measured by including humidity, above 100 degrees, forecasters said.

Fort Lauderdale tied its record high for the date, 95, according to the weather service.

Nonstop heat started in Texas in mid-June, before June 21's summer solstice.

Scientists say the prolonged summer warmth is consistent with the effects of global warming, which they say is creating more intense, frequent and prolonged bouts of extreme weather for much of the globe.

This article was originally published on NBCNews.com