Tornadoes in December: are they a sign of climate change?

In the last week, one of the deadliest tornado outbreaks in U.S. history has ripped through several mostly Midwestern states killing at least 90 people, sending record tumbling.

The tornado activity is virtually unprecedented, although the data is limited due to a lack of study of tornadoes which are by their very nature an elusive weather phenomenon.

The tornado that hit western Kentucky on Dec 19 was packed winds of up to 165 miles per hour. With a footprint of up to three-quarters of a mile wide, it tore through homes and businesses along a path of more than 200 miles. Officials in Kentucky say they were the most devastating in the state's history.

States including Illinois, Arkansas, Oklahoma and Tennessee were also hit by the powerful storms.

Heat wave fuels storms

Tornadoes don’t normally hit in December, a signal that climate change might be a factor, some scientists are saying. “A key driver of weather events is heat energy,” said John Englander, an oceanographer and global warming expert who is the author of a new book ‘Moving to Higher Ground.’

“Just like hurricanes, tornadoes are spun up by the atmosphere. These are all symptoms of a warming planet so it’s not surprising we are seeing more of them as we get more heat in the system,” he added, noting the warmer than usual weather across the United States.

Scientists point especially to the warmer than usual temperatures in the Gulf of Mexico, as well as a December heat wave over much of the contiguous U.S. leading several cities and states to set records for the warmest first month of winter on record.

The Houston area tied its hottest recorded temperatures seen for the month in the mid-to-upper 80s. According to the National Weather Service, some areas may see temperatures running 20°F to 40°F above average for this time of year.

While a warm December might be welcome to some, heat can trigger extreme weather events. The moist, warm air coming off the Gulf of Mexico becomes the heat energy, or fuel, that allows large thunderstorms to form. “The atmosphere works to bring equilibrium to unstable conditions and thatis what storms do (…) they distribute excess energy. When the unbalance is big so is the storm,” said Bob Bunting, founder of the Climate Adaptation Center in Sarasota, Florida.

When the warm air comes into with the Jet Stream, fast-moving bands of strong wind in the upper atmosphere, it can cause the dramatic spinning effect that makes tornadoes and hurricanes so destructive.
“When the Jet Stream dips both in latitude and in height above the ground in an unstable air mass, then the creates the vertical wind shear both directionally and speed. So there was big shear in directional change and in wind speed too,” said Bunting.

Lack of data

But scientists warn it may be too early to reach any form conclusions as reliable tornado data, unlike hurricanes, does not go back more than 20 years. So far, scientists say they have not detected a long-term observed trend in the frequency of the most violent tornadoes.

“Tornadoes are hard to observe,” said Jeff Masters, a meteorologist with Yale Climate Connections. “If it doesn’t run over a building we don’t know how strong it was and that makes it hard to see trends.”

Because tornadoes appear with little warning, move fast and can last for only a few minutes or seconds, that makes them much harder to study than hurricanes that move slowly and can hang around for days, if not weeks.

Tornado measurements began to improve after the 1996 blockbuster movie, Twister, awoke greater public interest in the deadly weather phenomenon. But data has not reliably shown – at least until this week – a greater intensification in the number of strong tornadoes, as is the case with the recent history of major (Category 3 and above) hurricanes.

The intensity rating for tornadoes also changed in from the F (Fujita) scale to the EF (Enhanded Fujita) scale in 2007, which added new data fields in order to align wind speeds more closely with storm damage.

Friday’s tornadoes were unusual in that they lasted for one to two hours and one is recorded to have traveled 140 miles. “It is possible that tornadoes are getting stronger. What we have observed is that tornadoes tend to be occurring in larger batches, bigger outbreaks,” said Masters.

Forecasters have also noticed that tornado activity has also moved several hundred east away from the traditional ‘Tornado Alley’ in Texas and Oklahoma, to the Mississippi Valley. Tornadoes are also now occurring all year-long, rather than just in the spring. “Winter tornadoes are becoming more common. That’s a climate change signal we can believe in,” said Masters.

Doppler radar advances

Reliable data on tornado history only goes back to around 2004, after the introduction of ground level Doppler radar in the 1990s which were able to not just detect but also track the motion of an echo.
This allowed forecasters to measure the wind inside of a storm, to distinguish between rain, snow or hail and to see and predict the formation of tornadoes.

But with only 158 of them around the country, hundreds of miles apart, that leaves gaps. The curvature of the earth also often makes it hard to see what’s going on at the surface.

Since recent introduction of mobile Doppler trucks have allowed closer up views, though only a handful are in operation. Since 2017, the ‘Doppler on Wheels’ fleet maintained by the Center for Severe Weather Research (CSWR) in Boulder, Colorado, has collected data in 200 tornadoes.

Weather satellites allows scientists even more visibility into the formation of tornadoes. The National Oceanic and Atmospheric istration operates geostationary satellites, but their high altitude (14 miles/22 kms) provides poor resolution.

Climate adaptation

One of the lessons from the latest tornado activity is the need to address building safety codes to include the necessity of a ‘Safe Room’ in offices and factories. The candle factory in Kentucky where eight people died received a tornado warning but did not have Safe Room where workers could take refuge. Production at the factory had been going around the clock to meet Christmastime candle demand.

That especially applies to the new areas being impacted in the Mississippi Valley where tornado safety measures are relatively new. “You don’t have as many Safe Rooms and basements in these regions,” said Masters. “One of the things we are learning with climate change is that our infrastructure was built for the 20 ^th century, not the 21 ^st century. We need to adapt to this new climate.”