A relentless heat wave that has had a swath of the southwestern United States in its chokehold is stretching into the Midwest on its way to the Northeast this week.

The National Weather Service has issued heat alerts that are in effect on Tuesday for 45 million Americans across a dozen states, from Southern California to Montana, through the Great Plains and back down to South Florida.

The weather forecast for Tuesday shows temperatures will reach or exceed 110 degrees in cities like Palm Springs, California; Phoenix, Arizona; and Las Vegas, Nevada. Temperatures in the 100s are expected from Texas to Nebraska.

Heat index values -- a measure of how hot it really feels when relative humidity is combined with the air temperature -- are forecast to be in the 100s for even more places, including Kansas City, Missouri. It's the hottest time of the year for this area and these high temperatures aren't too out of the ordinary there.

Arizona's capital is on a record stretch of 25 consecutive days with temperatures at or above 110 degrees. Overnight temperatures in Phoenix have also not dropped below 90 degrees for at least 15 days.

Staff at Valleywise Health Medical Center in Phoenix told ABC News on Monday that the burn unit has been consistently full amid the brutal heat. They said about a third of the burn patients they are currently treating are people who have contact burns from falling and getting burned from the hot ground. The Arizona Burn Center is a standalone facility within Valleywise Health Medical Center and is the state's only nationally-verified burn center.

Doctors there told ABC News that a majority of the weather-related contact burns they are seeing are to homeless people who have been outside for a prolonged period of time or, in some cases, are on drugs or alcohol, which is extremely dangerous in scorching temperatures. Valleywise Health Medical Center has taken a portion of its emergency room -- previously used as an overflow unit for COVID-19 patients during the peak of the pandemic -- and converted it into an area to treat the most severe or near death cases of heat exhaustion. About a handful of people a day meet that criteria, doctors said.

Meanwhile, Tucson, Arizona, has been at or above 100 degrees for 39 days, tying its record set in 2013. The city is expected to break that record on Tuesday.

El Paso, Texas, has been on a record-smashing stretch of 39 straight days with temperatures at or above 100 degrees. This is expected to continue this week and may finally come to an end over the weekend. The city's previous record of 23 consecutive days was set in 1994.

Miami, Florida, has had a heat index high of 100 degrees for a record 44 days in a row, well past the previous record of 32 days set in 2020.

Marathon, Florida, hit 99 degrees on Monday, tying its record set earlier this month for the hottest temperature ever recorded in the city.

For many Americans, temperatures will only get hotter as the week goes on. By Thursday, heat index values are forecast to top 100 degrees in Kansas City, Missouri; reach 100 in Minneapolis, Minnesota; and be near 100 along the Interstate 95 travel corridor from Richmond, Virginia, to Philadelphia, Pennsylvania, and New York, New York. It could be the first heat wave of the summer in the Northeast with three days in the 90s, though it will be short-lived.

The weather forecast for next week shows temperatures will remain hot in the South and West, while the Midwest and Northeast get a reprieve from the extreme heat.

ABC News' Mola Lenghi and Alyssa Pone contributed to this report.