Late Tuesday night, on September 10th, 2019, three tornadoes touched down and lifted over the course of five and a half minutes. All three were listed as an EF-2 on the Enhanced Fujita scale, and each tornado lasted roughly a minute. However, a tornado warning was not issued until two of the three tornadoes had already touched down and lifted, causing significant damage to the city of Sioux Falls. This delay in warning was due to the fact that tornadoes produced quasi-linear convective systems, or QLCS, are considerably harder to forecast than those formed in supercell thunderstorms.
Figure 1: Doppler radar (left) and storm relative velocity (right) of a supercell producing a tornado, Raleigh NC, April 16th 2011. Source: USTornadoes.com
Unlike supercells (Figure 1), where the location and formation of these tornadoes can be more easily seen on radars with enough time to issue a warning, a QLCS can produce tornadoes with little to no warning. Some of these tornadoes can form in the time it takes the radar to complete a cycle the area, and disappear just as quickly. In Sioux Falls, they experienced just that. An initial scan with a storm relative velocity radar shows an area of developing circulation at 11:25pm (Figure 2, left), but was significant enough to send out a warning. One minute and 26 seconds later, the following scan at 11:26pm (Figure 2, right) indicated a tornado had already formed, and by the time the tornado warning was sent out another minute and 35 seconds later, a second tornado had touched down. Luckily, while the city suffered considerable property damage, no deaths or serious injuries have been reported due to this outbreak of tornadoes in Sioux Falls.
Figure 2: Storm Relative Velocity at 11:25pm (left) and 11:26pm (right), Sioux Falls, September 10th 2019. Green indicated movement toward the radar site, while red indicates movement away from the radar site. Source: NOAA/GR2 Analyst/Matthew Cappucci