(CNN) Nearly 60 million people across the nation's midsection are under some type of heat watch, warning or advisory from the National Weather Service as the worst heat wave of 2018 arrives.
That number will only increase this weekend as the axis of extreme temperatures shifts toward the big cities of the Northeast.
Over 120 million people in the United States will see the temperature climb above 95 degrees Fahrenheit during the next week.
On Thursday, the extreme heat can be found over the Rockies and Central Plains, with Denver expecting a record high of 103 degrees, the city's hottest day since it tied the all-time Denver maximum of 105 in late June 2012.
The Midwest will see its hottest days on Friday and Saturday, when the combination of temperatures in the upper 90s and high humidity will create a heat index of up to 115 degrees for places like Chicago, Kansas City and Minneapolis.
Excessive-heat watches and warnings are posted for these locations, where "high temperatures and high humidity will combine to create a dangerous situation in which heat illnesses are possible," according to the National Weather Service.
Residents are warned to "drink plenty of fluids, stay in an air-conditioned room, and stay out of the sun," the agency says.
Unfortunately, not everyone in these northern climates has the luxury of central air. According to the US Energy Information Administration, about 2 million household units across the Midwest do not have air-conditioning equipment.
Overnight low temperatures are expected to dip only into the upper 70s, which does not allow buildings (and their occupants) much of a chance to cool off. It is the lack of overnight cooling that has proved deadly in past Midwest heat waves.
Fortunately, a round of thunderstorms will mark an end to the extreme temperatures for the Midwest on Sunday, limiting the duration of this round of heat.
For the Northeast, heat will build into the weekend, with temperatures well into the 90s into next week.
New York City's expected high of at least 94˚ F on Sunday would be the hottest day of the year there. The city could top 90 for five consecutive days through Monday; the last stretch longer than that was a seven-day streak in July 2013.
Though the temperatures over the Northeast will not reach the extreme highs of the Midwest and Central Plains, the duration of the heat will be significantly longer.
The major metro areas of the Northeast look to remain above average temperatures for late June-early July until after the July 4 holiday, with very few chances for rain as high pressure remains the dominant weather feature.