Everything at practice should mirror your meets as closely as you can. Jud Logan discussed how he would practice early in the morning for Olympic Trials because they competed at 9am. Boxers train at night because the main event fight is often at 11pm. When it rains you should be getting out there to throw. This way you know what to do at a meet when it's raining. Rain, snow, hail, cold weather or whatever might come, if they don't cancel a meet for it don't cancel practice for it.

This is one detail that people at the top of their sport decide to replicate. It's not for convenience, they want to enter competition as prepared as they possibly can. Whether it be a week or month, training in conditions which replicate competition will make everything easier.

The less thought you need to put into the competition the better. If you go in prepared nothing will phase you.