Deadly Kentucky Tornado Had Roots in Weeks of Unusually Mild Air
(Bloomberg) -- The violent weather system that tore through the central U.S. last week got its start in a confluence of moist air and mild temperatures that fueled “supercell” thunderstorms, touching off potentially record-setting tornadoes at a normally calm time of year.
The exact power of the storms, which killed at more than 75 people, has yet to be determined, but forecasters already know it was at least an EF-3 on the six-step Enhanced Fujita scale, used to measure tornadoes, said Matthew Elliott, warning coordination meteorologist with the U.S. Storm Prediction Center, the nation’s tornado tracker. While it isn’t unusual to have tornadoes pop up in this part of the country, aspects of this outbreak may set records.
The seeds of the outbreak were planted in weeks of mild weather, which has persisted without the typical parade of cold fronts moving through the central U.S. to bulldoze low-level moist air -- a building block for tornadoes -- into the Gulf of Mexico, Elliott said. Adding to the destruction was the was far-reaching path of the storms, which rambled through the region unimpeded by other counteracting weather systems.
“It is not unheard of to see tornadoes in this area: the unusual thing here is the length of the supercell,” Elliott said. “It is possible the supercell thunderstorm may be one of the longest super cell paths we have in recorded history.”
Large, supercell thunderstorms can spawn tornadoes, and the path of such a system Friday night suggests it might be a record breaker because it managed to survive unimpeded for hundreds of miles, Elliott said. Scientists are still sifting the evidence, as well as trying to determine the paths of the tornadoes, especially the one that leveled Mayfield, Kentucky.
Finding the fingerprints of climate change in individual storms can be difficult, and for tornadoes it can be more challenging because the twisters are short-lived and relatively small. Less than 10% of supercell thunderstorms produce tornadoes, for instance, Elliott said.
Fuel for Storms
What is known is that in the past 40 years the number of tornadoes in an area roughly from Paducah, Kentucky, to Jackson, Mississippi, has increased by 10%, and most of that has been in winter, said Harold Brooks, a senior researcher at the National Severe Storms Laboratory. Meteorologists mark Dec. 1 as the start of winter, while the calendar will catch up on Dec. 21 this year.
“Whether that is related to climate change is not 100% sure,” Brooks said. “But it is likely.”
In addition, the number of days when tornadoes form has dropped, while the days with multiple tornadoes has increased. So on days when twisters form, there are generally more of them.
“The Gulf is well above normal temperature-wise,” Brooks said. “We haven’t had any cold fronts get to the Gulf yet – that is how we cool off the water and that just hasn’t happened. As a result, that body of water is warmer than normal and that is basically fuel for thunderstorms.”
The cell held together from near Arkadelphia, Arkansas, to Cincinnati, which is about 650 miles. The killer tornado didn’t travel that far, but it is possible that its track too may be among one of the longest recorded.
The extended path contributed to its deadliness and its destruction, Brooks said. A long-lasting tornado in less populated areas of Kansas or Oklahoma can travel far without hitting anything, for example, but “you cannot do that in the east,” Brooks said. “When very strong tornadoes hit populated areas in the dark, the outcome of that can rarely be good.”
Elliott said it will take time for scientists and engineers to know exactly how long specific tornadoes survived across the ground and how powerful they were when they hit. Engineers will have to not only inspect the damage, but also determine how well a building was constructed to be able to say how much force was required to knock it down.
“It was an unusual event for any time of the year,” Brooks said. “December is usually the quietest month.”
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