What they can't predict is how toxic it might be.
The National Oceanic and Atmospheric Administration and research partners released their annual algae forecast for the lake, where blooms on the shallowest and warmest of the Great Lakes have flourished for more than a decade now.
The forecast comes a day after Ohio Gov. John Kasich announced plans to increase regulations on farmers to reduce the phosphorus from fertilizer runoff that feeds the algae.
If approved, the regulations would affect nearly 2 million acres and an estimated 7,000 farms, according to the state's agriculture department.
Researchers, using a scale for rating the severity of the bloom, expect it to be a 6 this year while the bloom last year was an 8 - matching the third-most severe bloom in 15 years.
Still, this year's bloom will be big enough to produce potentially harmful toxins that can foul drinking water or kill fish.
Scientists aren't able to forecast a bloom's toxicity and its size doesn't dictate how toxic it will be. The bloom in 2014 that caused a two-day shutdown of drinking water in Toledo was smaller in size than in several recent years.
"The blooms are a reminder that there is still work to do," said Crystal Davis of the Alliance for the Great Lakes.
Researchers say that the forecast of a smaller bloom this year doesn't mean enough progress is being made.
"Until the phosphorus inputs from agriculture are reduced significantly and consistently so only the mildest blooms occur, the people, ecosystem and economy of this region are being threatened," said Don Scavia, a University of Michigan scientist.
Patches of algae have been showing up since late June, a week to two earlier than normal because of higher temperatures this summer.
But warmer water temperatures this year won't mean there will be a more severe bloom because the key factor is the amount of phosphorus that's in the lake, said Rick Stumpf, of the National Oceanic and Atmospheric Administration.
Based on past years, researchers are starting to see that the blooms tend to peak in August before dropping a bit and then grow again in September, Stumpf said.
Where the bloom will go on the lake's western end along Ohio and Michigan will vary based on wind direction, but researchers point out that most of the lake will be fine for boaters and swimmers.
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