The first extreme heat wave of the year is set to sizzle in the Southwest next week, with temperatures topping 120 degrees in spots. Excessive heat watches and warnings already blanket portions of Arizona, California and Nevada.

The heat wave, predicted to last many days, will only intensify record-setting drought conditions plaguing many locations in the West.

Temperatures at least 20 degrees above average are possible for several days next week, and could persist even longer. Conditions this hot are “rare, dangerous and deadly,” wrote the National Weather Service in Phoenix.

The exceptional temperatures will bolster wildfire risk and exacerbate the potential for a long and significant fire season in the West as the heat saps more moisture from the ground.

Wildfire season is a natural part of many ecosystems, but climate change has these fires burning hotter and longer throughout the year. (John Farrell/The Washington Post)

Instigating the blazing temperatures is a ridge of high pressure currently centered over West Texas and Chihuahua, Mexico. The “heat dome” will become established over the southwestern United States, bringing sinking air and high temperatures. That sinking air will erode cloud cover, allowing more sunshine to pour in and heat the ground further.

The setup will also divert the jet stream well to the north, steering any inclement weather into the northern Intermountain West and Canada as much of the West bakes.

Temperatures are expected to climb by a few degrees per day through early next week, when triple-digit numbers will become widespread. In Phoenix, where the average temperature in mid-June is about 106 degrees, the week will end near normal — before Saturday climbs to 110 degrees, Sunday to 113 and Monday to 115 degrees.

By the middle of next week, the nation’s hottest large city could see multiple days at or above 116 degrees. That means Monday could tie a record, and Tuesday and Wednesday may break records if highs top 115 degrees.

Equally problematic will be the overnight lows, which by the middle of next week may not dip much below 90. That poses a grave danger to anyone without air conditioning, particularly the homeless and other vulnerable groups, whose bodies won’t be able to cool off at night.

“[An] increase in heat related illnesses, including heat cramps, heat exhaustion, and heat stroke,” is likely, the Weather Service in Phoenix wrote in an excessive-heat warning issued for the city. “Heat stroke can lead to death.”

The Salvation Army has announced plans to open cooling shelters throughout the Valley of the Sun, and other local agencies are likely to follow suit.

By Monday, Las Vegas could be flirting with 110 degrees, with highs topping 105 degrees possible in Albuquerque. Salt Lake City may nick 100, while Denver and Casper, Wyo., climb into the mid-90s. Even higher temperatures are expected Tuesday, with a high of 101 predicted in Salt Lake City. Even Billings, Mont., could hit the century mark.

Las Vegas could make it to 115 degrees next Wednesday, which would beat the current record of 114 degrees set in 1940.

Computer model guidance suggests the core of the heat should park over the Four Corners region by the middle of next week and actually strengthen, lending confidence to the extended duration of blisteringly hot weather predicted. There is a chance that Salt Lake City could log five consecutive days with highs in the triple digits, which would be a first in the month of June.

Death Valley could see highs approach 123 degrees.

The Weather Service has declared a high risk of excessive heat persisting into late next week for almost the entire Southwest, including interior California, the southern two-thirds of Nevada, western New Mexico and Colorado, all of Arizona and much of Utah.

The heat is amplified by anomalously dry conditions in the West, which allow for temperatures to climb higher. That in turn reinforces the existing drought by evaporating more moisture from the ground, a vicious self-perpetuating cycle that continues to wreak havoc on the West.

As of Thursday, drought covers more than 88 percent of the West according to the federal government’s U.S. Drought Monitor. In California, the drought is classified as either extreme or exceptional in 85 percent of the state, the largest area on record.

“Abnormal dryness and drought are currently affecting over 143 million people — about 45.9% of the population,” tweeted the National Centers for Environmental Information at the National Oceanic and Atmospheric Administration.

Meteorologists have been highlighting one particularly important number on weather maps that captures the true magnitude of the forthcoming heat event — 600. That corresponds to 600 dekameters, or 19,685 feet, which is the height that weather models simulate the lower atmosphere’s “halfway mark” will bubble up to.

Warm air expands the atmosphere vertically, while cooling air causes it to shrink downward. The same premise is what causes balloons to expand if you heat the air inside them. It’s extremely rare for halfway heights to reach 600 dekameters, highlighting just how incredibly warm next week’s air will be.

The potential for record-setting heat fits into a trend toward higher and higher temperatures in the Southwest, fueled by human-induced climate change.

Jason Samenow contributed to this report.