Once you've determined (hopefully in advance) that you are hiking in an area with flash-flood potential, keep note of these tips and you'll be able to stay ahead of the flood.
Cloud build-up, sounds of thunder in the distance, visible rain are all precursors to flash flooding.
If you are hiking near water already, keep an eye out for any sudden changes in water clarity from clear to muddy. This means the rain is heavy enough to cause runoff.
Floating debris in the water is equally alarming and is a sign of the rain's force.
Rising water levels or stronger currents both mean a greater volume of water is heading your way.
It's also possible to hear the roar of water as it approaches (e.g. in a canyon).
If you observe any of these signs, seek higher ground immediately, preferably a location that would allow you to leave the area entirely if necessary. Even climbing a few feet may save your life. Remain on high ground until conditions improve. Water levels usually drop within 24 hours. Flash floods do occur in the park during periods of low flash flood potential. A moderate or higher flash flood potential should be a serious cause for concern.