Forecasters looking in wrong place when predicting tornadoes

Weather forecasters may be looking in the wrong place when working to issue tornado warnings, according to a study. Historically, there have been a wide number of conflicting theories about how tornadoes form, said researchers from the Ohio University in the US.

However, the most widely accepted was that they form from the top down, based on work done from the 1970s through the 1990s, they said. For the first time, new observational evidence shows that tornadoes actually form from the ground up, which could have a profound impact on the way warnings are issued in the future. It's the first time these hypotheses have been able to be evaluated observationally, thanks to a modern radar system that collects data very rapidly.

"We need to reconsider the paradigms that we have to explain tornado formation, and we especially need to communicate this to forecasters who are trying to make and issue warnings," said Jana Houser, an assistant professor at Ohio University. "Based on our results, it does not look like you are going to really ever be finding strong evidence of a tornado descending, so we need to stop making that a priority in our forecasting strategies," said Houser.

The team demonstrated that tornadoes actually form at the ground and move up rapidly, contrary to the long-held hypothesis that most tornadoes form at cloud level and descend to touch the earth. This evidence, the first of its kind, was gathered after Houser observed an EF5 tornado in May 2011. Those findings were subsequently confirmed through observations from several other tornadoes, including a very compelling visual and radar analysis of the deadly El Reno tornado of May 2013. They 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 pickup vortex signatures at or above cloud level.

"We need to strategise 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. Radar observations of the wind field were used to track tornadic signatures in the core of the storm.