Brown pow is a term commonly used by mountain bikers, dirt bikers etc to describe the awesome riding conditions after a good rain. Rain in the summer or fall is to a mountain biker or dirt biker what fresh snow is to a snowboarder, skier or snowmobiler. Fresh snow makes a good powder day. Rain gives us "brown pow".
Now go send it!
It's raining! Gonna be some brown pow tomorrow! Who wants to go ride?