As if today’s cool temperatures and cold water weren’t enough, the National Weather Service is warning Chicago-area swimmers to stay out of Lake Michigan because of rip currents and dangerous waves.

The high today is only expected to reach the lower 70s, and while lake water temperature at Ohio Street Beach was 66 at 9 a.m., it was only 56 degrees at Osterman Beach and 57 at 63rd Street and Montrose, according to the weather service.

The weather service’s lakeshore hazard alert is in effect for all of southern Lake Michigan, including Illinois, Michigan and Wisconsin, through Thursday morning.

“High wave action and dangerous swimming conditions expected,” the weather service warned in its alert.

Waves could reach 11 feet at times, according to the weather service.

“Rip currents are powerful channels of water flowing quickly away from shore which occur most often at low spots or breaks in sandbars,” according to the weather service statement. “Structural currents form along piers where longshore currents and wave action flow into the structure. Rip currents and structural currents can sweep you into deeper water.”

The Chicago Park District’s beaches website didn’t say swimming is banned today except at Calumet Beach, which was expected to have a high bacteria count. But the Park District mentioned the hazard statement, and told would-be swimmers to check individual beaches’ flag color and “listen to lifeguard’s instructions.”

The district uses a three-color flag system to indicate whether swimming is allowed--green for no restrictions, yellow for caution and red for swim bans.

For updated information on beach swim bans, check the Park District beach site and Illinois Department of Health's beach site.

Daytime temperatures are expected to hover in 70s until Saturday, when highs are expected to reach the low 80s. For updated forecasts, check the Tribune's weather page

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