Rain can ruin a crop. It can create mold problems and the weight of the water can break branches. I would also watch out for wind too. I like to put cages around my plants. When the plants start to flower then it is a good idea to support the bigger branches with bamboo sticks. I have about 10 sticks per plant. I will probably pick up some more soon.

Maybe make a plastic tipi for the plants if your really worried about a big storm. I don't know if it would be worth the effort to make one for each plant though. I have no idea if rain can wash off the trichomes either. I hope not because after spraying my plants with neem oil. I like to come back later and rinse them down with the hose. Sometimes I spray the plants with the hose during the veg cycle. I like to think it helps make the plant stronger. During the flower/bud cycle I try to keep the colas as dry as possible.