When you think about tornadoes, you probably think about spring thunderstorms and eerie black skies pitted against fresh green grass. Indeed, many of the most devastating tornadoes that took Hoosier lives have happened in March, April and May. Spring is a period of unrest in the atmosphere most years.
However, when one studies tornadoes in Indiana, season is really irrelevant to when they can develop. A tornado can form at any time of the year if the proper atmospheric conditions exist.
How tornadoes formWINTER SURPRISE: Believe it or not, tornadoes can form in Indiana at any time of year, including during the winter. (Photo: deepspacedave/iStock/Thinkstock)
To generate a tornado, a thunderstorm must form first. Then within that system, changes in wind speed or direction with altitude — that is, wind shear — create a rotating updraft that develops into a tornado. This wind shear is most common when a cool, dry air mass passes over and on top of a warm, moist air mass.
For a tornado to form in winter in Indiana, the jet stream must be positioned to draw in warm air from the Gulf of Mexico and cold air from Canada to clash in the Midwest. Coincidentally, winter tornadoes are more common in the Gulf states and other Southeastern areas where warm air, moisture and storms are more abundant in the winter.
But just because winter tornadoes are more common in the Gulf states doesn’t rule them out from occurring in the Midwest, specifically in Indiana.
Indiana’s winter tornadoes
In Indiana, since 1950, there have been 76 tornadoes in the months of December, January and February. These tornadoes have ranged in strength from EF0 to EF4. An EF0 is a very weak tornado, but an EF4 is a very strong tornado.
These winter tornadoes since 1950 have combined for a total of three fatalities and 20 injuries in the state. All the fatalities occurred in one outbreak on Jan. 29, 2008, in two separate tornadoes.
In the Midwest, most winter tornadoes occur in bursts or widespread outbreaks; lone twisters are much less common. The most recent winter tornadoes in Indiana occurred during the Dec. 23, 2015, outbreak. Five tornadoes hit central and southern Indiana. The last time a major tornado, classified as an EF3 or higher, touched down in Indiana in winter was when an EF4 tornado injured five people in Knox County on Jan. 7, 1989.
Eggert is employed at the Indiana State Climate Office. He writes from West Lafayette. Facts and figures regarding tornadoes in Indiana were taken from tornadohistoryproject.com.