One key point is what is "severe". As pilots it's usually the top level of something bad, however nature does not have a rheostat dial that works on 1-10, with 8,9,10 being the severe setting, rather anything above severe becomes infinite. So the dial can be turned up infinitely beyond the severe setting and what could happen is just purely unknown.
Severe turbulence is the point you lose control of the aircraft either momentarily or totally or it simply just falls apart as the loading is so 'severe' it exceeds all limits, picture flying into a tornado. Which is what a CB can generate given the right conditions, and anywhere, not just the midwest USA.
Severe Ice is another thing, it starts at a point your ice protection starts to be overwhelmed slowly to an aircraft becoming an ice block in a few seconds. I have seen the few second thing and it should scare the hell out of any pilot, get out quickly or more importantly avoid.
Point is no aircraft is certified for flight into "severe" conditions, because it is a place that has no limits.