Background
Flash Flood Climatology
Flash flooding is New Mexico's second greatest weather hazard behind lightning. Eleven flash floods occur in an average year. Flash floods develop most frequently during July and August between  the hours of 3 and 8 pm.

Flash floods differ from ordinary flooding in that they rise and fall quite rapidly. Flash floods are the most common type of flooding in New Mexico. Historically, New Mexico's flash floods have favored locations that receive heavy rainfall and/or have steep terrain. Flash floods also tend to occur in urban areas, where impermeable land surfaces result in high runoff through heavily used areas. In some parts of New Mexico, flash floods go unreported and/or undetected due to low population density.

Weather Radio Coverage

The National Weather Service rapidly and directly informs the public of flash floods and other threatening weather conditions through NOAA Weather Radios, which may be purchased through electronics retailers. The weather radio network covers 50 percent of the state's land area and reaches 70 percent of the population. When a flash flood warning is issued, a tone will automatically sound on NOAA Weather Radios located closely enough to a transmitter. Tone alert only reaches 15 percent of the state's land area. The map below illustrates that although most New Mexico residents live and work within the coverage area of a weather radio, many parts of the state are not covered, including many mountain areas and recreation areas susceptible to flash floods. People need to be weather wise and manage risk, especially while enjoying the back country.

Image of New Mexico depicting areas covered by NOAA Weather Radios and areas not covered

   

Meteorological Factors for Flash Flooding

  • High rain rate + slow movement = flash flood (given aggravating hydrologic factors discussed next)
    • Rain rate depends on vertical moisture flux and evaporation of rain below cloud
    • Slow movement depends on mean wind speed, system size, & location of new development
  • Exceptions:
    • Ordinary storm can flood a steep and narrow canyon
    • Ordinary storm can flood a recent forest-fire-burn scar
    • Ordinary storm can flood arroyos and low water crossings quickly
    • Rain and warm weather over a snow field can flood

Relatively quickly moving storms may also produce a flash flood if they flow downstream along an elongated drainage basin. Flood waters may travel miles from their rain source and cause problems far downstream.

   

Hydrologic Factors for Flash Flooding

Many factors play a role in the distribution and severity of flash floods in New Mexico, including:

 

 

  • Terrain gradients
  • Soil permeability
  • Time of year
  • Soil moisture
  • Drainage basin size and orientation
  • Vegetation
  • Snow pack
 

Sharp terrain gradients, impermeable soils, frozen ground, excessive soil moisture, narrow drainage basins, less or dormant vegetation, and thick snow cover all increase the potential for flash flooding from a given storm.

 

Now that you understand where and how flash floods occur, and how NOAA Weather Radio's help alert people to flash flooding, let us look at the National Weather Service's flash flood detection capabilities.


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