Where I live can get lines of storms any time of the year, as long as there is enough heat energy. It's the difference in temps on each side of a front that determines the strength, not the absolute temperature.

I've seen lines of storms cross the country as long as TV has had radar, and they start out very strong in the midwest and weaken as they approach the east coast. The same system that flattened buildings in KY just gave us a little wind and rain.