Terrence Cook
EF2
I disagree with the message of this article, although you normally witness these clouds after the passage of a storm and they can occur during weakening. I still believe that they are an excellent indicator of the severe potential (vigorous, persistent updraft). Here is the article I found...
[url=http://www.usatoday.com/weather/tg/wmamatus/wmamatus.htm]
[url=http://www.usatoday.com/weather/tg/wmamatus/wmamatus.htm]