A stolen car that sank through ice on a Minneapolis lake Thursday vividly illustrated the fact that lake ice across much of southern Minnesota isn't safe for any kind of recreation.

Recent heavy snow last weekend wiped out thin skins of ice on some lakes, while the remaining blanket, along with warm conditions, will slow the thickening process for existing ice, said Kara Owens, boat and water safety specialist for the Minnesota DNR.

Lake Minnetonka remained open Friday. Lake Waconia froze over Monday after the snow. But it still wasn't thick enough to walk on Friday, said naturalist Jim Gilbert. Owens noted that at least 4 inches is needed for that.

Heavy snow on ice also pushes water up through cracks, resulting in a layer of slush beneath the snow.

"It kind of wrecks the lake for anything," Gilbert said.

Lower temperatures and less snow across northern Minnesota have produced much better lake ice. Owens said a DNR officer reported that 250 ice-fishing houses were out on Upper Red Lake on Friday, atop more than 12 inches of ice.