A “very unique event”: Rare anticyclonic tornado touched down in southeastern Tennessee Wednesday

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HUNTSVILLE, Ala. — The National Weather Service in Nashville confirmed Thursday that two tornadoes touched down in southeastern Tennessee on Wednesday, April 5, 2017.

Bedford County, Tennessee tornadic radar imagery from 3:55pm through 4:39pm CT on April 5, 2017. Click the animation for a larger image.

In Warren County, an EF-1 tornado packing estimated 90 to 95 mph winds touched down 8 miles south of McMinnville, Tennessee, and traveled a path of 3 miles.

The tornado injured one person in the Irving College community; it also heavily damaged a large barn, uprooted and snapped trees, and it destroyed a mobile home.

It touched down at approximately 4:59pm and lifted approximately 4 minutes later.

In southern Bedford County, the National Weather Service rated the tornado damage found southeast of Shelbyville as an EF-1 tornado with estimated peak winds of 95 mph. The tornado touched down approximately 5 miles south-southwest of Shelbyville, traveling approximately 5.4 miles while damaging trees, homes and barns along the way.

The NWS classified the Bedford County tornado as a “rare and unique anticyclonic landspout tornado [that] touched down in southern Bedford County off a left splitting supercell and an anticyclonic rear flank downdraft coupled with strong southwesterly surface flow.”

The NWS in Nashville consulted the opinion and expertise from weather forecast offices in both Norman, Oklahoma as well as Huntsville in order to determine the “characteristics of this very unique event“.

What is an anticyclonic tornado?

In the northern hemisphere (where we live) the vast majority of tornadoes are considered cyclonic, which means they spin in a counter-clockwise direction.

However, a tiny fraction of twisters actually rotate in the opposite direction: these are considered anticyclonic, because they spin in a clockwise fashion.

A few such twisters probably occur each year, but they account for only perhaps 1% of all U.S. tornadoes—and that’s a very rough estimate,” tornado climatology expert Harold Brooks of NOAA’s National Severe Storms Laboratory told wunderground.com’s Bob Henson.

Brooks explained further for wunderground.com:

“What appears to be the main driver is the wind shear that produces rotating supercell thunderstorms.

In the Northern Hemisphere, a blossoming supercell will often split in two, with one cell angling to the right of the mean upper-level wind, spinning cyclonically, and the other angling leftward and rotating anticyclonically.

Rightward-moving, cyclonically-spinning cells are the ones better positioned to ingest warm, moist air and grow more vigorously. Thus, most tornadoes are cyclonic, spawned by mesocyclones within cyclonically rotating supercells.

Toward the outer edge of a storm’s rear-flank gust front, there can be anticyclonically rotating features; very rarely, one of these will spin up an anticyclonic twister.”

What is a landspout?

The American Meteorological Society defines a landspout as a tornado that forms without a parent mesocyclone. In other words, the twister develops within the 1 to 3 miles of the atmosphere above the ground, but does not descend from a “preexisting midlevel mesocyclone.”