Summer will make a dramatic entrance in the U.S. this week with a heat dome that will bring stifling temperatures and uncomfortable humidity to millions.

The heat will be particularly worrisome this weekend across wide stretches of Illinois, Iowa, Nebraska and Wisconsin.

This will be the first stretch of true summertime weather for many from Midwest to the East Coast, said Tom Kines, a meteorologist at the private weather company AccuWeather.

“A lot of those folks have been saying, where’s summer? Well, buckle up, because it’s coming,” said Kines. The humid conditions will make places that exceed 90 degrees feel as much as 20 degrees hotter, he said.

A heat dome occurs when a large area of high pressure in the upper atmosphere acts as a reservoir that traps heat and humidity, said Ricky Castro, a National Weather Service meteorologist in Illinois.

According to the NWS Weather Prediction Center, daytime temperatures will be in the 90s and overnight temperatures will only drop to the mid-70s from the Great Lakes to the East Coast during the heat wave that’s expected to last into next week.

Moisture blown northward from the Gulf of Mexico is fueling the muggy weather, said Jacob Asherman, a Weather Prediction Center meteorologist. This influx of moisture is fairly typical during late spring and summer, he said.

The heat will be widespread into next week.

On Friday, Denver could reach 100 degrees, according to the weather service. Chicago temperatures could reach 96 degrees Sunday. On Tuesday, Washington, D.C., could see a high of 99 degrees, and New York City could reach 96 degrees.

Several states in the Midwest could see dangerous temperature impacts over the weekend, according to a weather service measure that rates the risk from 0 to 4. Parts of Kansas and Nebraska will be in the highest category on the scale Saturday, meaning that anyone without effective cooling or sufficient hydration could face health risks. On Sunday, Illinois, Iowa, Missouri and Wisconsin also could see a Category 4 rating.

Humidity is the amount of water vapor in the air, and the heat index is what the temperature feels like when the humidity outside is factored in, according to the weather service.

When humans sweat, it cools the body because it absorbs and removes heat as it evaporates off the skin. The air is saturated with water on humid days, which makes it harder for sweat to evaporate. Hot and humid days can be dangerous when the body is unable to cool itself off and can exacerbate preexisting health conditions and even lead to heatstroke.

Minimizing sun exposure, wearing loose and light-colored clothing, staying hydrated and spending time in air-conditioned spaces are ways to cool down during extreme heat, according to the NWS.

Some parts of the U.S., such as Arizona, are hot without the mugginess. Phoenix and nearby desert regions experience so-called dry heat due to being far away from large water bodies, mountains that block moist air masses and weather patterns that bring scarce precipitation.