A waterspout briefly touched down in the Tennessee River near Decatur just after 7 PM Sunday evening. The waterspout developed in response to a pop up thunderstorm in the area at the time. There was no threat of a tornado at any time Sunday evening for areas along the Morgan/Limestone County line or in Decatur. However, there were plenty of photos of funnels sent to our newsroom that resembled funnels associated with small tornadoes.
While Sunday evening's waterspout is technically considered a type of tornado, it develops much differently and is far less damaging and less harmful than typical tornadoes we are used to seeing in North Alabama. Sunday evening's waterspout is considered a "fair weather waterspout," and is similar to landspouts.
Landspouts and fair weather waterspouts form in nearly the same way, with the only difference being that one occurs on land and the other occurs on water. They start to form when subtle boundaries of air at or near the surface start to collide with each other. If the atmosphere is unstable, with warm temperatures and ample humidity, these colliding boundaries begin to spin. Updrafts associated with nearby thunderstorms pick up this spinning at the ground and send the rotation into the air. A funnel develops as a result of this air rushing up to the cloud base of the storm. Although we typically see the visible funnel near the cloud base before we see it at the ground, the circulation begins at the ground as opposed to a tornado which starts in the clouds and drops downward from the storm. Given that this circulation occurred over water, it will technically be considered a waterspout as opposed to a landspout.
Click on the video to watch a visual representation of the differences between the formation of tornadoes compared to fair weather waterspouts and landspouts.