Dense fog advisory for all of SE WI until 6 a.m. Monday

Hey look at that tornado / waterspout / landspout

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Look up there in the sky, it’s a bird, it’s a plane, it’s… a tornado-water-land-spout? Since the start of August southern Wisconsin has seen two cases of a tornado or funnel cloud not attached to a powerful thunderstorm. But if and when these funnels touch down and become tornadoes, are they the same tornadoes spawned by a supercell thunderstorm? Of course the answer is yes AND no!

Let’s start with the classic supercell thunderstorm that spawns a tornado. These are the “Cadillac” of tornadoes. A powerful updraft at speeds up to 100mph pulls air off the ground below and into the thunderstorm. Due to wind shear, winds moving at different speeds at different altitudes, the air flowing into the storm is already rotating horizontally. The updraft grabs this rotating air and pulls it into the storm’s mesocyclone or wall cloud, tilting it upward on one end. Nearby, the downdraft, full of cool air, rain, and hail falls onto the rotating air pushing the other end downward. Thus, the updraft and downdraft help tilt the rotating air into the storm, and down towards the ground to complete the connection. And since the parent thunderstorm is a supercell, it may last for several hours spawning the tornado over and over again or keep it on the ground for an +hour!

Now let’s cover the funnel cloud near Oakfield on August 1st, and the landspout over Madison’s Lake Monona on August 8th. These two are less “Cadillac” and more “go-kart”. Sure they have 4 wheels, a gas pedal, and engine. But that’s where the similarities end. In these two cases we do not have a powerful supercell but simply a rain-free cumulus cloud or small rain shower. It’s updraft speed may be 20mph or less. The wind shear needed isn’t coming from the large scale windflow, meaning it has nothing to do with the jetstream overhead or the strong winds produced by an approaching low pressure system.

(Radar image showing only a small shower in Madison during the time of the “tornado”)

Instead, the wind shear is produced locally and on a much smaller scale. The shift in winds may be produced by a lake breeze from Lake Michigan as was the case in Fond du Lac county on August 1st. Sometimes the wind shift/shear is so small and last for such a short amount of time, forecasting it is nearly impossible. When Madison’s landspout touched down on Lake Monona the rotation in the cloud above was so tiny it could not be seen on radar so a tornado warning could not be issued.

Finally you may wonder why I keep calling Lake Monona’s weak twister (it was rated an EF-0) a landspout and not a waterspout. We’ll it’s nit-picky but any landspout over a smaller lake not designated a “marine zone” by the National Weather Service is not considered a waterspout. But the only difference between this occurring over Lake Monona vs. occurring over Lake Michigan is the name.