In the central waters a long fetch of strong easterlies or northeasterlies can generate high seas along the Michigan shore, which run athwart the N-S traffic through the lake. Southerlies can be dangerous particularly near the converging N shore. If the fetch and duration are sufficient, waves of 10 feet or more can be generated in open waters by winds from any direction once they reach 20 knots or more. This occurs most often during October, November, and December, when waves of 10 feet or more can be expected 2 to 4 percent of the time in the NW and S parts of the lake and 4 to 7 percent in the wide central portion. Extreme waves of 20 to 22 feet have been encountered throughout the lake.
Dense fog plagues the mariner most often in spring and early summer over the open lake waters.
From April into July visibilities drop below 0.5 mile upto 11 percent of the time. May and June are the worst times, and the cold, central waters are the most likely place. These fogs are usually the result of warm air moving across the still cold lake. Fog is most prevalent and thickest during the morning hours. Rain, blowing snow and low clouds also reduce visibilities, particularly from late fall through early spring.