COLUMBIA — Tornadoes in December are more common than you might think.
The United States averages 1,200 tornadoes a year, most occurring in spring and summer.
Last month, the National Weather Service documented 44 tornadoes nationwide. That compares with 52 documented during December 2009, another 45 in December 2008 and 19 in December 2007, according to the weather service.
The 18 tornadoes that hit Missouri on New Year's Eve surpassed the record for the most tornadoes on a single December day in the state. The old record was 15, on Dec. 2, 1982.
Winter thunderstorms that develop into tornadoes occur under the same weather conditions as thunderstorms year-round. Pressure is created in the atmosphere when cold and warm air fronts meet, causing severe weather.
The conditions on Dec. 31 were ideal for tornadoes to form, with a cold weather system moving northeast hitting the already warm, humid air in the atmosphere.
"It was a combination of features," said Scott Truett, senior forecaster with the National Weather Service. "It was a very strong weather system. Winter systems tend to be very strong."
In addition, the pattern was complicated by unstable air entering the system.
"When you have the strong weather system itself, and you have the unstable air that's fueling that system, you have the ingredients for the situation we had on New Year's Eve," Truett said.
That contributed to spin in the atmosphere, which pushed the thunderstorm into tornado formations.
The National Weather Service rated Friday's Fort Leonard Wood tornado a 3 (severe) on the 5-point Fujita Scale. Winds were estimated at 136-165 mph along a path about 4 miles long.
Another tornado that hit just north of Rolla was also rated 3 (severe), but it traveled for 11 miles. Other tornadoes in Lecoma, Christian and Webster counties, as well as near Table Rock Lake, were rated 0 (gale) or 1 (weak).
This system in its entirety was the only incidence of tornado activity in Missouri this December. The monthly record for tornadoes in Missouri during December was 28, set in 1982.
According to Pat Guinan, extension climatologist with MU's Commercial Agriculture Program, the state of Missouri averages two tornadoes each month from July to February.
"Tornadoes can occur, and will occur, any time of year," Guinan said, "You just have to have the right ingredients."