You could collect some of the rain water for magickal workings. If the quality of the rainwater in you area is good, not heavily polluted or acid rain, you could collect it for almost any ritual use. Many witches work with fresh water. If you can't get to a body of fresh water, rain water is just as good. If it's storming as well as raining, the collected "lightning water" or "storm water" can also be used in protective magick. Here's an article by my favorite author, Scott Cunningham, about using rain water, spring water and dew.
http://www.nmrjournal.com/zine/archive/ ... 16/185.htm
If you work with elementals or other beings associated with water, such as undines, merfolk, or sea monsters, this would be good time to connect with them.
Some magick associated with water are divination, dreams, psychic powers, astral travel, healing, and fertility.