Early heat wave could shatter records across California and the Southwest

It feels like summertime in March across much of the western United States this week, as an extraordinary, early heat wave sends temperatures skyrocketing in California and the Southwest.

Highs above 90 degrees Fahrenheit are expected to be widespread in Southern California, with parts of Arizona and Nevada reaching the triple digits. Some 39 million people in the region are under heat alerts Monday, and the heat wave is likely to expand and intensify as the week goes on.

The National Weather Service said the “anomalously early” heat wave is caused by an unusual pattern in which a dome of high pressure builds and settles over the West, baking the region in sweltering conditions.

“Daily record-tying/breaking highs will become more common across southern California, the Desert Southwest and into the Great Basin over the next couple of days,” the agency said Monday in its short-term forecast. “These temperatures will serve as a baseline for even more anomalous, more widespread record-breaking heat expected later this week.”

In Los Angeles, highs were forecast to be in the mid- to high-90s all week, with an expected peak Thursday of 98 degrees. Temperatures in Phoenix should climb into the triple digits by Wednesday, with Thursday expected to reach 105. Multiple cities in southern Nevada, meanwhile, may climb to 107 by later this week — blisteringly hot conditions for this early in the year.

“Temperatures will soar to 20+ degrees above normal making it feel more like early June than mid-March,” the Las Vegas office of the weather service wrote Sunday in a post on X.

Cities that could set new all-time monthly records in the coming days include San Francisco, Los Angeles, Sacramento and Palm Springs in California; Phoenix and Tucson in Arizona; Las Vegas; Salt Lake City; Albuquerque; and Colorado Springs and Denver in Colorado.

Heat domes occur when a strong and persistent region of high pressure sits over land, trapping hot air like a lid on a pot.

In addition to its unusual timing, the heat wave will be notable for its duration. The weather service said this week’s heat dome will likely be “one of the strongest ever seen during this time of year,” with little relief in sight as high temperatures persist into next weekend.

While it can be tricky to directly attribute individual extreme weather events to climate change, studies have shown that global warming is making heat waves more frequent, more intense and last longer.

The prolonged heat could be dangerous for high-risk groups, including children, older adults and people with pre-existing health conditions — particularly because most are not yet acclimated to summerlike conditions.

“Moderate to major heat risk is expected. Please check on family and friends without A/C and avoid outdoor recreation during the afternoon hours this week,” the San Diego office of the weather service wrote Sunday on X.

The March heat wave will exacerbate one of the worst snow droughts in decades in the West, depleting the region’s snowpack, which was already below average, and raising concerns about wildfire risk and water availability later in the year.

As the West bakes, a major winter storm is walloping the Upper Midwest and the upper Great Lakes regions, with heavy snow expected across Wisconsin and Michigan on Monday. More than 120 million people from the Mississippi Valley into New England are also under wind alerts, with destructive wind and tornadoes possible in some areas, including Pennsylvania, New Jersey and Washington, D.C.

Meanwhile, a powerful atmospheric river has been lashing Hawaii with relentless rain over the past five days, causing widespread flash flooding and landslides.

This article was originally published on NBCNews.com