It's what those tired of winter weather have been waiting for.
"It's almost like we've come back to life. The Chicago winter just drags on and on, and it's just so nice to get a spring-summer tease like this," Steve Pavilanis said.
Steve and Beth Pavilanis, among plenty of others in the Chicago area, have been trying to shake cabin fever, and Sunday's weather conditions were the perfect cure.
Unseasonable warm weather had both Lake Michigan and the lakefront crowded with people looking to enjoy temperatures in the high 80s.
"I think it's a great day. I'm actually from Detroit, Michigan, and I am visiting a friend for her bachelorette weekend, and we are enjoying this gorgeous day that we have here in Chicago," Rachel Orlawski said.
"Winter months drag on. It's really cold, it makes me want to hibernate, but the minute it's nice, I go outside," said Columbia College student Shay Turner.
But weather forecasters say that our day of summer weather conditions are likely to change as a cold front moves in bringing the possibility of severe thunderstorms.
"When you have these very strong winds forcing the atmosphere, combined with what we call unstable air -- the warm, humid air mass -- the atmosphere can become very, very volatile," said Mike Bardou, a meteorologist National Weather Service.
But Sunday's Shamrock Shuffle, which is the unofficial start to the running season, wasn't affected.
"This is my first time outside. I have been on the treadmill all winter long. I am very excited. I cannot wait to go back home and continue my running outside," said Darcy Soltys, who participated in Sunday's Shamrock Shuffle.
Officials from the city's Office of Emergency Management and Communication are warning Chicago residents to be prepared for the possibility of bad weather.
"Contrary to popular belief, tornados can strike major cities," said Jose Santiago OEMC executive director.
Authorities will continue to monitor the area's weather conditions as most prepare for a return to reality when temperatures drop drastically back into the 50s.
But a one-day tease is just fine for all those dining outside on Rush Street Sunday night.
"Oh my god, it feels like heaven. Are you kidding me? This is the best," said Contessa Helena.