NWS Burlington: Flash Flooding (short)

What can produce Flash Flooding? Very heavy
rainfall associated with thunderstorms produces most of our flash flood events in the North
Any thunderstorm can produce copious amounts of rainfall and an associated flash flood
risk; which can be enhanced by thunderstorms that are stationary, slow-moving or training
of thunderstorms, which is when storms move over the same area repeatedly often in sequence. Tropical cyclones with their torrential downpours
associated with heavy rainbands and/or embedded thunderstorms in the tropical cyclone itself
can cause flash flooding.  
Though not necessarily weather-related, the failure of a dam will cause an immediate,
sudden release of water prompting flash flood warnings.
 Some areas are more vulnerable to flash flooding than others.
  Prior rainfall events can saturate the ground,
taking less amount of rainfall to produce flash flooding.
  Urban areas are susceptible to flash flooding,
since rain can’t absorb into the ground if it is covered by pavement.
  Mountainous terrain, canyons and valley locations
are also prone to flash flooding as rainfall runs down the mountain and pools into low-lying
valleys. The threat to human life is significant even
in the most minor of flash flood events with field and culvert flooding and water on roadways.
In higher-end events, significant structural damage to infrastructure, such as road washouts,
or complete property losses can occur.   
Though flash flooding is potentially dangerous at any time, it is especially so at night
simply because it is difficult to see advancing floodwaters until it is too late.

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