Also, remember that lenses are at their sharpest at moderate apertures, and usually are less sharp at either end of their range. Most lenses I have are tack sharp at f/8 - f/11 *IF* that aperture allows for a fast enough shutter speed at the aperture/ISO I have selected. In the days of film and full size sensors there was an old rule-of-thumb that you need at least a shutter speed of 1/X where X=the focal length of your lens in order to hand hold the shot. With the APS sized digital sensors it is a good idea to go one stop faster than that.
Considering this is storm photography, you might not have the luxury of using f/8. My opinion is to bump the aperture at least one stop up from wide open if you can, then compensate with the ISO. In my opinion, a little more grain is much more acceptable than a soft shot due to camera movement.