The tornadoes that ripped through multiple states, killing more than 100 people, were created by a very specific set of climate conditions, according to two Tennessee climatologists.
Kelsey Ellis, an associate professor of geography at the University of Tennessee, explained that December tornadoes are not unusual in the Southeast, and said these types of storms occur when cold, dense air interacts with warm air, creating unstable climate conditions. However, Ellis noted the fact that one of the powerful tornadoes traveled more than 230 miles is, as she put it, an "extreme anomaly."
"The time of year is not, the time of day is not, even though both of those can surprise people, that it's a cool season and that it's overnight," she said, "but the long track of this one is extremely rare, and it requires a very specific set of environmental conditions."
The previous record for a long-track tornado is from 1925, when an F-5 "Tri-State Tornado" traveled 219 miles through Missouri, Illinois and Indiana. Now, the so-called "Quad-State Tornado" is expected to break that record.
In the Department of Geosciences at Middle Tennessee State University, professor Alisa Haas pointed out that because tornadoes can occur any time of day, it's important to have multiple reliable methods for receiving warnings, since power and cellphone service often go down during severe weather. She added that increasing research on these events could help with preparedness.
"We don't know every single thing that goes into the formation of a tornado," she said, "and that can affect whether we have timely warnings or not."
So far, the National Weather Service has reported at least 38 tornadoes occurred simultaneously during the outbreak.