Wet conditions are part of the soccer season. There's no avoiding it
1) Bring a Rain Jacket.
Wearing a rain jacket when possible will help goalkeepers keep under layers drier for longer. Try to keep your base layers dry throughout the game so that you can perform at your best. Once your base layer is wet, you will always be reminded of an uncomfortable cold shock whenever you move, or the wind blows.
Goalkeeper hack: rain jackets allow for amazing slides because they are low-friction.
2) Bring a towel to dry your soccer goalie gloves
Champion-level goalkeepers do this all the time- watch the pros during rainy games. They will bring a towel (or several) out to the game and place it on a goal net. Towels are multi-purpose in a game environment, that's true, but during wet weather, towels are a lifesaver, and they remove excess mud or water from your goalie gloves. When the latex in goalkeeper gloves becomes saturated with water, they will lose their grip.
3) The tighter the soccer gloves, the better.
Getting the correct size of goalkeeper gloves is extremely important. The 90's- the days of giant goalie gloves- are over and keepers should aim for a tighter fit that also gives the player excellent movement. Big gloves become cumbersome in wet conditions.
4) Consider goalie gloves with finger spine protection
This is not the favorite of older keepers, but soccer gloves with spine protection in the fingers are useful for wet conditions. When it's wet, the ball is likely to skid off of surfaces faster. This puts the keeper in a precarious position- you may not get your hands in place or take the ball early enough to get enough tip around the post.
5) Wear surgical gloves under your GK gloves
It sounds strange- but this is such a great little trick! Keep your hands drier and warmer during rainy-day games. The thin nature of the surgical gloves allows for goalkeepers to retain their feel for the ball while keeping your hands dry through the entire match.
6) Watch the Bounce
A ball that is wet and skidding around will pick up speed as it connects with the ground. Goalkeepers should stay aware of their penalty area and prepare for the ball to act differently in different areas in the box. This is of lesser concern on turf, but it should still be trained for. Proper warmups before game time will provide the keeper with the experience needed to conquer problems with skidding balls before game time.