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A trio of tornadoes tore up Sioux Falls, S.D.

The twisters were all rated EF-2 on the Enhanced Fujita scale.

A tornado left severe damage to homes, structures and trees in Sioux Falls, S.D., on Sept. 11. (Video: City of Sioux Falls via Storyful)

Tornado triplets touched down in Sioux Falls, S.D., Tuesday night. The three twisters, all rated EF-2 on the Enhanced Fujita scale, with winds about 130 mph, brought significant damage to the city of 175,000 people.

Drone footage in the video above offers an aerial glimpse of the damage to trees and homes.

A severe thunderstorm watch had been in effect since 6:25 p.m. Tuesday. Although a tornado was listed as possible, the primary focus was on the potential for damaging winds.

A “bow echo,” the storm’s leading edge arching forward where it is pushed outward by strong descending winds, entered Sioux Falls about 11:18 p.m. Several bursts of wind punctuated the line in various locations. It took on the form of a quasi-linear convective system, or QLCS. Occasionally, QLCSs can spin up tornadoes.

It’s easy to forecast the development of sporadic QLCS tornadoes, but issuing warnings for them is a completely different ballgame. They usually form in the time it takes the radar beam to sweep around once, appearing suddenly and dissipating just as quickly as they came.

Tuesday night was a case in point. The radar scan at 11:24 p.m. shows a broad area of inbound winds in blue, marking strong southerly winds flowing toward the radar. One or two pinkish-red pixels dot the map just west of the more intense blues. That marks a subtle, but important, change in wind direction over a very short distance, indicative of some rotation.

A tornado warning had been in effect since 11:04 p.m. as the storm’s circulations entered the southern Sioux Falls metro from the south. A downwind warning was issued at 11:27 p.m.

The 11:24 p.m. radar scan shows tightening circulation, but nothing conclusive that screams “tornado.”

Now let’s fast-forward to the next scan 1 minute and 26 seconds later. Can you spot the difference? That’s a full-fledged tornado. The warning came 95 seconds later at 11:27 p.m. By then, the tornado had lifted, another one had developed, before it, too, had lifted. Both were on the ground for under a minute and a half, traveling less than one mile.

A third funnel touched down at 11:28 p.m. near 41st Street in Sioux Falls.

All three twisters were dissolved by 11:30 p.m. The entire barrage to Sioux Falls lasted five and a half minutes — less time than it would take to sing Billy Joel’s 1973 classic “Piano Man.”

“We also had significant straight-line wind damage” said Todd Heitkamp, the meteorologist in charge at the National Weather Service in Sioux Falls.

Forecasters dread QLCS environments. The systems are notoriously tough to predict and put meteorologists in the “darned if they do, darned if they don’t” situation. QLCS tornadoes often spin up from signatures that in one radar scan may be very weak. But most of those weak signatures do not produce tornadoes. Warning on every single one would produce exorbitant false-alarm rates.

Two of the three tornadoes that occurred did so within tornado warnings issued by the National Weather Service; the first came with 20 minutes warning, an exceptionally long lead time for a QLCS environment.

Fortunately, the weather in Sioux Falls in coming days will be much more tranquil. Temperatures will hang in the 70s Thursday and Friday before they warm in time for the weekend.

Editor’s note: This article has been corrected to reflect that the first tornado did occur with warning from the National Weather Service office in Sioux Falls; it was originally reported that the tornado had struck without warning.

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