It does not necessarily need to be a thunderstorm.
Mother Nature can be a bitch!
Some years ago I was on the mid north coast of NSW driving south on one of those days you are glad not to be on a golf course.
Scattered cloud at around 5,000 ft.intermittent rain and cold,
I saw two cloud formations converging on each other, and as a gliding man stopped to watch.
It was truly amazing when the two formations collided, the result was an instantaneous downward flow of cloud, not unlike a waterfall. The speed was astonishing, and so was the damage.
About two miles further down the Pacific Highway, a swathe had been cut about 200 yards wide. An uprooted, very old fig tree a few cows, power lines and general bush had been demolished.
I truly hate to think what could have happened to any aircraft hitting such a system.