Weather. Lake Superior is renowned for its cold temperatures, rough seas, fog, and sudden squalls. Boaters should monitor marine weather forecasts and be constantly alert to changing conditions. Average daytime high temperatures range from 60 degrees Fahrenheit in May, to the upper 70s in mid-summer, to the mid-60s in September. Average lows vary from 40 degrees in May, to the upper 50s in mid-summer, to 50 degrees in September. Average water temperatures in May and June are only in the 40s. Even in late summer, surface temperatures rarely exceed 60 degrees, except in protected bays. Average summer winds blow at from 5 to 20 knots with waves of from one to four feet. Winds of 30 to 40 knots and 6 to 12 foot seas are possible.
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