MARYLAND HEIGHTS, Mo. – The St. Louis area experienced two destructive tornado outbreaks during the last three months of 2021. Extreme Weather Specialist Chris Higgins showcases the technology that allowed us to confidently track those tornadoes, even in the darkness of night.

The Tornado Debris Signature is something that is fairly new to the science of meteorology. It only became possible with the rollout of dual-polarized Doppler radars over the past decade.

Kevin Deitsch is the Warning Coordination Meteorologist at the National Weather Service office in Weldon Spring. He says the dual-polarized data was especially helpful during the last two outbreaks because they both occurred at night.

“It’s crucial because it allows us to confirm tornadoes without actual reports from somebody on the ground. Obviously, we love our spotters and need our spotters but at nighttime, it is just hard to see these things,” said Deitsch.

So how does the dual-polarization radar see tornadoes? Just like any radar, it sends a beam of energy out into the clouds. When that energy hits a raindrop or a snowflake then some of that energy bounces back to the radar. That’s how we can tell there is something out there.

The new dual-polarization radars actually send out two pulses, one that is horizontal and one that is vertical. That allows us to measure the consistency of the shapes and sizes of the targets. That’s the big breakthrough!

Falling raindrops are very consistent in shape and size. But debris from a tornado is not. Because it’s made up of pieces of homes, trees, buildings — anything the tornado can loft up into the air.

The dual-polarization data is also useful after a storm during the forensic reconstruction of a tornado event. By tracing out the TDS signatures, survey crews have a much better idea of where to look for damage. What they are unable to inspect from the ground they sometimes can see from space by using hi-resolution satellite data.

“If the tornado goes through areas that are heavily wooded with lots of leaves, cornfields, things like that, we can see the circulation in some of the debris,” said Deitsch.

This is exactly what happened after the December 10th storm. By combining the TDS signature with satellite data the weather service was able to confirm the first of the tornadoes was on the ground continuously from Defiance all the way into Maryland Heights, a total distance of 25 miles.