I rarely blog about anything outside of IT topics, but today I’m going to write about tornadoes.
After the EF-3 tornado in St. Louis on March 16th, I thought I’d post a little bit about the things I’m looking for when I’m watching for tornadoes. I don’t qualify as even an amateur – I’m more more like a weather enthusiast. But here are some of the things I look for. These screenshots come from an iOS app called RadarScope.
The top part of of the first image, labeled Reflectivity, shows a classic hook echo. Rotation causes rain, hail, and possible debris to rotate around a supercell, creating this curved reflectivity. A hook echo doesn’t happen for all supercells, and even if it does show up, it doesn’t guarantee that there is a tornado. But it is cause for alarm, if I see this I’m going to head for cover immediately, even if a warning hasn’t been issued yet. I’ll figure out if there’s a tornado from the safety of the basement.
The bottom part of the first image, labeled Velocity, is a velocity couplet. It’s in the same map location as the hook echo. Green means movement toward the radar, red means away from the radar. Green and red right next to each other is evidence of rotation. The hook echo and the couplet are in the same spot on the map; combined, they are strong evidence of a tornado. I’d also be heading for cover for a velocity couplet that looks like that, even if there was no hook echo.
The top part of the second image is the same hook echo as the first image. The bottom part of the second image, labeled Correlation Coefficient, is a measure of how consistent the shapes and sizes of objects are within the radar beam. If it was all red, the same type of precipitation is detected i.e. all raindrops are pretty much the same. The blue spot in the middle means the reflected images are not consistent with each other. It is potential evidence of a debris ball kicked up by a tornado.
Radar can’t actually see a tornado, it can only report on atmospheric evidence of the likelihood of one. In this case, there is a hook echo, a strong velocity couplet, and a debris ball all in the same location on the map. This is about as strong of a radar guarantee you can get that there is a tornado on the ground.
The third image is zoomed out, you can see the red polygon indicating the area where the National Weather Service issued a tornado warning. At this point it was a still considered a ‘radar-indicated’ tornado. In the next radar pass, it was upgraded to ‘radar-confirmed’ tornado – all of the evidence combined was enough for the NWS to authoritatively state that there was a tornado on the ground. A few minutes later it was upgraded again to a ‘trained-spotter confirmed’ tornado – trained volunteers or law enforcement call to report visual confirmation of a tornado on the ground.
Of course I always rely on actual meteorologists as the authority on interpreting radar, but I feel a lot better during storms because I am able to understand some of what the radar is saying. The National Weather Service published a damage survey two days later giving details of the tornado.
