The only sure way I know to make it rain in Montana is to go on a trail ride without proper rain gear. Murphy's Law in action, I guess.
So what constitutes proper rain gear? This really depends on your personal preferences, but whatever you choose obviously needs to be of a material that will keep you dry in a downpour, but yet the style is such that you can still move freely in the saddle. When I ride out, I usually wear shotgun chaps treated with chap wax and a cowboy hat, regardless of weather. Then I bring a half-length raincoat and gloves in case the weather turns. Other people may choose to bring a Pommel Slicker or Saddle Slicker because it can protect both your upper body and your legs with its split design that snaps around your legs. The Pommel Slicker is also designed to allow you to snap it up such that you can also cover the front and cantle of your saddle. Either way, the most important thing is to be sure that you have protection for your head, body, legs and gloves.
It is a good idea to have a coat that will lay over your cantle to keep your seat dry. It is also important to be sure that the sleeves on your coat are long enough to cover your wrists when you ride. Just remember that if you get wet, you get cold. And that can be not only miserable, but dangerous.