Just wanted to share an interesting article found at:
http://www.stormtrack.org/forum/arch...php/t-716.html

Here are some key figures extracted from it:
- non-severe thunderstorm: 15 to 30 MPH (1,200 - 2,500 fpm)
- severe "Multicell cluster" thunderstorms: 60 MPH (5,500 fpm)
- supercell: typical 70 or 80 MPH with extremes of 150 to 175 MPH
(12,000 - 15,000 fpm)

Cheers,
- Ido