The first heat wave of the season is bringing triple-digit temperatures earlier than usual to much of the U.S. Southwest, where forecasters warned residents Tuesday to brace for dangerously hot conditions.

By Wednesday much of an area stretching from southeast California to central Arizona will see “easily their hottest” weather since last September, and record daily highs will be in jeopardy, the National Weather Service said.

Excessive heat warnings were issued for Wednesday morning through Friday evening for parts of southern Nevada and Arizona, with highs expected to top 110 degrees Fahrenheit (43.3 Celsius) in Las Vegas and Phoenix. The unseasonably hot weather was expected to spread northward and make its way into parts of the Pacific Northwest by the end of the week.

“We’re looking at high temperatures well in the 90s and 100s, temperatures well above average for the time of year — some spots as much as 10 to 20 degrees above average,” said Marc Chenard, an NWS meteorologist Marc Chenard in College Park, Maryland.

The hot temperatures’ persistence over several days, with even nights staying warm, will increase the impact, he added.

On Monday the U.S. Border Patrol reported that four migrants died last weekend from heat-related causes while attempting to cross into the country in southeast New Mexico, near El Paso, Texas. Anthony Good, the agency’s El Paso sector chief, urged migrants not to risk the extreme heat.

“The desert environment is extremely unforgiving, especially during the summer months,” Good said. “We urge anyone considering crossing illegally to understand the severe risks involved.”

Fire crews will be on high alert especially in Arizona, where fire restrictions went into effect before Memorial Day in some areas and will be ordered by Thursday across much of the western and south-central parts of the state, authorities said.

Fire forecasters at the Southwest Coordination Center in Albuquerque, New Mexico, said typically it does not get this hot until mid- or late June.

“It does seem like Mother Nature is turning up the heat on us a little sooner than usual,” said Tiffany Davila, a spokesperson for the Arizona Department of Forestry and Fire Management.

“We can’t back down from a fire just because it’s pushing 113 degrees outside. But we do keep a close eye on everybody in the field. Make sure they are keeping hydrated and taking more breaks than they normally would,” she said.

Monday highs reached 110 F (43.3 C) at Death Valley National Park in California near the Nevada line, 103 F (39.4 C) in Phoenix and 105 F (40.5 C) in Needles, California.

In Las Vegas, which topped out at 103 F (39.4 C) that day, the mercury was forecast to hit at least 108 F (42.2 C) on Wednesday and could then go even higher, according to the NWS.

“A new record high looks almost certain for Las Vegas on Thursday with an 80% chance of reaching 112 degrees (44.4 C). This would tie the earliest date for reaching 110 degrees (43.3 C) which previously occurred June 6, 2010,” the weather service said Tuesday.

Forecast highs for Thursday included 120 F (48.8 C) at Furnace Creek in Death Valley and 113 F (45 C) in Phoenix, the latter of which would break a record high for the date of 111 F (43.8 C), set in 2016.

Last summer the city saw a record 31 straight days of at least 110 degrees F (43.3 C), stretching from the last day of June through the entire month of July. At least 400 of the year’s 645 heat-related deaths were during that period.

Phoenix, Maricopa County and Arizona state officials are striving to better protect people from ever higher temperatures. Those most in danger from the heat are people outdoors, especially homeless people in downtown areas who often lack sufficient access to things like water, shade and air conditioning.

This year governments are setting aside more money to keep cooling stations open longer and on weekends, including two that will stay open overnight.

Mayor John Giles of the city of Mesa, just east of Phoenix, said officials are “committed to ensuring that those most vulnerable to heat exposure have access to essential life-saving services, including hydration and cooling stations and daytime respite centers.”

(AP)