An rare November tornado killed 22 and injured over 200 as it ripped through mobile home park near Evansville, Indiana at 2 am Sunday morning. A preliminary damage survey of the tornado indicates that it was an F3 on the Fujita damage scale. Winds in an F3 tornado range from 158 mph to 206 mph. The tornado was 1500 feet wide, and had a 15-20 mile long damage path. The squall line of supercell thunderstorms that spawned the tornado was very low-topped, with echo tops of only 10-15 thousand feet. More typically, severe thunderstorm echo tops reach 40,000 feet. Forward motion of the line was exceptionally fast, estimated at 70 to 75 mph.
A separate strong F2 tornado touched down in central Kentucky in Munfordville. This tornado had a 200 yard wide path and traveled one mile through the center of Munfordville, causing significant damage to roofs in the downtown area. No injuries or deaths occurred with this tornado. A third tornado, also rated at F2 with 140 mph winds, touched down in Kentucky about 50 miles southwest of Evansville, Indiana. This tornado injured five as it cut a 150 yard wide, 11-mile long swath of damage. Figure 1.
Damage to a mobile home park near Evansville, Indiana. Image credit: Paducah, KY NWS
Tornado outbreaks in the Fall are uncommon compared to the Spring in the Midwestern United States. The most recent Fall tornado to affect the Midwest occurred when an F4 tornado struck Ohio near the Indiana border on November 10, 2003
, killing four and injuring 26. One might expect Fall to have a similar level of tornado activity compared to Spring, since strong cold air masses interact with warm, humid air masses in both seasons. However, there are important differences between the seasons. For example, the amount of sunlight is much lower in the Fall, when we are approaching the annual minima of sunlight on the December 21 Winter Solstice. With autumn's longer nights, morning temperatures are usually cooler than in Spring. This cooling causes more atmospheric inversions, where the temperature at the surface is cooler than it is aloft. This increases the stability of an air mass, and stable air is a strong deterrent to storm formation. Of the 25 most deadly tornadoes
to affect the U.S., only one has occurred in the Fall.Figure 2.
The annual cycle of tornado activity for southern Indiana shows strong peak in Spring, and just a slight peak late in the Fall. Image credit: NOAA Storm Prediction Center.