Well, that bothered me (the fact that they couldn't tell it was going to rain before hand), but what really bothered me is that when it started to rain, somehow the odor completely faded away.
They covered that by the dude noting that it was a fast moving rainstorm which are very common in the South. They come out of nowhere, drop torrential rain for a short period, and then are gone just as fast. That made sense.
The smell being gone didn't make total sense, though the "disguise" was tenuous at best from the beginning (you could tell the walkers knew something was up from the minute they walked out there) so the rain washing off the gore and exposing them isn't too far-fetched.