Having done a 2 day with Scott Jedlinski on Red Dot pistols in the weather that changed from rain, to cold rain to sleet and snow the most useful item for the 2 days was hand warmers. Also knowing the weather was going to be bad I wore wool socks and waterproof Wolverine boots. These two things helped me a great deal. It's a good opportunity IMO to train in inclement weather though, it'll show you where your weak areas are with your gear.
David