Heat wave threatens nearly 40 million across Southwest

Forty-two million people are under heat watches and warnings in the Southwest into the weekend as a heat dome strengthens over the region, with temperatures soaring to hazardous levels from Texas to Arizona and northwestward into California's Central Valley.

Why it matters: Extreme heat is the leading weather-related killer in the U.S., and the first such event of the season can be particularly dangerous since people are not yet accustomed to such high temperatures.

Threat level: Temperatures are forecast to climb into the 110s Fahrenheit in Phoenix through Sunday, with the National Weather Service warning of “very high heat risk” and cautioning the heat will be excessive “even by local standards.”

• Daily temperature records will be threatened in Las Vegas through the weekend.
• In Death Valley, California, the hottest place in the U.S., temperatures could reach or exceed 120°F for the first time this year, tying or breaking daily records there.
• The heat extends into California where triple-digit temperatures are forecast in Sacramento for the next three days. Outdoor labor will be especially dangerous, the NWS warns.
• Overnight low temperatures are forecast to remain elevated in Central and Southern California, likely breaking records Thursday and Friday nights.
• Overall in the U.S., overnight low temperatures have been increasing faster than daytime highs, and when the overnight temperature remains particularly warm it can be hard for the human body to cool down and get relief. This increases the risk of heat-related illnesses, which can be fatal.
• Urban residents and those without air conditioning are at the greatest risk.

Zoom in: In Phoenix, the heat is forecast to peak on Saturday, with a high of 112-116°F. The daily high-temperature record for that date is 114°F set in 1918.

• In Las Vegas, the high temperatures this weekend may reach 109°F, which would tie the daily record on Saturday. The heat is likely to continue there through at least Sunday. Overnight lows are also forecast to be in the mid-80s, close to record levels for this time of year.
• In Sacramento, three days of triple-digit heat are likely including the Thursday through Saturday period, along with unusually warm nighttime temperatures.

What's next: Beyond Sunday, the high-pressure area or "heat dome" associated with this event is forecast to meander eastward into Texas, where sultry 100-degree temperatures have been occurring in Houston, and there are signs that next week could feature a significant heat event in the South, Southeast and potentially the Mid-Atlantic.

Meanwhile... in areas along the periphery of the heat dome, complexes of severe thunderstorms are likely to develop each afternoon into the overnight, with storms barreling across several states with high winds, hail and torrential rainfall.

• This type of weather pattern is known to produce particularly widespread areas of severe weather and is referred to as a ring of fire since the storms erupt on the dividing line between the high heat and areas of relatively cooler conditions.

Context: Dry soils from the global warming-related megadrought in the region will help send temperatures soaring.

• More frequent and severe waves are one of the clearest manifestations of human-caused global warming, studies show.
• Climate change is causing heat waves to be more frequent, intense and longer-lasting.