I'm curious to know if this is one of the pre-U.S. Music heads or not... I have an old WT800A that was left outside one night after a gig. It ended up raining that night, and I walked out my front door to find it sitting in the middle of a wet deck. I promptly opened it up got as much moisture out as possible, then let it sit to dry for a few days. Aside from needing a new power switch it powered right up and ran flawlessly - and is still doing so 8 years later. These things are pretty impervious to the elements.
...and remember, NEVER rely on your guitar player to make sure your gear makes it back inside after load out.