NEW YORK (WPIX) — A heat wave is expected to blanket much of the U.S. this week, trapping some in a “heat dome.” And yes, it’s sweltering.

The National Weather Service warned Sunday that record-breaking heat was forecast from the Midwest and Great Lakes to the Northeast through the week and into next week.

“The duration of this heat wave is notable and potentially the longest experienced in decades for some locations,” the NWS said in a tweet.

Hot and humid weather is in the forecast for many, even prompting air quality alerts in parts of Illinois, Michigan, New York, New Jersey and Connecticut. Heat index values in New York City alone could hit 100 degrees Tuesday and Wednesday, according to the National Weather Service.

On Monday alone, more than 70 million people were under extreme heat alerts.

These sweltering temperatures are part of a heat dome — but what exactly does that mean?

A heat dome is when high pressure prevents warm air from rising, trapping it as if it were a dome, according to the American Meteorological Society. It’s as if a lid was placed on a pot of boiling water on the stove — the warm air cannot leave, causing temperatures to skyrocket, making you feel like you’re in an oven.

That doesn’t necessarily mean that every time it’s extremely hot out, you’re in a heat dome. It’s more about the lingering, no-relief heat. It can last a few days or a few weeks, an NWS spokesperson previously told NPR.

This week, the dome is expected to sit over Illinois, Michigan and Ohio.

Parts of Iowa, Missouri, Wisconsin, Indiana, Pennsylvania, New York, Vermont, New Hampshire and Maine could see maximum daily HeatRisk (an experimental tool used by the NWS) levels reach “extreme.” That’s the highest level on the four-point scale, defined as a “level of rare and/or long-duration extreme heat with little to no overnight relief” that can affect “anyone without effective cooling and/or adequate hydration.”

By this weekend, the NWS says the “most intense heat will shift south from New England towards the Northeast urban corridor and Mid-Atlantic.”

Last year the U.S. had the most heat waves — abnormally hot weather lasting more than two days — since 1936.

Meanwhile, the National Hurricane Center has issued its first tropical storm watch of the season. A disturbance in the Gulf of Mexico could become a tropical storm by Wednesday, the NHC said Monday, prompting alerts in parts of Texas and Mexico.

The Associated Press contributed to this report.