Biggest, so late in the season: May 1-3, 2013 in Minnesota/Iowa
Blooming Prairie, MN and Owatonna, MN unofficially had 20" of snow. Osage, IA set the state snowfall record for May with 13". Chariton, in southern Iowa, had 12". Impressive for being so far south.
https://www.weather.gov/dmx/maysnow
https://www.weather.gov/arx/may0213
Most insane winter event I've ever had the pleasure of covering.
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My in laws live in the Austin area and lost their machine shed that day. That was just too much wet heavy snow at once and the entire thing collapsed. To make it even worse their tractor and planter and other equipment was parked in there. But in the end it didn't matter as almost half of Mower county was prevent plant that year so nothing got planted. My brother in laws were moving snow off and away from an old hog building nearby and saw the whole thing come down.
Here in Iowa that was a very trying year as well. Had some corn in the ground from the 1 day in April that was fit that than got snowed on and flooded and finally spiked almost 30 days after planting. I never want to see a year like that again.