I think getting a bit wet, as long as they don't stay wet for too long, should be OK.There is a drain for the water and there is also a cave like space for 30 hens but the problem is that I fear if they would get wet and become diseased if storm comes. I tried using plastic tarps during last storm but it got torn in half.
My sister has a very open coop, and when she gets huge storms the chickens do get wet. But her storms last maybe 4 to 12 hours at the most.