The World Cup Sylt is one of the biggest and longest running windsurfing events on the planet. Featuring all of the three main disciplines, 200,000 spectators on the beach, and some of the most extreme conditions The Sylt World Cup is one not to be missed.
Westerland can be very tough in onshore winds to get out, due to the shore break. From 3-4m big waves with lots of power to small choppy waves. 3m tide. Be aware of long rock lines (ca. 20m into the sea) every 100-200m from Westerland to Kampen. You can't see them during high tide! But they are marked with a sign on the beach. Strong current when it's coming from South or North.
Organizer's website: http://www.windsurfworldcup.de/
Current weather conditions for Sylt http://en.windfinder.com/forecast/list_sylt
You can see a list of the equipment that the slalom sailors have registered here.