Flash Floods vs Flood Advisories

During the Summer months when thunderstorms with heavy rain occur, the National Weather Service issues a variety of flood-related products. But what are the differences between them.

Essentially, the NWS places flooding into two categories: 1) threatening to life and/or property, or 2) "nuisance" flooding.

Flash Flood: Potential Threat to Life and/or Property

A "Flash Flood Watch" will be issued when the potential of flash flooding is in the upcoming future. A "Flash Flood Warning" is issued when Flash Flooding is imminent or occuring. By Nationa Weather Service definitions, the following constitute a Flash Flood:

During a rapid period of time, usually less than 6 hours,

  • River or stream flows out of its banks and is a threat to life or property.
  • Person or vehicle is swept away by flowing water from runoff that inundates adjacent grounds.
  • A maintained county or state road is closed by high water.
  • Approximately 6 inches or more of water flows over a road or bridge.
  • Dam break or ice jam release causing dangerous out of bank stream flows or inundates normally dry areas, creating a hazard to life or property.
  • Any amount of water in contact, flowing into, or causing damage to an above ground residence or public building and is runoff from adjacent grounds.
  • 3 feet or more of ponded water that poses a threat to life or property.
  • Mudslide, rock slide, or debris flow caused by rainfall.



Spring Creek Flash Flood
Ft Collins, CO
July 27-28, 1997

Flash Flood Watches and Warning will be updated or canceled by "Flash Flood Statements".

 

 

  Urban and/or Small Stream Flood: "nuisance" flooding

An "Urban and/or Small Stream Advisory" will be issued when some flooding is about to occur but generally doesn’t pose a threat to life or property. Small streams or creeks may leave their banks but don’t threaten structures. Also, these events may include standing water in parking lots, streets, or backed up storm sewers. However, if rain amounts increase these advisories may be upgraded to a "Flash Flood Warning".



Return to News Archive
USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.