This article originally appeared in the But Why? column of the January/February 2016 Iowa Outdoors magazine.
Because tornadoes usually require warm, humid air to form, it’s not surprising winter tornadoes are rare in Iowa. But they can happen, and occur more frequently in the southern states during winter versus the Upper Midwest.
One of the best known winter tornados in Iowa occurred Jan. 24, 1967. A large storm developed in the Rocky Mountains and by noon, it was moving over central Iowa. A warm front stretched across central Iowa, and temperatures in southeast Iowa warmed into the 60s; a few places even set record highs.
But it was a different story in northwest Iowa, where temperatures were in the teens and single digits—with a snow storm in progress. As the storm moved northeast, thunderstorms began to erupt in southern Iowa, moving east, producing a dozen tornadoes and killing one person in Lee County. The storm produced tornadoes into Wisconsin. The biggest tornado didn’t occur in Iowa, but in the suburbs of St. Louis, where an F4 tornado tore a 21-mile path through suburban neighborhoods, killing three people and injuring more than 200.
Winter tornadoes can be particularly dangerous, not because they’re stronger, but because they tend to move faster on the ground. The upper atmospheric winds that lead to winter tornadoes move faster than during warm months, and faster moving storms can mean less time to get out of the way.
According to the Weather Channel, the months with the highest average number of U.S. tornadoes are June (211), April (227) and May (270). Winter tornadoes are much rarer, with the nation averaging only 23 tornadoes in December, 39 in January and 96 in February—mainly in southern states.
