A flash flood is a rapid rise of water into a normally dry area, or a rapid rise on a stream or creek, that begins soon after its cause, often within minutes to a few hours. Intense rain is the usual trigger, but a dam or levee failure or the sudden release of water held by an ice jam can also cause one.
How fast is flash
National Weather Service (NWS) sources describe the time window in different ways. The NWS glossary says a flash flood begins within six hours of the causative event, then adds that the actual threshold may vary in different parts of the country.
The NWS manual of definitions instead says minutes to multiple hours, and an NWS safety page says flash floods generally develop within 6 hours. Six hours works as a common working rule, not a hard line.
What produces the rain
Rain that falls hard and keeps falling on the same ground is the core problem. NWS Pittsburgh notes that most flash flooding comes from slow-moving thunderstorms, storms repeatedly moving over the same area, or heavy rain from hurricanes and tropical systems.
NOAA's National Severe Storms Laboratory (NSSL) lists how quickly a storm moves and how intense its rainfall rates are among the key questions. Ice jams and snowmelt can also help cause flash floods.
What decides whether rain becomes a flood
NSSL notes that a given amount and duration of rain may or may not cause a flash flood, depending on the watershed. Soil moisture, vegetation, pavement, steep terrain and recent wildfire all change the outcome.
Pavement and roofs matter a great deal. NWS Pittsburgh says urbanization increases runoff 2 to 6 times over natural terrain.
In the West, NSSL notes that canyons, small streams and dry arroyos are often not easily recognizable as a source of danger.
How the Weather Service warns on it
Local NWS offices issue Flash Flood Warnings for short-term events that need immediate action, such as dangerous small stream or urban flooding and dam or levee failures. The Flash Flood Guidance and Excessive Rainfall Outlook entries cover two of the main forecast tools behind those decisions.
NSSL notes forecasters can usually tell in advance when conditions favor flash floods, but there is often little lead time for an actual warning.
Where the picture gets complicated
Flooding that is already under way can turn into flash flooding. The NWS glossary notes this can happen when intense rain drives a rapid surge of rising water.
A flash flood also does not require a thunderstorm overhead. Water from a storm upstream, or from a failed dam, can reach places where no rain fell.