ST. LOUIS, Mo.- Monday night’s deadly and destructive storm was referred to as a ‘Derecho’. It is a large scale thunderstorm, typically extending from north to south, that forms a “bow” segment.
“Bow” – think bow and arrow, a long line the protrudes out. The storm must have winds up to 58 mph, that’s enough to issues severe thunderstorm warning. And typically extending top to bottom by 250 miles, these typically extend north to south.
The damage is typically straight-line wind damage, which can be as bad as an EF or even EF 2 tornado. Usually winds stay under 100 mph but have been clocked up to 120 mph, that’s about as strong as a strong EF-2 tornado.
The issue with Derechos, is it can be extremely widespread damage. Even though wind damage can only amount to weak tornadoes, the damage can produce a much wider swath.
Typically, tornadoes occur on a localized, smaller-scale area. Derecho damage can take up an entire viewing area. Kind of like what we saw with last night’s storms.
There’s a reason the NWS issues severe thunderstorm warnings out ahead of the heavy rain and lightning. Sometimes warnings are issued 20 to even 40 minutes before you see the first raindrop or strike of lightning—that’s because wind associated with these bowing segments can arrive before the actual thunderstorm itself. This is called an outflow boundary.
At the core of the thunderstorm warm moist air cools within the center of the storm, the cool air sinks down to the ground and rushes out. The air hits the ground and forms a mini front that can rush out miles ahead of the actual storm. This is why often before the storm arrives it feels like the temperatures have dropped 20 degrees and the winds pick up. That is the cool air from within the t-storm.
Even out ahead of the main line, meteorologists watch for small areas of spin. These are mesovortices, and minor turbulent spots. They can look ominous and scary, but many times they don’t form anything with these types of t-storms. If this small spin in the atmosphere gets organized enough ahead of the main line of storms, it can produce a very weak, spin up tornado.