Remember that rain and snow are essentially distilled water. They do pick up some contamination from atmospheric pollution, but not much. Distilled water is not terribly harmful to most electronics equipment. It certainly *CAN* damage some components, but at work we have had quite a few different systems go swimming in rain water, and only a very few have suffered significant permanent damage. I had a Lynx Wireless DMX Tx/Rx dongle get flooded this year, and it was still working even when the case was full of water. I came across the issue when troubleshooting another problem.
Sitting the board in uncooked rice is a good idea. 'Silca gel if you have it.