The stall speed for something small like a Cessna 172 is in the ballpark of 60 mph, so those definitely need to be strapped down if there's a chance of even a simple severe thunderstorm. But for something like a Boeing 737, stall speed is more like 125-150 mph (depending on flaps configuration). If surface winds that high are a real possibility, airlines will do their best to relocate the planes, because there are too many ways they can be severely damaged even without taking off while stationary.

I just cannot for the life of me stop laughing at the image this evoked. Just 400 planes sliding sideways across the desert as pilots are chasing after them on foot.