If your scene is calling for rain, don't wait for Mother Nature to get her act together.
You can create your own rain and not spend a ton of money to do it. Here's a tutorial by Tom Antos that shows you how to construct two different rain machines: a $15 rig made out of a flat sprinkler hose and a sturdier $60 to $70 rig made out of PVC pipe.
Before you get too far into making either of these rigs, here are a few things you should know. First of all, movie rain is not like real rain. Real rain tends to be more sparse with smaller droplets and tends to not show up on-camera. This is part of the reason why filmmakers will use/create rain machines that disperse huge, thick water droplets that will register on-screen.
Secondly, getting that sexy movie rain may require more than just a simple garden hose or PVC pipe -- something more along the lines of a fire hose. That's not to say that you can't get great results from Antos' rig, but just be aware that it may not look as thick or full as you might hope.
Lastly, you might want to use a few other things to help your movie rain look more dynamic, like smoke, debris (leaves, etc.), or Fullers earth, as well as a fan to blow all of that around.
In the end, if all you have is a regular old hose, don't fret. If you light your scene well and turn that spigot up full blast, you should be alright.