Monthly projections of Great Lakes water levels are jointly produced by the U.S. Army Corps of Engineers Detroit District, and Environment and Climate Change Canada over a six-month forecast horizon.

The Great Lakes generally follow a seasonal cycle where during the fall and early winter, water levels decline due to an increase in evaporation as temperatures decline and cold air moves over relatively warm surface waters. In the spring and early summer, water levels typically rise due to increased precipitation and enhanced runoff from snowmelt. The combined effect of precipitation over the lake, evaporation from the lake, and runoff to the lake is referred to as "water supply" or "net basin supply."

Water levels follow a seasonal cycle where during the fall and early winter, the lakes generally decline due to an increase in evaporation as temperatures decline and cold air moves over the relatively warm lake waters. In the spring and early summer, water levels typically rise due to increased precipitation and enhanced runoff from snowmelt. We refer to the combined effect of precipitation over the lake, evaporation from the lake, and runoff to the lake as Net Basin Supply (NBS). This edition of the Water Level Future Scenarios incorporates the projection of water levels if NBS values are those like years where low ice cover (< 25%) occurred and warm water temperatures. Eight NBS sequences are incorporated into the analysis represented by the purple plume. The eight NBS sequences incorporated into the purple plume are based on years with max annual ice coverage not exceeding 25%. Additionally, three years from the purple plume have been called out to highlight conditions that could occur over the next 12 months. These years span from July through the following June and include 2006-07, 2012-13, and 2017-18 NBS sequences. These years are shown in the colors pink, blue, and green, respectively. The gray shaded region of the plot represents the full range of possible outcomes using historical NBS sequences from 1900 to 2022. The most recent coordinated six-month forecast is also shown for comparison.