I have never heard of Illuminations being cancelled due to the weather. Jonette was right, we had a pretty fierce storm blow through the night of our wedding. During our reception, my EM told me we had to move the desserts inside because of the weather, so we ate inside at the Norway Pavilion. It wasn't raining yet at 8:50, so I opted to make the walk to the Italy Isola. It was windy and thundering and lightning at that point. Disney handed out ponchos (for free!) to the guests that wanted them, and right when Illuminations started the rain came down. It made the event that much more memorable!