Rain or Shine! No matter what the weather, the show must go on! Be prepared for bad weather just in case, and if you are dressing up in costume, be sure it will be able to weather the storm! And note if it does rain, even though the party goes on, the fireworks or parade may be canceled.
If you are leery of buying tickets far in advance because you are worried about inclement weather, rest assured most night of the party do not sell out. Historically, the first the night and Halloween night sell out, but others do not. You can usually buy tickets at the ticket counter on the night of the party.