Looking in wrong place when predicting tornadoes

The findings indicate that a tornado-strength vortex can be active on the ground for a minute or more before the deeper tornadic column forms and is picked up by conventional radar. However, tornado warnings are issued based on radar readings that pick up vortex signatures at or above cloud level.

“We need to strategize how we’re issuing warnings a little bit differently. The way we’re doing it, we’re never going to get an improvement on our warning system,” Houser said.

The challenge is in getting those kinds of readings quickly. Conventional radar can’t get ground-level readings over a broad area because objects in the way, such as hills, buildings and trees, disrupt the data, and they collect data slowly. Houser’s data came from the University of Oklahoma’s Rapid-scan, X-band, polarimetric mobile radar (RaXPol). Radar observations of the wind field were used to track tornadic signatures in the core of the storm. In the 2013 El Reno tornado, observations showed evidence of a tornado that was only visible in radar data less than 20 meters above the ground, but nowhere else, at the beginning. The next elevation angle above showed no tornado-strength rotation.

In all four cases, none of the tornadoes formed from the top down. Three formed from the bottom up, and one contracted almost simultaneously over the depth of the column being observed.

 The tornadoes formed in 30 to 90 seconds, making the prospect of forecasting tornadoes with 100 percent accuracy seem bleak.

In the largest tornado the team studied, the deadly 2013 El Reno tornado, video documentation showed that a funnel cloud was visible for one minute and 40 seconds before the development of a radar-derived, vertically-continuous vortex. That storm ultimately had a width of 2.6 miles and winds exceeding 300 miles per hour.

When radar data were re-examined, it was determined that the only sign of tornadic-strength rotation that existed at the beginning occurred in the lowest elevation data, less than 20 meters above the ground, which was present for about one minute before the tornado’s formation and persisted until the rest of the tornado developed. This provided the first confirmation that this tornado began at the ground and was able to persist at that level for almost two minutes before the formation of the deeper vortex.

Houser said that by better understanding the mechanisms of tornado formation, it’s possible forecasters can generate short-term, high-resolution models of storms just hours before they occur to determine their likelihood of producing a tornado, efforts already being pursued by the National Weather Service and the National Severe Storms Laboratory.

Other efforts could include stationary, ground-based radar systems, though there would need to be a blanket of them to prove effective, as well as storm spotters relaying visual responses to forecasters.

“If we can get a concrete picture of what’s happening in tornado formation, we’ll have better insight into the processes and mechanisms in predicting tornadoes,” Houser said.