Tens of millions of Americans across the south and central U.S. – many of them in Texas – have endured a brutal heat wave over the past couple of weeks as temperatures soared to record levels, including some above 110 degrees.

Forecasters expect the intense heat to continue in Texas for much of this week in and expand north into the Plains and east into the Southeast. Meteorologist Scot Pilie warned on Twitter that more temperature records could topple, leading to dangerous heat index values.

In all, as of Monday, more than 45 million Americans live where some level of heat alert is in effect, according to the National Weather Service. How hot was it last week?

Several Texas cities set or tied all-time record-high temperatures: Del Rio (115 degrees), Laredo (115 degrees) and San Angelo (114 degrees). The heat index, which is how hot it feels to the human body when humidity is factored in, reached an unofficial record of 125 degrees in Corpus Christi.

This cauldron of misery is courtesy of a sprawling heat dome that has parked itself over portions of Texas and Mexico this month, sending temperatures skyrocketing. What is a heat dome?

A heat dome occurs when a persistent region of high pressure traps heat over an area, according to William Gallus, professor of atmospheric science at Iowa State University.

“The heat dome can stretch over several states and linger for days to weeks, leaving the people, crops and animals below to suffer through stagnant, hot air that can feel like an oven,” Gallus said in an article in The Conversation.

WFLA-TV chief meteorologist Jeff Berardelli tweeted last week that this heat dome, “likely enhanced by climate heating, is fueling a record heat dome so extreme that even experts are astonished!” He added that the heat dome is “basically impossible” without climate change. Texas heat dome to expand

Millions more will be sizzling this week as the heat dome expands north, according to AccuWeather forecasters.

Meteorologists said some states that have largely avoided record-high temperatures this month will begin to feel the extreme heat as the dome expands north and east.

“The heat that has gripped Texas for days on end will extend into [this] week as an intense core of high pressure high up in the atmosphere expands in coverage from northern Mexico into more of eastern Texas,” explained AccuWeather Meteorologist La Troy Thornton. Persistent heat wave conditions

AccuWeather forecasters predict that more locations across the central U.S. will suffer this week because of the weather pattern that brought the record-breaking heat to Texas last week.

Forecasters say that as the jet stream nudges slightly northward and eastward in the coming days, the heat will build into a portion of the Plains, Mississippi Valley and Southeast. Heat index chart calculates danger of humidity, heat

The National Weather Service warns that “extreme heat and humidity will significantly increase the potential for heat-related illnesses, particularly for those working or participating in outdoor activities.”

“Please stay safe in this ongoing dangerous heat wave!” weather service meteorologist Kwan Yin Kong said in an online forecast Friday.

This chart shows how hot it feels when the effects of humidity and high temperatures are combined: