I have smoked in all kinds of weather and ended up with good results and interesting stories to share. If you can provide some sort of shelter for your WSM it will help. The rain will cool the dome temp and alter the cook times. I also find that due to the design of the joints in the WSM some water runs down the sides and ends up in the bottom of the smoker. The biggest problem is keeping the meat dry when I open the lid. Having an assistant hold an umbrella helps and adds to the fun.
Mike