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| Flood Safety Awareness Week |
| Flooding and Related Phenomena
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When it comes to tropical cyclones, a generic term for a hurricane,
typhoon, or tropical storm, wind speeds do not tell the whole story.
Intense rainfall, not directly related to the wind speed of a tropical
cyclone, often causes more damage. Since the 1970s, inland flooding
has been responsible for more than half of the deaths associated with
tropical cyclones in the United States. Typically, greater rainfall
amounts and flooding are associated with tropical cyclones that have
a slow forward speed or stall over an area.
What do I need to know about inland flooding from tropical cyclones?
Inland freshwater floods accounted for more than half of U.S.
tropical cyclone deaths over the past 30 years. Rainfall is typically
heavier with slower moving storms, as slower moving tropical cyclones
allow heavy rain to persist over a location.
What types of inland flooding are caused by tropical cyclones?
- Flash flooding: Occurs in creeks, streams, and
urban areas within a few minutes or hours of excessive rainfall.
Rapidly rising water can reach heights of 30 feet or more. Streets
can become swift moving rivers and underpasses can become death
traps.
- River flooding: Occurs from heavy rains associated
with decaying hurricanes or tropical storms, and in extreme cases,
river floods can last a week or more.
How do I know how severe a flood will be?
Within flood warning products, the NWS conveys the magnitude
of observed or forecast flooding using flood severity categories.
These flood severity categories include minor flooding, moderate
flooding, and major flooding. Each category has a definition based
on property damage and public threat.
- Minor flooding: minimal or no property damage,
but possibly some public threat or inconvenience.
- Moderate flooding: some inundation of structures
and roads near streams. Some evacuations of people and, or transfer
of property to higher elevations are necessary.
- Major flooding: extensive inundation of structures
and roads. Significant evacuations of people and, or transfer
of property to higher elevations.
The impacts of floods vary locally. For each NWS river forecast
location, flood stage and the stage associated with each of the
NWS flood severity categories are established in cooperation with
local public officials. Increasing river levels above flood stage
constitute minor, moderate, and major flooding. Impacts vary from
one river location to another because a certain river stage (height)
in one location may have an entirely different impact than the
same river stage at another location.
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