CHICAGO (CBS) — Chicago’s 27 beaches open on Friday, kicking off the Memorial Day weekend and the unofficial start of summer, but you still might want to stay out of the water for a while.

Although the beaches were open, the water in Lake Michigan is still incredibly cold, and can be life-threatening if you do much more than dip your toes in. Chicago Park District officials said people should only swim when lifeguards are on duty — 11 a.m. to 7 p.m. — and only where lifeguards can see them.

According to the Great Lakes Surface Rescue Project and other sources, at least 13 people have drowned this year in Lake Michigan, including two people who died on May 16.

Tianna Hollinside, 13, drowned after she went into the water off Rogers Park Beach and didn’t resurface. Fire Department divers found Tianna, but only after she’d been underwater for at least 45 minutes. She was rushed to the hospital, but was pronounced dead a few hours later.

The same evening, 24-year-old Juan Cornelio drowned after he and another person went into the water Montrose Beach. The other swimmer was able to make it to the pier, but Cornelio never resurfaced, and was underwater for more than 30 minutes before divers pulled him out. He was later pronounced dead at Weiss Memorial Hospital.

No lifeguards were on duty when those victims went into the lake.

While lifeguards will be on duty 7 days a week through Labor Day, the water is only about 55 degrees in Lake Michigan, which is still very dangerous for swimming, and can quickly cause a person to go into shock.

“The water is still pretty chilly. That water’s only in the 50s. Now, the air is going to be chilly today, too, but if it gets warm out, pay attention to the water temperature, too,” said Chicago Park District director of cultural and natural resources Cathy Breitenbach.

According to the National Oceanic and Atmospheric Administration, hypothermia can occur with prolonged immersion in water below 70 degrees.

The city does not prohibit swimming based on water temperature, but does issue bans based rough surf on high bacteria levels.

This year, the city will be using a new and improved method of testing water for bacteria. Results will be available within a few hours, rather than a day, so advisories and warnings can be issued more quickly when necessary. Tests will be conducted daily at about 8 a.m., with results by the afternoon.

“It used to take us a full day to get test results back. With our new test that we’re doing in partnership with UIC, we’re able to get results in two to three hours,” Breitenbach said.

She said people can help keep the water cleaner by not feeding seagulls, and by using the trash cans at the beaches.

As of 7:45 a.m. Friday, the only beach with a swim ban in effect was Fargo Beach, due to hazardous surf and high water levels.

For a full list of Chicago’s beaches, click here.

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