We're building a small rain machine (approximately 6'x6') and have to record a few lines of dialogue during the scene in which it is going to be used. Is the sound of the rain going to ruin our audio (and require us to use ADR) or is it possible to get high quality sound even with the rain? My concerns are the distance the mikes are going to be from the actors and the sound of the rain muffling their voices. Thanks, Ryan

...Is the sound of the rain going to ruin our audio (and require us to use ADR) or is it possible to get high quality sound even with the rain?
Maybe. How loud is the device? How close is the noise source to the actors? How distant will the microphones be? You probably don't yet know the answers to these questions, so neither does anyone else :)

...My concerns are the distance the mikes are going to be from the actors and the sound of the rain muffling their voices.
Sorry to restate what you probably already know - you may need to get the mics closer. You may need to work in a plant mic. You may need to use lavs. You may need to protect mics from water. You may need to redesign the rain machine to move noise sources further away from actors.

And now, the real answer: test. Test before all the crew and all the actors and the director are standing around giving you da' stink eye while you're sweating...

PS. it is quite possible to get good dialog in the rain, especially when there isn't wind. Clothing people are wearing can also help or hinder. For example, soft clothing doesn't make noise when a raindrop hits it. Rubber/plastic coated raincoats do, and creak besides. Under the brim of a baseball cap can be a great place to hide a lav and protect it from raindrops. A condom can be used to protect lavs & plant mics. Transmitters are frequently sealed in the smallest pelican cases, with holes drilled for antenna and mic, and a drop of silicone caulk to seal the opening once the antenna or cable is in it.

As said there is always the ADR route if it is too noisy, also beware taking different shot angles as the rain will change phase and levels into a mic.

You may want to put something on the ground that will quietly absorb the raindrops if you can't see their feet in the shot.

Another option includes using very closely placed lav mics that are properly rigged and protected.