2 twisters possibly merged over S. Carolina
Monday morning’s tornado outbreak across the Carolinas was one to be remembered. At least 16 tornadoes had been confirmed across South Carolina alone, a number that could grow as National Weather Service survey teams continue to inspect damage.
It was part of the same tornado outbreak that left 34 dead and spawned at least 105 tornadoes from the South to the Mid-Atlantic Sunday and Monday.
Four of the tornadoes in South Carolina reached EF3 status on the 0 to 5 scale for twister intensity, with winds topping 136 mph.
But two of the EF3 tornadoes did something nobody was expecting and few have observed before — combined, with one strong twister essentially getting absorbed by the other’s circulation. It’s an unusual event that’s left many experts scratching their heads.
Many of the tornadoes, especially across the Midlands and Piedmont, struck in the dead of night. But what Doppler radar illuminated was staggering.
“It looks like (this outbreak) is going to have the most significant tornadoes for us since 1984,” explained Frank Alsheimer, science and operations officer at the National Weather Service office in Columbia, S.C. “Looking at the … setup, the (parameters) were the top of tornado situations around here. We were ramped up and expecting this.”
At one point, four tornado debris signatures, indicating tornado damage lifted into the air, lined up along a 20-mile stretch of storms, illustrating four simultaneous twisters. Perhaps the most remarkable were the northern two bright areas of debris, known as debris ball signatures, which danced around each other before combining — hinting at a potential merger of two strong tornadoes. But there’s more to it than that.
“We’re not used to seeing things along those lines,” said Alsheimer, who couldn’t remember an episode like this in his area before. He says that, while it’s impossible to confirm whether the tornadoes’ circulations merged without a detailed computer modeling study, there’s evidence to support that the twin funnels got extremely close to each other before some sort of interaction occurred.
There are two distinct explanations about what may have happened. While the damage track from the first tornado doesn’t lead continuously straight into the second, there may still have been a merger of their areas of circulation. That could have strengthened the second tornado and may explain the sharp curve in its path as well.
The first tornado apparently dwindled some in intensity as its vorticity — or spin energy — was affected by the competition nearby. This may have allowed the other twister to usurp its vorticity, strengthening the parent circulation of tornado 2 and intensifying the surface circulation as well.
“It certainly appears there was ingesting of vorticity into that larger one,” said Alsheimer.
What may also have happened could be attributed partly to cool, descending air — a downdraft — within the storm, specifically north of the first tornado. This could have choked out the first tornado and given the other nearby twister a bit more of an eastward shove. This is possible, but less likely.
What happened Sunday night in South Carolina may be an event that stands on its own.
Only a few instances of merging tornadic circulations have occurred before — mostly with rotating supercell thunderstorms as a result of a tornado family process. That was the case on March 13, 1990, when the remnants of a former F5 tornado were drawn into a new, strengthening tornado near Hesston, Kan. The new tornado produced F5 damage to the northeast.
A stunning example of two powerful tornadoes merging occurred in Pilger, Neb., on June 16, 2014 — and was broadcast live on The Weather Channel.
But Monday morning’s episode in South Carolina wasn’t a tornado family, and wasn’t from a supercell but rather two different areas of spin embedded within a squall line. Therefore, it may be an event scientists study for years.