We are in the same boat (ha!) here (as is about half the state it looks like). I avoid getting woodwinds out in ANY drizzle, rain, etc. They keep cases with them at all times. We have a pretty woodwind-heavy show - I told the kids they may be just "singing" their part tonight. If it was the game before contest I might go ahead and use the woodwinds through light sprinkles/ very light drizzle. Otherwise not worth losing pads and rusting up rods.
We stay in the stands playing brass/drums (just making sure to dry them off good before packing them in cases) through "light" rain (if it's not too cold). Once we hit "medium/heavy rain" we'll hit the buses and wait it out...until I get a principal/sup to say "pack it up and go home". Luckily I have a GREAT principal, sup, and athletic director - all are behind whatever decision I make for the kids' sake.
GOOD LUCK - remember - we LIKE the rain here in Texas.