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OTHER EFFECTS A HURRICANE CAN CAUSE

Hurricanes are strong storms that, because of their nature, can bring about multiple weather effects and cause numerous different kinds of related damage to land and structures.  These weather effects include strong winds, tornadoes, storm surges, and inland flooding.

Strong Winds
The intensity of a hurricane is expressed in terms of categories that relate to wind speeds and potential damage at the time of landfall.  This is explained in the Saffir-Simpson Hurricane Scale.  Even though a storm may be less intense on the scale, it is important to properly prepare for the storm to hit.  A less intense storm can produce significant damage in areas that have not prepared in advance.

Tropical storm-force winds are strong enough to be dangerous to those caught in them.  In addition to the winds themselves, often there are various kinds of small debris blowing around, which can cause damage to structure, well as injury to anyone who is unprotected.  It is because of this that many emergency plans require evacuations to be complete and personnel to be sheltered prior to the onset of tropical storm-force winds, not hurricane-force winds.

Hurricane-force winds can easily destroy poorly constructed buildings and mobile homes.   Larger debris, including signs, roofing material, and small items left outside become flying missiles in hurricanes, and can do extensive damage.  Winds can also do widespread damage to trees, towers, water and underground utility lines (from uprooted trees), and fallen poles, causing considerable disruption.  High-rise buildings are also vulnerable to hurricane-force winds, as wind speeds increase with height.  Most often windows are blown out, making surrounding areas also quite dangerous.  If in a high-rise, it is recommended to stay below the tenth floor, but above any floors at risk for flooding.

In a hurricane, the strongest winds usually occur in the right side of the eyewall of the hurricane.  Wind speeds typically decrease significantly within 12 hours of landfall, however winds can stay well above hurricane strength inland.  Inland wind estimates can only be made shortly before landfall, as this is when the windfield forecast errors are relatively small.  This information is useful in deciding what inland areas might be most vulnerable.

Tornadoes
Hurricanes can also produce tornadoes, adding to the storm’s destructive power.  Tornadoes are most likely to occur in the right-front quadrant of the storm, though they can often be found embedded in the rainbands, well away from the center of the hurricane.

Some hurricanes seem to produce no tornadoes, while others develop multiple ones.  Studies have shown that over half of the landfalling hurricanes produce at least one tornado, though in general, tornadoes associated with hurricanes are less intense than those that occur in the Great Plains region.  Despite this, the effects of tornadoes, added to the larger area of hurricane-force winds, can produce substantial damage.

Presently, there is no way to predict which hurricanes will produce tornadoes.   New Doppler radar systems have greatly improved the forecaster’s warning capability, but the technology usually provides lead times from only a few minutes up to about 30 minutes.  This means that preparedness is critical.

Storm Surges
A storm surge is simply water that is pushed toward the shore by the force of the winds swirling around the storm.  This advancing surge combines with normal tides to create a hurricane storm time, which can increase the mean water level 15 feet or more.  Additionally, wind-driven waves are superimposed on the storm tide.  This rise in water level can cause severe flooding in coastal areas, particularly when the storm tide coincides with the normal high tides.  Because much of the United States’ densely populated Atlantic and Gulf Coast coastlines lie less than 10 feet above mean sea level, the danger from storm tides is immense. 

Storm surge levels in particular areas are determined by the slope of the continental shelf.  A shallow slope off the coast will allow a greater surge to overwhelm coastal communities.  Communities along a steeper shelf will not see as much inundation, but they do have a greater risk of large breaking waves. 

Wave and current action associated with the tide also cause extensive damage.  Water weighs approximately 1,700 pounds per cubic yard, and extended pounding by frequent waves can demolish any structure not specifically designed to withstand such repeated forces.  Beaches and costal highways often get eroded by waves, and many buildings can withstand hurricane-force winds, but collapse when their foundations become unstable as a result of being worn down by the storm waters.  Storm tides, waves, and currents in confined harbors also do severe damage to ships, marinas, and pleasure boats.

In general, the more intense the storm, and the closer a community is to the right-front quadrant of the storm, the larger the area that most be evacuated.  The biggest problem is the uncertainty regarding how intense the storm will be when it finally makes landfall.  It is this uncertainty that is often weighed against the human and economic risks to the community, but it has become a precautionary rule for many community managers to plan for a storm that is one category higher than the forecast.  This helps minimize the loss of life from hurricanes.

Inland Flooding
While storm surge is always a potential threat, more people have died from inland flooding from 1970 up to 2000.  Intense rainfall is not directly related to wind speed, and some of the greatest rainfall amounts occur from weaker storms that drift slowly or stall over an area.  Inland flooding can be a major threat to communities, even those hundreds of miles from the coast.

In a study from 1970 to 1999, freshwater flooding accounted for 59% of United States tropical cyclone deaths.  These floods are why 63% of US tropical cyclone deaths during that period occurred in inland counties.  At least 23% of those deaths were the result of people who drown in, or attempting to abandon, their vehicles, and 78% of children killed by tropical cyclones drowned in freshwater floods.