If you see rain in the distance, get to high ground immediately? ;-)

Winter is a great time to visit DV, as the temperatures are more friendly and there may even be clouds to decorate the sky. Flash floods, however, are a real concern in many areas.

There are excellent maps of both areas in PDF format on the respective NP sites. If you can print 11x17, they come in quite handy.

Have a great time. Both areas offer excellent photo opportunities.