A National Weather Service watch is a message indicating that conditions favor the occurrence of a certain type of hazardous weather. For example, a severe thunderstorm watch means that a severe thunderstorm is expected in the next six hours or so within an area approximately 120 to 150 miles wide and 300 to 400 miles long (36,000 to 60,000 square miles). The NWS Storm Prediction Center issues such watches. Local NWS forecast offices issue other watches (flash flood, winter weather, etc.) 12 to 36 hours in advance of a possible hazardous-weather or flooding event. Each local forecast office usually covers a state or a portion of a state.
An NWS warning indicates that a hazardous event is occurring or is imminent in about 30 minutes to an hour. Local NWS forecast offices issue warnings on a county-by-county basis.
Many more watches are issued than warnings. A watch is the first sign a flood may occur, and when one is issued, you should be aware of potential flood hazards.
Be aware of flood hazards. Floods can roll boulders, tear out trees, destroy buildings and bridges, and scour out new channels. Flood waters can reach heights of 10 to 20 feet and often carry a deadly cargo of debris. Flood-producing rains can also trigger catastrophic debris slides.
Regardless of how a flood or flash flood occurs, the rule for being safe is simple: head for higher ground and stay away from flood waters. Even a shallow depth of fast-moving flood water produces more force than most people imagine. The most dangerous thing you can do is to try walking, swimming, or driving through flood waters. Two feet of water will carry away most automobiles.