Autocross is a rain or shine sport, the only thing that shuts an event down is lightning in the area. For rain days plan to bring some plastic bins to keep your stuff dry, a towel to dry off hands and feet before driving and something to protect you from the elements. An umbrella does ok, a poncho is a little better, but the seasoned competitors will all be in full rain suits. A rain suit can be had for under $100, and will provide you with many years of service and comfort.