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Basic Heavy Rain and Flash Flood Concepts


Prolonged heavy rain from low pressure storm systems and especially thunderstorms can pose a potential flash flood threat. Flash flooding is a serious threat to life and property requiring immediate action if flooding is occurring or imminent. Never drive across flooded roads. Move to higher ground if necessary. Be particularly careful at night when it is difficult to see or assess flood waters. NWS Louisville staff members monitor heavy rain and flash flood potential during significant storm systems. The WSR-88D Doppler radar can estimate rainfall. When Louisville forecasters believe that flash flooding is imminent or if a flash flood report is received, then they issue a flash flood warning for the area concerned. Below are some basic heavy rain and flash flood concepts.

A simple rainfall concept:

  • The heaviest precipitation occurs where the rainfall rate is the highest for the longest period of time.

Factors that affect heavy rainfall production:

  • Precipitation efficiency: How efficient is the thunderstorm in converting water vapor condensates into rainfall that reaches the ground.
  • Rainfall rate: How intense is the rainfall at any one spot.

Factors that contribute to efficient rainfall production and high rainfall rates:

  • Moist, deep-layered air mass: provides moisture needed for heavy rainfall and limits rainfall evaporation (if dry air were present aloft).
  • Deep low-level warm layer above 0 deg C: allows warm air mass to contain more moisture and enhances warm rain processes.
  • Strong inflow: produces rapid moisture advection and continual moisture source for storms.

To assess HEAVY RAINFALL POTENTIAL, one should consider:

  • Available moisture: surface, 850, and 700 mb dewpoints, precipitable water values, K index. (For more information, consult "Convective Season Parameters and Indices.")
  • Instability: CAPE, Lifted Index, Total Totals Index, Showalter Index. (For more information, consult "Convective Season Parameters and Indices.")
  • Average layer relative humidity: surface-700 mb or 1000-500 mb RH.
  • Strength of inflow: use surface, 850, and 700 mb upper-air charts and/or isentropic charts.

To assess FLASH FLOOD POTENTIAL, one must ALSO consider 3 other factors:

  • Topography: flash flooding is more likely in hilly and mountainous terrain than in flat areas.
  • Antecedent conditions: flash flooding is more likely from future rain if the soil is nearly saturated and/or streams are running high from recent past rain.
  • Storm propagation: extremely important in determining whether heavy rain will fall over a relatively large area (for moving storms) or across the same area (for stationary or regenerative storms).
    • a. Forward propagation: heavy rain is progressive so that flash flooding is less likely unless topography and/or antecedent conditions dictate otherwise.
    • b. Slow moving/Backward/Regenerative propagation: individual convective cells move forward but continued cell redevelopment upstream causes a thunderstorm complex to exhibit little or no overall (net) movement (i.e., some cell movement but little or no system movement); flash flooding is very possible in these situations.

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