DULUTH, Minn. — An expert on Minnesota lakes is predicting that despite a slightly warmer-than-average winter in the northern part of the state, ice-out in the Northland this year will be later than average.
John Downing, director of the University of Minnesota's Sea Grant program that's headquartered at the University of Minnesota Duluth, has developed what he calls an "ice-out widget" that people can use to get an estimate of when ice-out will occur in their area.
Downing, a professor of biology, an expert in limnology and freshwater lakes and whose family has had a cabin on a northern Minnesota lake for 120 years, wanted to find out what the biggest factors were when the ice melts on lakes.
It turns out it's heat.
"That may seem obvious. ... But there are a lot of factors involved. How thick the ice is. Snow depth. Wind. The size of the lake. ... Rain can be a big factor; it really speeds it up," Downing said. "But we found that the most critical factor is thawing-degree days."
In other words, how warm it gets each day and how often it gets warm.
Thawing-degree days are figured on a complicated formula: Add the daily high temperature to the daily low temperature, divide by 2 and then subtract 32. For every positive number, those are thawing-degree days. So if the air temperature is an average of 10 degrees above freezing during a day, that accumulates 10 thawing-degree days. (You don't need to know that formula; the ice-out widget does it for you.)
The widget starts measuring any thawing-degree days in January. The number should pick up in February, speed up in March and rocket up in April.
But, so far this winter, that hasn't been happening.
"For those big, north-central Minnesota lakes, I use the data from the Hibbing airport. And they are at zero thawing-degree days," Downing noted earlier this week. "Normally, by this time in March, you should have several accumulated already — probably 10 by now."
And that's why Downing is predicting a later-than-normal ice-out for the Northland. How late, and whether the May 13 walleye opener might be impacted, depends on how fast those thawing-degree days start picking up. The one good thing this winter, Downing noted, is that ice thickness on many Minnesota lakes is generally less than usual, which should make the thaw happen quicker — once it starts.
Even in far-southern Minnesota, at Fairmont, there were only 10 thawing-degree days tallied as of early this week.
When thawing-degree days hit 220 for the weather station closest to your lake, that's when ice-out is likely to occur in the Northland.
"At that point, it becomes 50/50 that ice-out occurs," Dawning noted.
The faster you see that number approaching 220 in spring, the sooner ice-out is likely.
Downing admits he's a bit of an ice-out, phenology nerd, especially as climate change increases its impact. But he figures a lot of other Minnesotans may want to look ahead to when ice-out might occur in any given year.
Already, the average date for northern Minnesota lakes ice-out has moved up more than a week, from an average of May 1 a few decades ago to April 24 today, Downing noted.
"But what's really noticeable now is how much it swings back and forth now. It used to be fairly consistent, within a week or so. Now, we get big fluctuations, as much as seven weeks difference from year-to-year when ice-out will occur," Downing said. "It's now happening as early as March 25 and as late as May 15."
This new widget should let people know within about a week when ice-out will occur.
"But we are still working out the bugs," Downing noted. "It's not perfect."
"I know if I'm eager to know when I will be able to put my dock in, and put my boat in, that there are other people who have cabins or resorts who want to know when they can get in and when people are going to start coming north. ... I think it can be a valuable tool," Downing noted.
You can find the ice-out widget at z.umn.edu/widget
Look for the weather record station closest to your lake and follow the directions. Ice-out should occur when the thawing-degree days for that station reach 220, give or take a week.
