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Scalding heat killed at least four people this weekend in Arizona and was stoking at least five major wildfires across the desert Southwest, authorities said Sunday.

Excessive heat warnings blanketed Southern California and Arizona, where temperatures soared into the triple digits Sunday and were expected to remain there until Monday, according to the National Weather Service. "The interior areas will remain dangerously hot," the NWS said.

The heat will remain constant in the West and expand into the middle of the country by midweek, said Reynolds Wolf, a meteorologist for The Weather Channel.

"It's like you take a salt shaker of madness and sprinkle it" over the western half of the country, Wolf said.

At least four people have died in Arizona from separate heat-related emergencies, authorities said.

The first, a 25-year-old man, died while hiking the Peralta Trail in Pinal County on Saturday, Sheriff Paul Babeu said.

Sunday, a 28-year-old woman who worked as a personal trainer died during a morning hike along the Desert Vista Trail in Maricopa County, Phoenix fire officials told NBC News. The heat overcame her so swiftly that she died despite having taken along plenty of water and being immediately treated by a doctor in her hiking group.

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Later Sunday, two hikers, a man and a woman in their early 20s hiking in Pima County left the trail without taking along water and had to be rescued by helicopter, authorities said. The woman died before deputies arrived, and the man was being treated at a hospital, the Pima County Sheriff's Department said Sunday night.

And one man died and another remained unaccounted for Sunday night after their hiking group was overcome in Ventana Canyon near Tucson, the Pima sheriff's office told NBC News. The man who died was believed to be from Germany, but that hadn't been confirmed, the sheriff's office said.

The crushing heat is expected to get even worse Monday, when highs in Phoenix could hit 120 degrees — the hottest in more than 20 years.

New Mexico, southern Nevada, Southern California and other parts of Arizona are expected to see temperatures as much as 20 degrees above average — up to 110 degrees, said Michael Palmer, The Weather Channel's lead meteorologist.

Related: California Fire Crews Fight Sherpa Wildfire, Brace for 120-Degree Temps

Fire officials said the excessive heat would hamper efforts to quell five explosive wildfires that were forcing evacuations in three states.

In New Mexico, the Dog Head fire, burning about 30 miles south of Albuquerque, had already damaged 24 homes. The fire was only 9 percent contained Sunday, according to incident commander Rich Nieto.

The fire, which started Tuesday, tripled to 17,000 acres Saturday, forcing evacuations. But it hadn't grown much by Sunday, partly because of rising humidity that came with the rising temperatures in the area, Nieto said.

Another fire burning in the state, near Cibola National Forest, had ballooned to more than 35,000 acres Sunday and was 30 percent contained.

"The weather forecast ... calls for unseasonably warm air temperatures along with a drying trend which will continue to develop over the region," fire officials said in an incident report. "This pattern will continue to increase fire behavior and smoke production."

Heat was also feeding the Sherpa fire in Santa Barbara, California, according to Los Padres National Forest officials.

The more than 2,000 firefighters battling the flames were focused on saving 60 homes in Refugio Canyon and 80 homes in El Capitan Ranch, which were under a severe threat, officials said. Those areas and other surrounding areas were under mandatory evacuation warnings, and officials said Highway 101 could be shut for a second time this week because of the blaze.

Crews were able to make some progress overnight when expected "sundowner" winds, which rush down the mountains in 40-mph gusts, didn't materialize. But they will likely strengthen again, meeting low humidity and high temperatures and preventing crews from quelling the fire, which has already burned 7,800 acres.

A new fire broke out about 11:30 a.m. (2:30 p.m. ET) Sunday along the U.S.-Mexico border near Potrero, California, about 45 miles east of San Diego, and within just a few hours, it had exploded to 1,500 acres, the California Department of Forestry and Fire Protection said.

The entire population of Potrero, about 700 people, was being evacuated for what was being called the Border fire on Sunday night, CalFire said.

Winds weren't as much of a concern for crews fighting the Cedar fire in eastern Arizona on Sunday, but the flames would be propelled by heat. "Over the next 24 hours, fire will continue to spread on the south and east flanks due to burnout operations, resulting in increased smoke and acreage growth," an incident report said.

The fire had burned 12,000 acres and was 40 percent contained. Adam Wolfe, the communications manager for the county, told NBC station KPNXof Phoenix that people in the affected city of Show Low should be ready to flee.

"If they have medication, if they have family members, make sure everything that is important can be put in cars and moved," he said.