Everything depends on the weather as you well know. I always plan worst case scenarios.......Cold, windy, rainy is a distinct possibility. Everyone has wool undergarments, top notch raingear and glacier fingerless neophrene gloves if it were my group going in may.........A variety of good hats would also have to pass muster.....hats to sleep in, hats to paddle in the rain.......

What are the experience level of your group? Can they handle two or three days of cold rain? Do they have the right gear to stay safe from hypothermia? Will they have the right mental attitude if it gets nasty and stays nasty?

My planning would include a flexible time frame. Make sure your in a good position for departure especially if anyone has to be back at work on such and such a date. If your on big water and it gets snotty you might leave when you shouldn't........little margin for error in the cold water.

On the plus side (I am generally an optimist  ;) ) if the weather is nice and the fish turn on you might have some of the best fishing of your life. Never know.........just be safe and plan for the worst is my advice.