More Heat, and Little Respite, Is Expected in the Southwest

The soaring temperatures baking much of the desert Southwest this week are not only strikingly high but also unusually persistent.

In a forecast for the Phoenix area, where the temperature on Monday was expected to reach 106 degrees, the National Weather Service said the duration of the heat wave would be “one of the longest, if not the longest, depending on how it’s measured.”

Sweltering temperatures are likely to continue for at least a week in the area, possibly approaching record-breaking levels by the weekend, according to Tom Frieders, a meteorologist with the Weather Service in Phoenix.

Nighttime temperatures there this week will barely dip below 90 degrees, and even the cooling thunderstorms that typically accompany monsoon season, which starts around this time of year, have been lagging.

“This year, we are seeing a bit of a delay in the evolution into the monsoon thunderstorm season,” Mr. Frieders said. “And so with that, we ended up seeing temperatures a little bit hotter during the day.”

The heat wave now stationed over the desert Southwest — caused by a “heat dome” of high pressure — has also brought sweltering temperatures to parts of Texas and New Mexico. Experts estimate that more than 50 million people across the United States live in the areas expected to have dangerous levels of heat.

“It looks like it’s going to be pretty oppressive around here,” said Capt. Darren Noak of the Austin-Travis County Emergency Medical Services, which conducted a helicopter rescue on Sunday to treat a person who had been overcome by the heat in a popular hiking spot west of Austin, Texas.

As Captain Noak spoke on Monday afternoon, the heat index in the city — a measure of how the air feels that takes into account both temperature and humidity — was climbing toward an expected high of 109 degrees.

Captain Noak said Austin residents had enjoyed some relief last week, when high temperatures fell short of 100 degrees for several days and the E.M.S. made only 39 heat-related calls, compared with 74 the previous week.

But now, “the weather predictions are for above 100-degree temperatures for at least the next two weeks straight,” he said. “I would imagine this week is going to shoot back up there again.”

Orlando Bermudez, a forecaster for the Weather Service in Austin, said residents of central Texas could “copy and paste” the same basic forecast for the next couple of weeks: hot and dry.

“The story of the week into the weekend into next week is heat,” he said, predicting a virtually unbroken pattern of triple-digit temperatures most likely extending at least through July 20.

Julie Engel, the president of the Greater Yuma Economic Development Corporation in southwest Arizona, said residents in the area had been dealing with these kinds of temperatures for a long time.

“It’s really not anything out of the ordinary for us,” she said. Construction crews and farmworkers know how to adapt, she added, for example by starting their workdays hours before dawn.

Still, temperatures across the region are dangerous for many people, according to Zack Guido, an assistant research professor at the Arizona Institute for Resilience in Tucson.

“Most challenges associated with these extreme climate conditions fall on people with less resources,” Dr. Guido said, adding that high nighttime temperatures only added to the risk.

“If you don’t have air-conditioning at night, and the temperatures remain above 90 degrees Fahrenheit, that has been attributed to physiological stress that can be quite harmful,” he said.

John Washington contributed reporting.