Why So Many Illinois Tornadoes Were Spawned on a December Day

A swarm of tornadoes tore through parts of Illinois on Dec. 1, 2018, destroying several dozen homes and damaging hundreds more, particularly in the town of Taylorville, about 25 miles southeast of the state's capital, Springfield.

(NEWS: Tornadoes Leave Extensive Damage)

National Weather Service damage surveys confirmed 27 tornadoes tore through parts of central and southern Illinois.

This broke the record for the most tornadoes in any Illinois December outbreak, previously 21 tornadoes spawned in a destructive Dec. 18-19, 1957 outbreak, according to the NWS office in Lincoln, Illinois.

Unless you live in Florida, Southern California or the Caribbean, December brings thoughts of snow and cold, so let's explore how a tornado outbreak flared up in Illinois in this month.

The Ingredients

There is certainly a peak time of year for tornadoes in the U.S. – typically from April through June.

However, tornadoes can be spawned any time of year when the right conditions overlap. Over a 20-year period from 1997 through 2016, the U.S. averaged 31 December tornadoes per year.

(MORE: Your Average Tornado Risk By Month)

In Saturday's case, a bullish swirl of low pressure aloft was spinning over the Missouri Valley near St. Joseph, Missouri.

A strong jet stream rounded the base of that upper low from the Red River Valley of Texas, punching northeastward into the mid-Mississippi and Ohio Valley.

Closer to the surface, a complex frontal system was tapping milder and somewhat more humid air northward from the Deep South into central and southern Illinois.

But is it really warm enough in December for severe thunderstorms?

After all, high temperatures in central Illinois only topped out in the 60s – not exactly the hot, humid day you might think of before severe thunderstorms break out in, say, late spring or early summer.

What's important isn't the actual surface temperature or dew point, but rather the relative difference between the air near the ground and aloft, which meteorologists refer to as instability.

Warmer and more humid air near the ground topped by colder and drier air aloft is an environment with more instability to generate thunderstorms.

In this case, the previously mentioned upper-level low over the Missouri Valley provided enough colder, drier air aloft to overlap the milder and somewhat more humid air.

It wasn't summertime instability, but it was sufficient to generate thunderstorms.

"In fact, the relatively modest amount of moisture may have allowed some of Saturday's tornadoes to be more visible than in a more soggy setup, where rain often wraps around tornadoes and obscures them," said Weather Underground meteorologist and Category 6 blogger Bob Henson.

What's often key with tornado potential is wind shear in the lowest levels of the atmosphere. This low-level wind shear produces horizontal spin.

You can visualize this by placing a pencil between your hands, then sliding each hand in the opposite direction. Each hand is analogous to winds at a different height of the atmosphere. The pencil then rolls between your hands.

This spin can be tilted and stretched into a tornado by a strong thunderstorm.

In this case, east or southeast winds near the ground were topped by stronger south to southwest winds about 5,000 feet above the ground, providing ample low-level wind shear to generate tornadoes.

According to NWS-Lincoln, a supercell tracking from the far eastern St. Louis metro area to just east of Bloomington-Normal, Illinois, might have been responsible for multiple tornadoes, including Taylorville – a distance of roughly 140 miles.

December Tornadoes in Illinois Aren't Unusual

Tornadoes have struck Illinois in every month of the year.

According to the State Climatologist Office for Illinois, 57 December tornadoes have been documented in the state in the 67-year period from 1950 through 2016, an average of one tornado every one to two Decembers.

The last December tornadoes in the state occurred on Dec. 23, 2015, when six were recorded in far southern and western Illinois, according to data from the Tornado History Project.

Despite being one of the least tornadic months of the year, there have been 19 deaths and 327 injuries from December tornadoes in that same period, more than in either June or March.

The majority of these were from the state's most violent December tornado outbreak in modern times. On Dec. 18, 1957, a swarm of 25 tornadoes tore through parts of Illinois and Missouri, claiming 17 lives and injuring 273.

Two of the tornadoes, affecting Mt. Vernon, Gorham and Murphysboro, Illinois, were rated F4. The Gorham and Murphysboro tornado alone claimed 11 lives and injured 200, moving at a forward speed of 60 mph, which likely contributed to the death and injury toll.

Most of Gorham and much of Murphysboro had been destroyed just 32 years earlier by the 1925 Tri-State Tornado, the deadliest and longest-tracked tornado in U.S. history.

A separate Dec. 18, 1957 tornado in Perry County, Illinois, was rated F5, killing one and injuring six.

These remain the only F/EF4- or 5-rated December tornadoes of record in the state.

While the number of tornadoes from this December Illinois outbreak may be on par with the 1957 outbreak, given the three violent (F/EF4 or 5) tornadoes in 1957, and the tendency for more weaker tornadoes to be detected in recent times from better technology compared to earlier decades, known as tornado inflation, the 1957 event might be on a pedestal above the other December Illinois outbreaks as far as impact is concerned.

Other deadly December Illinois tornadoes occurred in 1950, 1951 and 1982.