The severe thunderstorms that moved across the Northland early Thursday were spurred by a huge well of warm, moist air in North Dakota with surface temperatures near 100 degrees and dewpoints near 80.
Dan Miller, science and operations manager for the National Weather Service in Duluth, said the storms started in the afternoon and continued into the overnight hours.
“The hotter and more moist the air is near the surface, the more unstable the air becomes. That’s what triggers the thunderstorms,” Miller said.
The Duluth airport reported an official wind gust of 69 mph, with a ship on Lake Superior reporting a 72 mph gust. Miller said that based on the damage he saw, winds probably gusted as high as 90 mph. A weather station in downtown Duluth reportedly measured a gust of 100 mph.
The wind snapped off some trees mid-trunk, while others were completely uprooted. Miller said many trees likely uprooted because soils were saturated - most of the Northland received several inches of rain last week - with the wind essentially just pushing the tree over, and many trees were top-heavy with foliage that became watered down quickly.
Miller said there were two primary corridors of storm damage - one along the Canadian border and another along the U.S. Highway 2 corridor between Bemidji and Duluth, from Cass and Itasca across Aitkin, Carlton and southern St. Louis counties and into Wisconsin's Douglas, Bayfield and Ashland counties.
The northern storm started in North Dakota and moved across Koochiching, northern St. Louis, Lake and Cook counties.
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