30 July 2004

A Remarkable Storm

George E. Hrabovsky, President, MAST

I have been a storm-chaser for nearly twenty-five years. In that time I have seen over fifty tornadoes. My wife, Dianna, has been chasing for just over a year and has encountered six tornadoes, two this year alone. This report is about our latest tornado.

The Situation

The surface synoptic-scale weather map for our region at the time of the tornado is shown at left. As you can see, there is a cold front pushing in from the west and a warm front about two-thirds of the way across the state of Wisconsin to the north. Warm temperatures are south of the warm front.

There is a low pressure center in Canada and a jet streak (a very fast moving current of air aloft) rotating around this low. This jet streak has winds of up to 90 knots (about 100 miles per hour). This jet streak would add a great deal of energy to any thunderstorm that developed in the region. 

The Storm Prediction Center noted that this could enhance the amount of energy available for convection and predicted the possibility of isolated super cell thunderstorms. Such a storm has a strong, persistent rotating updraft and can contribute to the formation of significant tornadoes.

To be honest I was not too excited about the situation until about 5 PM local time when I noticed some interesting thunderstorms beginning to form in southeastern Minnesota. One of these storms was moving right for Madison.

Here is what the storms looked like on the satellite image.  The cluster of lakes in the center of the picture is Madison. 

I thank the Space Science and Engineering Center of the University of Wisconsin, Department of Atmospheric Sciences, for the satellite image. This image was taken at 7:55 PM local time.

The storm to the northwest of Madison is the one that produced the tornado.  As the storm approached we saw this impressive wall cloud to our west.

A wall cloud signifies a very strong updraft.  Such updrafts are an important ingredient for tornado formation; thus wall clouds are a good indicator of where tornadoes might form. Unfortunately, only about 10 percent of wall clouds are associated with tornadoes. 

All of the wall clouds we saw were rotating, thus indicating that they were possibly tornadic. There were two other wall clouds at the same time, and both are shown below.

The Tornado

Here is what the radar looked like as the tornado formed,  I thank the National Weather Service for making this radar image available.

MSN is the call sign for Madison on this radar image, and you can see what looks like a slight hook echo on the cell just passing over Madison. In reality, while this is an indication of a rotation in the updraft of the storm, the tornado formed to the southwest of this feature.

As the rotating wall cloud that would produce the tornado approached us, it spawned a pair of funnel clouds right in front of us. In the photo below you can see a funnel cloud on either side of the radio antenna.

Shortly after this image the rotating wall cloud passed directly overhead and the tornado formed literally on top of us.  

Neither Di nor I we were injured and the car; though rocked severely, sustained no damage.  The photograph below shows the tornado as it entered the city of Madison.  The tornado is in the middle of the picture, above and to the right of the time-stamp (which is ten minutes too fast).

The Science

Through a combination of reading weather maps correctly, interpreting radar and satellite information, and a thorough knowledge of storm structure and dynamics, Di and I were able to put ourselves in a position to find the tornado. The fact that we got considerably closer to the tornado than we wanted is irrelevant. 

Here is what happens in a tornado:

1) A low-level circulation is started. Often this circulation will be weak.

2) An updraft (or other vertical forcing mechanism) stretches this circulation vertically.

3) As the vortex is stretched, it amplifies the rotation of the circulation.

When the rotation reaches a point where the winds climb above 40 miles per hour, and the rotation reaches the cloud base of the thunderstorm, you have a tornado. 

You can have tornadoes with no visible funnel (this can happen if the air is very dry).  In our situation we saw no funnel because the tornado was weak enough that it could not produce a sufficient pressure drop to supersaturate the air within the funnel. Only after it passed over us did it gain enough strength to produce the impressive funnel seen above.