• A heat dome is descending on a swath of the nation and will slowly spread East.
• Temperatures will reach 100 degrees in some areas and feel even hotter.

Almost 100 million Americans were under National Weather Service extreme heat warnings, advisories or watches on July 23 as a heat dome descened on a swath of the nation and slowly spread East.

The high temperature in St. Louis on July 23 was forecast at 98 degrees − with a heat index that could reach a stunning 116, the weather service said. The forecast high in Little Rock for Wednesday and Thursday was 101 degrees, with heat index values nearing 110.

“Just absolutely ridiculous heat index values here of well over 100 degrees, and 110,” Weather Channel meteorologist Jim Cantore said. “We have the combination of high temperatures, 90s in most of these cases, even 100 as we showed you in Little Rock, with surging moisture, deep moisture and dew points in the 80s.

“That’s not air you can wear anymore, that is air that is going to give you a shower whether you like it or not.”

The Northeast was given a couple days reprieve, with highs in the 80s and relatively low humidity. But the dome will spread across the region Friday, driving high temperatures deep into the 90s that will feel like more than 100 degrees as the humidity builds there.

Heat dome is scorching the South, moving toward the East: What you can expect

What is an extreme heat advisory?

The weather service issues an Extreme Heat Warning when “extremely dangerous” heat conditions are expected or occurring. People in those areas are urged to avoid outdoor activities, especially during the heat of the day, and to check on family and neighbors. An Extreme Heat Watch is issued when conditions are favorable for an extreme heat event but its occurrence and timing are uncertain. People are warned to suspend all major outdoor activities if a warning is issued.

A Heat Advisory is issued for dangerous heat conditions that are not expected to reach warning criteria.

What is a heat dome?

Heat dome is not actually a scientific term, according to climatecheck.com, which says the term does effectively describe the “oppressive” high-pressure atmospheric systems that cause warm air to be pushed to the Earth’s surface and trapped there for long periods of time.

“The dome traps high-pressure air in one place, like the lid on a pot,” the website says. “These large zones of hot air result in a combination of blistering temperatures, devastating wildfires, and drought conditions.”