Rainy tracking
1) Get on the trail of your wounded deer immediately.
2) Use a gps and make a waypoint every drop of blood you see.
3) No GPS? Use trail markers, red tape, surveyors tape, arrows or plain string to mark each blood drop.
4) Call friends and hunting partners to assist. Send one or two guys in the general direction of travel to jump ahead of the trailers.
5) I've trailed deer in the rain this way and it has worked without fail.
Wounded deer will make for a water source if the wound is not immediately fatal (liver/kidney/small artery). If not pressured, they will lie down sometimes. If you discover small pools of blood and an obvious bed, slow down and look ahead. Your deer is not far and will soon die in his bed.
Generally wounded deer will go downhill. If not pressured, they almost never go uphill. Notice I said almost. Their will to survive is strong so they will do whatever that keen sense of survival tells them to do (escape).
After the blood has washed out, use the GPS waypoints or your other markers to focus your direction of travel. Use expanding circular paths once the trail goes cold. Sometimes a deer, full of adrenalin will stumble another 40-50 yards after they have bled out.
Good luck.