I have been in very severe weather have yet to see them fully cancel the special event activities.
If it rains, there are several versions of the parades that can be run. Fireworks are almost never cancelled, but they may be delayed (especially if the first parade run is delayed)
For the stage show, unless there is continual rain, they will either run the complete set if the stage is dry, or they can run an abbreviated version which usually consists of just the final song.