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Below are the logs from Tornado Tim's storm chase in 1998.
Supercell storm on Colorado/Nebraska border.

Wall cloud seen here indicates a Mesocyclone - a rapidly rotating air mass within a thunderstorm that often gives rise to a tornadoOn May 21st 1998 I filmed a powerful supercell on the Colorado/Nebraska border. When I left for the chase I picked out a cloud mass building over Yuma, Colorado. This was all done by eye and instinct as I had no radar or internet information on this chase. In many ways, learning to read the clouds while chasing taught me more than many books I have read.

I proceeded northeast on I-76 exiting at Julesburg. I then drove south toward Amherst Colorado to get in front of the rapidly building clouds I had been watching  for almost 2 hours now. Since I was without radar everything had to be done by sight so I had made this choice based on ground disturbance below the storm, low level wind speeds, and the storms postition along the front. I sat for another 45 minutes northeast of the storm watching the clouds build. Consistently this one storm kept putting out strong down drafts sending dirt flying everywhere. This is one of the signs I look for when chasing, strong down drafts beneath a building wall cloud. As I watched the small, disorganized wall cloud become a well-defined wall cloud it seemed as though the winds around me grew stronger at each passing minute. This picture is the beginning of a powerful super cell. The winds now were a steady 45 miles per hour feeding into the cloud.
The entire base of the storm was rotating, a classic wall cloud. This is a dangerous storm.

Strong Downdrafts from the center of the storm indicate a dangerous storm     Wall cloud seen here indicates a Mesocyclone - a rapidly rotating air mass within a thunderstorm that often gives rise to a tornado

From my vantage point there wasn't much lightening. On the radio I heard that this storm was putting out tennis ball size hail just to the south and west of this cloud. In the center of the rotating base the ground disturbance begin to get stronger and steadier. Remember the entire base, about 2 miles wide was rotating. The winds around me were a steady 40 miles per hour and rocked the car while wind gusts were over 60 mph.
Down drafts continued from the center of the wall cloud.

strong winds beat ground below  straight line winds pound ground

Doppler radar is still indicating a tornado while these down drafts blasted out.
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