In my most memorable ride in a properly heavy thundery downpour I rode down the pavement/verge on a 60mph rural road as I thought it would be safer than riding on the road in bouncing rain.
It ended abruptly with me going over the bars because the pavement/verge I was riding on suddenly narrowed. I landed hands first on the road, and looked up to see my bike bounce of a passing car, which turned out to be an unmarked police car*.
I can’t guarantee that this happens in all thundery downpours, but it is a good reason to take the train home.
*NB:- best type of car to hit. I didn’t pay for their dented front wing and missing wing mirror, and got a lift most of the way home.