A bit more about how I have been taking photographs in the rain... I normally use a 16-35mm wide-angle lens on a full-frame body, and so I can't get much protection from a lens hood. In any case, I normally use Lee slot-in filters which could not be attached at the same time.

My current approach is to clamp an umbrella onto the tripod and to use that to keep most of the water off, at least when setting up the camera and composing the image. I've got two Manfrotto Super Clamps with the "061" joining stud connecting them back-to-back at a slight angle: one clamp goes on a solid metal piece of the tripod, and the other clamp goes around the umbrella.

I start out by setting up the camera and hanging my equipment bag from the hook underneath. After that I put on the clamp and attach the umbrella, then attach the camera, and set up the filters. The umbrella is good enough to keep off most of the water, but before taking the shot I will usually wipe off any spots from the filter and switch to hand-holding the umbrella (to get the placement better to keep the rain off, to reduce movement on longer exposures, and just to make sure it stays out of view).

I hope it goes without saying—be careful about strong winds! I'd only try this on calm days, or where I have good shelter.