Since non of the services use exactly the same techiques, the best way to get an accurate forecast is to check more than one site. Then you can make an educated guess on how to pack.

Accuweather	Side by side comparisions of various forecasts
Intellicast	Category forecast skiing, golf, kiting, etc.
Rain Or Shine	Links to weather cams around the world
Weather Planner	Select fair-weather dates for outdoor activities. Is about 83% accurate forecasting a year in advance