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
Factors that affect
heavy rainfall production:
- The heaviest precipitation
occurs where the rainfall rate is the highest for the longest
period of time.
- Precipitation efficiency: How efficient is the thunderstorm in
converting water vapor condensates into rainfall that reaches
- Rainfall rate: How intense is the rainfall at any
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
- 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
- Instability: CAPE, Lifted Index, Total Totals Index,
Showalter Index. (For more information, consult "Convective
Season Parameters and Indices.")
- Average layer relative
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
- 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.