(Flash) Flood Impact
rainfall of 6 to 12 inches or more is common during landfall, frequently
producing deadly and destructive floods. Such floods have been the
primary cause for tropical cyclone-related fatalities over the past 30
years. The risk from flooding depends on a number of factors: the
speed of the storm, its interactions with other weather systems, the terrain
it encounters, and ground saturation. Even storms with relatively light
winds can be very damaging–Tropical Storm Claudette dumped 45 inches of
rain near Alvin, Texas in 1979, contributing to more than $680 million*
Rains are generally heaviest with slower moving storms (less than 10
mph) or those which become nearly stationary. To estimate the total
rainfall in inches, one rule of thumb is to divide 100 by the forward
speed of the hurricane in miles per hour (100/forward speed = estimated
inches of rain). However, your local National Weather Service Forecast
Office will have a more accurate estimation for your local area.
The heaviest rain usually occurs near or along the cyclone track
in the period 6 hours before and 6 hours after landfall. However,
storms can last for days. Long after the winds of Hurricane Agnes (1972)
had died down, its remnants fused with another storm system over the Florida
panhandle and produced floods which extended all the way up to the northeastern
United States. The result: 122 deaths and $6.9 billion* in damage.
Other recent tropical cyclones which caused major flooding that resulted
in significant loss of life include Hurricane Floyd and Tropical Storm
Alberto. Hurricane Floyd (1999) brought intense rains and record
flooding to the eastern United States. Of the 56 people who perished
during Floyd, 50 drowned due to inland flooding. Tropical Storm
Alberto (1994) drifted over the southeastern United States and produced
torrential rainfall. More than 21 inches of rain fell at Americus,
Georgia. Devastatingly, 33 people drowned and damage exceeded $750
|Occasionally hurricanes produce little rain
where it is expected. Hurricane Inez (1966) produced only a small amount
of rain in Miami when the city was under the eyewall where torrential
rains usually occur. As a result of the absence of rain, the strong winds
blew salt spray many miles inland, causing severe damage to vegetation
from salt accumulation.
Large amounts of rain can occur more than 100 miles inland where
flash floods are typically the major threat along with mudslides in
mountainous regions. High winds generally become less of a threat the
farther inland a hurricane moves (although there have been several exceptions),
but the heavy rains frequently continue and even intensify as the dying,
but still powerful, hurricane is forced up higher terrain or merges with
other storm systems in the area. For example, Hurricane Camille (1969)
devastated the Gulf Coast, but weakened quickly as it moved northeast.
The storm combined with a cold front in the mountains of central Virginia
to produce an unexpected 30 inches of rain. As a result, 109 people died.
The possibility for heavy rain after a hurricane moves inland makes
monitoring changing weather conditions and maintaining contact with the
local National Weather Service Office very important.
* Adjusted to 1994 dollars