It's perfectly possible to surf when it's raining or snowing. You'll be wet anyway, so you might as well make the most of it. What's more, surfing in the snow is an exceptional experience.
In the event of complicated weather conditions including thunderstorms or strong winds, sessions may be cancelled for safety reasons. The session will be credited back to you if less than 45 minutes have passed. However, a session lasting more than 45 minutes is considered full and no refund will be given.