Ctitical strike attachement

[Nights-wolf]

Hi all,
I would like to create Windfury spell, based on critical strike, It's done but i don't know how to attach a model on it when it turn on.
I would like to attach "tornado <effect>"
"Abilities\Spells\Other\Tornado\TornadoElementalSmall.mdl"
Can someone help me please??
( I didn't find a post about it ='( )