The first heat wave of summer is expected to bring temperatures in the upper 90s that feel close to triple digits as Baltimore City hosts the AFRAM festival in Druid Hill Park this weekend.

Heat domes are broiling much of the Midwest and Northeast this week as almost 77 million people in the United States were under extreme heat alerts Tuesday, according to the National Weather Service.

“There is no real chance of rain to cool things down,” Kevin Witt, a meteorologist for the NWS Baltimore/Washington office, said Tuesday. “Saturday and Sunday will be the warmest for the Baltimore area over the next couple days. Humidity is going to increase and make it feel even hotter than it is.”

Witt said heat domes, like the one forming over Baltimore and elsewhere, are caused by high-pressure systems in the upper atmosphere that compress air closer to the surface and prevent clouds from forming.

Juneteenth in Baltimore on Wednesday is expected to be sunny with an afternoon high in the low 90s. Temperatures are expected to reach 94 for the summer solstice Thursday, and the NWS is calling for a high temperature of 97 on Friday.

The heat index — a measure indicating the level of discomfort the average person is thought to experience from the combined effects of temperature and humidity — is expected to hit 99 degrees Saturday and Sunday as temperatures in the upper 90s mix with relative humidity around 50%. There is also a 40% chance of rain and a chance of thunderstorms Sunday night.

The city issues code red extreme heat alerts and sets up emergency cooling centers when the heat index reaches or exceeds 105 degrees. Historically, the record high temperature for June 22 measured at BWI Thurgood Marshall is 100 degrees in 1988. For June 23 the record is 97 in 2010.

AFRAM, a free festival headlined by Busta Rhymes, is Saturday and Sunday from noon to 9 p.m. Last year, about 200,000 people attended what the city calls one of the largest African American festivals on the East Coast, featuring music, food, craft vendors and showcases of community organizations.

“The city is providing cooling buses, misting stations, and water and other drinks for attendees to stay cool and hydrated during AFRAM,” said Kevin Nash, spokesperson for the parks and recreation department.

The Baltimore City Office of Emergency Management is urging residents to avoid alcohol and keep drinking water during the heat.

Rapper Big Daddy Kane, funk band The Time, R&B singer Mýa, gospel vocalist Karen Clark Sheard, Grammy-nominated R&B star October London and singer Alex Isley are also scheduled to perform.

Climate change is making heat waves last longer at higher temperatures over larger areas, according to Associated Press, citing a study in the journal Science Advances that found the average duration of global heat waves increased from eight days between 1979 to 1983 to 12 days by 2016 to 2020. Last year, the U.S. saw the most heat waves — abnormally hot weather lasting more than two days — since 1936.

According to AP, the death certificates of more than 2,300 people who died in the United States last summer mention the effects of excessive heat, the highest number in 45 years of records.

Chicago broke a 1957 temperature record Monday with a high of 97 degrees. Hot and muggy conditions will continue this week with peak heat indexes near 100, the National Weather Service in Chicago said.

Extreme heat alerts stretched from Iowa to Ohio and even into the upper reaches of Michigan on Tuesday, canceling youth sports camps, nature walks and festivals across the region. The National Weather Service said the heat wave was expected to make its way across the country and into Maine until at least Friday.

In Toledo, Ohio, the city canceled a weekly fitness event and a neighboring suburb called off a street fair because of temperatures expected to reach the upper 90s. A food bank in upstate New York canceled deliveries for Wednesday out of concern for its staff and volunteers.

In New York, Gov. Kathy Hochul activated the National Guard to assist in any heat emergencies that develop over the next several days.

“This is a time of significant risk, and we’re doing our best to make sure that all lives are protected,” Hochul said Tuesday.

Schools in New York canceled field trips Tuesday to the Rosamond Gifford Zoo in Syracuse, where workers turned on water misters for visitors and the animals. Elephants and other animals were getting chunks of ice in their pools, said Ted Fox, the zoo’s executive director.

“Most of the species love them,” Fox said. “Even the tigers love to lick the ice and put their heads on them when it’s this warm.”

Baltimore Sun reporter Tony Roberts and AP contributed to this article.