You can never be sure what’ll happen when a cumulonimbus (‘Cu-nimb’) cloud like this comes your way, but you’ll be lucky if the answer is ‘nothing at all’. If the horizon beneath the ice-cream castle is obscured, you can be sure you’ll get rain and probably wind as well. Watch the cloud against a stanchion (or on the radar) to see if it’s on a collision course, just as you would with a ship. If you see lightning, it’s probably classifiable as an isolated thunderstorm, in which case you may avoid the worst winds by leaving it to port.
The best thing of all is to alter course so as to miss it. Where this is impracticable, take a precautionary reef, shut the hatch and clap on the oilskins.