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6/16/02         DLJ

(Flash) Flood Impact

Widespread 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* in damage.   

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 million. 

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
 

 

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