This article, first published Monday afternoon, was updated Tuesday afternoon.

A heat wave that will bring scorching temperatures from the Southeast to the Northern Rockies is kicking into high gear Tuesday. At least 30 million Americans will see temperatures hit 100 degrees or greater over the next several days as a sprawling heat dome parks itself over the middle of the nation.

Temperatures won’t be off the charts — in most places, they will range between 5 and 10 degrees above average. But the heat will combine with high humidity in some areas to yield heat index values well into the triple digits, spiking to hazardous levels in spots. Heat advisories stretch from Georgia and Alabama to central Minnesota, the Dakotas and parts of Montana. About 42 million people are under some sort of heat alert.

The potential for severe weather flanks the heat dome on its northern periphery, while along its southwest flank turbulent conditions have been both a blessing and a curse in drought-stricken states such as Utah.

As the heat dome developed in recent days, it was bordered by a cool pocket of air over the Southwest, which intensified monsoon rains. While the moisture was welcome in the drought-plagued region, the monsoon storms also triggered flash floods and blinding sandstorms. Over the next few days, the monsoon activity is predicted to settle down some.

However, through Thursday thunderstorms transiting the northern periphery of the heat dome could unleash damaging winds and hail across the Great Lakes, Ohio Valley and Northeast.

The heat

On Tuesday, the most exceptional heat will be concentrated over parts of the northern Intermountain West and Northern Tier, where highs will top 100 degrees.

Temperatures expected to climb to 106 degrees in Billings, Mont., and 103 in Bismarck, N.D. That’s just two degrees shy of Billings’s record; it will also beat out the daily high of 103 degrees set in 1947. It would be the hottest temperature ever recorded in Billings so late in the summer.

It will also be atypically hot in the Upper Midwest and the Corn Belt, including around Minneapolis and Des Moines.

“Extreme heat and humidity will significantly increase the potential for heat related illnesses,” warned the Weather Service in the Twin Cities, where forecast highs are in the mid-to-upper-90s through Wednesday. It also noted that overnight lows will struggle to fall much below the mid-70s, providing little nocturnal respite from the heat. That’s integral to the body’s ability to cool down.

The heat will expand well to the South and southeast through the Southern Plains and into the Southeast, with forecast highs also well into the 90s along with suffocating humidity levels. Dew points, a measure of how much moisture is present in the air, will surge to 70 degrees or higher over most of the eastern United States. That will bring a sultry, soupy feel to the air.

Omaha, Kansas City, Oklahoma City and Houston will all peak in the upper 90s on Tuesday; Dallas will nick 100. Midweek temperatures will remain similar, with a slight uptick in readings over the Pacific Northwest as a reinforcing shot of high pressure amplifies the heat dome.

The atmosphere’s halfway point will bulge upward more than 400 feet above typical, resulting from the hot temperatures expanding the atmosphere vertically. It’s the same premise behind a balloon growing in volume when heated or shrinking when chilled.

The heat will consolidate over the Deep South and the Southeast into the coming weekend.

Monsoon storms flank heat dome in Southwest

The desert Southwest, notorious for its blistering heat, has seen a reprieve from scorching temperatures in recent days but has been victim to an onslaught of stormy conditions associated with the seasonal monsoon.

The monsoon — driven by a wind shift that draws moisture from the Pacific over the desert — has been intensified by a pool of cold air at high altitudes over the region since late last week. Towering thunderstorms have dropped curtains of drenching rain punctuated by pinpoint bolts of lightning, leading to funnel clouds, flash flooding and washed-out roadways.

Social media video even captured a vehicle being swept downstream in raging torrents that an hour earlier had probably been a dry arroyo.

The downdrafts exhaled by thunderstorms over the desert Southwest have also kicked up sandstorms, or haboobs, that have brought abrupt drops in visibility. One photogenic haboob descended on the Grand Canyon on Friday.

The erratic thunderstorm outflow winds kicked up thick dust that crossed Interstate 15 in Utah on Sunday. Motorists encountered a sudden drop in visibility, and while some pulled over, others did not. That resulted in a more than 20-car pileup that claimed at least eight lives. Dry antecedent conditions stemming from long-standing drought in the West also were a contributing factor that allowed for dust to be lofted easily.

On Monday, monsoon rains brought severe flash flooding to Cedar City, Utah, 250 miles south of Salt Lake City, where numerous cars were swamped and basements inundated with water.

While highly disruptive in some areas, the monsoon rains have brought beneficial moisture.

Phoenix had received 1.84 inches of rain through Sunday because of the monsoon, which is more than all of its monsoon rain in 2019 and 2020 combined. The clouds and rain held high temperatures in the low 80s between Friday and Sunday, the first time it has been that cool over three consecutive days in July on record. Its average high at this time of year is 106 degrees.

Some of the moisture crept unusually far west, dropping 0.74 inches of rain on Death Valley, Calif., on Monday. That’s a figure greater than the combined July total to date in Portland, Ore., San Diego, Los Angeles, Honolulu, San Francisco, Spokane, Wash., and Denver, according to Brian Brettschneider, a climatologist in Alaska.

Los Angeles even received measurable rain Monday, which is unusual in July. The brief soaking, which deposited 0.12 inches, a record for July 26, was enough to push this month into third place among the wettest Julys on record.

While thunderstorm activity associated with the monsoon may ease some over the next several days, forecast models do show the possibility of it flaring up some later this week. Some moisture may even work north into parts of the Intermountain West, which have received little monsoon rain so far, and any rain would be welcome.

Nearly two-thirds of the West is under severe or exceptional drought, the two most significant categories highlighted by the U.S. Drought Monitor. Hardest hit thus far has been Utah, where government officials have taken to praying for rain; 70 percent of the state is experiencing an exceptional drought.

The predicted rainfall won’t cure the drought nor make a meaningful dent in the deficit, but it’s a small token of aid for the parched landscape. Any rain would also aid efforts to combat active wildfires.

Nearly 80 large wildfires are burning in a dozen states. The largest of the blazes, the Bootleg Fire, is still just over 50 percent contained as it continues to rage after having torched 400,000 acres in south-central Oregon.

The fires are sending copious amounts of smoke into the atmosphere, leading to sporadic air quality concerns. That smoke is also riding northward around the developing heat dome, surfing jet stream winds all the way to the East Coast.

That’s prompted air quality concerns in New England. Boston’s air quality plummeted into the “Code Red” category Monday for the first time since 2012.

‘Ring of fire’ storms possible in Great Lakes and Northeast

The northern periphery of the heat dome, sometimes referred to by meteorologists as a “ring of fire,” will also be a breeding ground for thunderstorms, some strong to severe. A weak, diffuse front will exist north of the heat dome, marking the edge of cooler temperatures banked to the north in Canada, along which storms may erupt. The jet stream riding along this transition zone will energize storms, which may produce damaging wind gusts and large hail.

The Weather Service’s Storm Prediction Center highlighted the zone from central Minnesota through New England as having an elevated risk of severe weather Tuesday, while the area to watch stretches from northern Minnesota into northern Virginia on Wednesday, with special attention focused on the Great Lakes.

By Thursday, widespread thunderstorms, some severe, could shift into the Northeast.

The Northeast is one part of the country that looks to remain untouched by the heat dome itself, with temperatures near or even a little below average this week.