"Dangerous and deadly" heat grips Texas to California, threatens 40 million

An unusually intense, early season heat wave is gripping areas from Texas to the entire Southwest, including major metro areas such as Houston, Phoenix, Las Vegas and Sacramento.

Why it matters: Daytime high and overnight temperature records began to fall Thursday, including in Phoenix, and many will be tied or broken Friday through Sunday. The National Weather Service (NWS) is warning of a "high" potential for heat-related illnesses.

By the numbers: 113°F-116°F forecast high temperature expected in Phoenix on Saturday. The daily high-temperature record for that date is 114°F set in 1918.

• 112°F: Forecast high temperature in Phoenix Friday.
• 109°F: Forecast high Friday in Las Vegas.
• 100-106°F: Forecast highs in Sacramento on Friday and Saturday.
• 122°F: Forecast highs Friday and Saturday in Death Valley, Calif., which would be a daily record.
• 10°F-20°F or greater: Temperature departures from average across the Southwest through Sunday.

Threat level: When overnight temperatures remain elevated it increases the heat wave-related public health risk, particularly for those without cooling access in cities.

• This is because the human body needs time to rest and recover, and studies have shown that multiple days without recovery boosts the odds of heat-related illnesses and heat stroke, which can be deadly.

The big picture: The heat wave is tied to a sprawling area of high pressure, also known as a heat dome, that is enshrouding the Southwest in a bubble of sinking, and therefore warming, air.

• Like an atmospheric detour sign, the high pressure area is deflecting storminess around the region, maintaining sunny skies in its sphere of influence, with areas of poor air quality in place for days.

What's next: The heat dome, intensified by climate change, will shift east next week, with temperatures well into the 90s reaching Chicago and possibly as far north as Minneapolis. The heat may then move east and dissipate slightly as it reaches the East Coast, based on computer model projections.

• During the next week, the NWS is predicting that about 223 million Americans, or 70% of the population of the continental U.S., will see temperatures exceed 90°F.

Context: Dry conditions from the global warming-related megadrought in the Southwest are helping to send temperatures soaring. This is a positive feedback loop, because the heat in turn dries the region out even more.

• More frequent and severe heat 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.
• In some cases, such as with last year's deadly Pacific Northwest heat wave, studies have shown that extreme heat events are occurring today that would be impossible without human-caused global warming.