Calculated annually by NOAA, the severity index is based on bloom’s biomass – the amount of algae – over a sustained period. An index above 4 indicates a significant bloom. The largest blooms occurred in 2011, with a severity index of 10, and 2015, at 10.5.

The size of a bloom isn’t necessarily an indication of how toxic it is. For example, the toxins in a large bloom may not be as concentrated as in a smaller bloom.

The Lake Erie forecast is part of a NOAA ecological forecasting initiative that aims to deliver accurate, relevant, timely and reliable ecological forecasts directly to coastal resource managers and the public. In addition to the early season projections from NOAA and its partners, NOAA also issues HAB bulletins twice a week during the bloom season.