NOAA's National Weather Service Weather Forecast Office

Forecast and Warning Definitions

The National Weather Service issues forecasts and warnings for the U.S. open waters (waters beyond five nautical miles from shore) and for the nearshore waters (waters within five nautical miles from shore) of the Great Lakes. The open waters forecast will cover the entire lake when the Nearshore is not being issued.

Lake Superior Forecasts for the open waters are issued four times daily throughout the year, at 4:00am, plus every six hours with intermediate updates as necessary. Nearshore Marine Forecasts are issued four times daily from late April to late October, at 5:00am plus every six hours with intermediate updates as necessary.

Forecast wave heights are for the average of the highest third of the waves at the end of the fetch in the forecast area. Wind directions and speeds are forecast for the 100-foot level above the lake surface in the open lake forecast and near the lake surface in the nearshore marine forecast to best reflect the conditions experienced by the primary vessels in these areas (e. g. recreational and charter crafts).

Marine warnings and advisories are headlined within marine forecasts:

• Gale Warnings for winds 34 to 47 knots inclusive
• Storm Warnings for winds of 48 knots or more
• Small Craft Advisories for winds 18 to 33 knots and/or average waves of four feet or higher for nearshore waters