Surely just putting the hood up would be better (and quicker) though
All down to personal preference I guess. I have a 3-zip tonneau cover in place when I'm touring so to put the hood up I have to first remove the tonneau cover and stow it before putting the hood up, which is a bit of a faff compared to putting the storm cover on - plus the hood doesn't stop rain getting into the engine bay.
On the topic of encountering thunderstorms, I always carry a golfing umbrella, then you can just park up for 20 minutes and wait for it to pass. That's me in the car in front.
