Juneteenth and Father's Day brought the start of a simmering heat wave, with temperatures expected near triple digits in the Twin Cities and much of the state.

Sunday's high in the Twin Cities hit the mid-90s with a dangerously high heat index — a measure of how hot the body feels outdoors when temperature and humidity are combined — of about 100 degrees, according to the National Weather Service's regional office in Chanhassen.

A high-pressure dome of hot air is expected to move across much of the state through Monday, bringing extreme heat and humidity. The highest air temperature was forecast at 104 degrees in west-central Minnesota. Tuesday will bring a little relief, with humidity decreasing and temperatures cooling slightly to the upper 80s or low 90s in the Twin Cities, but even then "we will be running above average," said Brent Hewett, a meteorologist with the Weather Service.

"It will be very hot across the whole state, other than the North Shore. That's really the only area where you'll see temperatures in the 70s and low 80s," Hewett said.

Extreme heat and humidity significantly increase the potential for heat-related illnesses, particularly for those working or being active outdoorss, authorities warn. In a typical year, more than 600 Americans die of complications from overheating, according to Hennepin Healthcare. Children, seniors, pets, and those who are ailing or overweight are the most vulnerable.

Christine Hill, a spokeswoman for HCMC in Minneapolis, said late Sunday afternoon that the hospital had treated a couple of patients with heat-related illnesses.

People must drink plenty of fluids, stay indoors when possible and be cognizant of the signs and symptoms of heat exhaustion and heat stroke, said Dr. Andrew Laudenbach, an emergency physician at HCMC. He urged people without access to air conditioning to find places to cool down and to check on relatives and the elderly, particularly those who live alone.

Extreme heat poses a lot of health risks, including organ failure, and "that can lead to your kidneys not functioning right, your liver not functioning right, your brain not functioning right, all of which can have serious and potentially permanent consequences," Laudenbach said.

Authorities have designated places around the Twin Cities as cooling centers, although some have entry fees. They include libraries, government buildings, movie theaters, shopping malls and recreation and community centers.

Despite the heat, people flocked outdoors Sunday to lakes and parks and outside coffee shops and restaurants.

A Juneteenth block party Sunday afternoon at George Floyd Square drew several people, including vendors and artists. Small business owners Tiffany Stevens and Tanesha Johnson each had a pop-up in an indoor space when traffic outside started to pick up. Without a tent to keep them cool, both women still took their business outside.

"It's a beautiful day. We can't complain," Stevens said. "We've got four months and it's going to be cold again."

But Nyqueela James and her fiance, Jerry Weldon, who had their first pop-up event decided indoors was the best option.

"I can feel the breeze coming through, but I can't do the sun," James said. "It's hot, it's sunny and I don't have a tent."

Johnson, selling charm jewelry, complimented Deangelo Christon for coming prepared. Christon, a native of Greenville, Miss., wore a big hat and sat under a tent next to a giant fan as he made beef hot tamales.

"This ain't nothing," he said, referring to Sunday's hot weather. "I'm a country boy, I know how to stay cool."

Artist Mary-Lydia Andersen came to her Stone Arch Bridge Festival booth prepared Sunday. Andersen is the owner of Bunny Bowls and brought a small fan, a cooler full of cold drinks, a jug of water and loads of sunscreen.

Having brought so much to protect against the sun and heat, Andersen said she was sharing with others who didn't bring enough.

After spending a few hours at the festival, Ben Michaels and Star Conarck sat under the shade of a tree. Both are fans of festivals and excited to see artists, attend more summer events and interact with others after more than two years of the pandemic.

"Now that things are going on again, I'm excited to see artists get this opportunity," Michaels said. "I'm excited for events to be able to happen. I'm excited to do things specifically outside."