Water Level Scenarios Summary:
This product is not an official forecast of Great Lakes water levels. Rather, this product is meant to illustrate outcomes that would occur under historical weather and water supply condition, with scenarios chosen based on similarities to recent conditions. For the official forecast, please see our Monthly Bulletin of Great Lakes Water Levels. This tool has been predominantly used to show the possible range of water levels in the upcoming year based on historical hydrologic conditions and water supplies. Beginning in 2021, we will be modifying our scenarios to account for different time periods based on the scenario that is chosen.
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 showcases the potential water levels if the water supplies that were received in ten of the most recent La Niña fall and winter seasons were experienced. These ten years are represented by the purple plume on the graphics. Two years are called out on the plot, 1975-76 and 2000-01, because these are the only other two times in recorded history that there have been three consecutive winters of La Niña conditions, which is also likely to happen in the winter of 2022-23. The green and blue lines represent the scenario years, 1975-76 and 2000-01, and indicate on the graphic the water levels that would occur if the NBS that was received in those two years were to occur. Please note that the year 2000-01 is included in the purple plume, but 1975-76 is not because it was not part of the most recent 10 years of La Niña conditions in the fall and winter. The most recent coordinated 6-month forecast is also shown for comparison.
Click on the individual graphics to view a larger, printer-friendly image (pdf).
*At this time, water level outlooks for Lake Ontario are still under development due to complexities of its weekly regulation process.
For the official 6-month forecast of all lakes, including Lake Ontario, see the Monthly Bulletin of Great Lakes Water Levels.