Flooding With Tropical Cyclones

The National Weather Service has declared March 16th through the 20th as Flood Safety Awareness Week. The National Weather Service in Charleston will feature a different educational topic each day during the awareness week. The topic for today is flooding associated with tropical cyclones.

When it comes to tropical cyclones, a generic term for a hurricane or tropical storm, wind speeds do not tell the whole story. Intense rainfall, not directly related to the winds 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.

Rainfall is typically greater with slower moving storms as they allow heavy rain to persist over a location for longer. Therefore, people living in areas well away from the coastline still need to be alert if a tropical cyclone makes landfall nearby.

Flash flooding occurs in creeks, streams and urban areas within a few minutes to a few hours of excessive rainfall. Streets can become swiftly-flowing rivers and underpasses can become death traps. Coastal flooding is also an issue when rainfall combines with a storm surge.

River flooding occurs from heavy rains associated with decaying hurricanes or tropical storms. Flooding associated with tropical systems can last for several weeks to more than a month.

The National Weather Service in Charleston issues two areal flood products for non-river flooding: flood advisories and flash flood warnings.

A flood advisory is issued when minor flooding is expected but is not expected to pose a significant risk to life or property. This can include areas that frequently flood during heavy rain, such as downtown Charleston and Savannah.

A flash flood warning is reserved for flooding events that pose a significant threat to life and/or property.

Minor Flooding: minimal or no property damage but possibly some public threat or inconvenience. Some secondary roads adjacent to the river may become impassable.

Moderate Flooding: some inundation of structures and primary roads near the river. 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 a flood vary locally. For each National Weather Service river forecast location, flood stage and the stage associated with each of the National Weather Service flood severity categories are established in cooperation with local public officials. Impacts vary from one river location to another because a certain river stage or height in one location may have an entirely different impact than the same level above flood stage at another location.