Storm hazards

HURRICANE HAZARDS

Tornadoes

Hurricanes can produce tornadoes that add to the storm's destructive power. Tornadoes are most likely to occur in the right-front quadrant of the hurricane. However, they are also often found elsewhere embedded in the rainbands, well away from the center of the hurricane.

Some hurricanes seem to produce no tornadoes, while others develop multiple twisters. Studies have shown that more than half of landfalling hurricanes produce at least one tornado. Hurricane Beulah (1967) spawned 141 according to one study. In general, tornadoes associated with hurricanes are less intense than those that occur in the Great Plains. Nonetheless, the effects of torna-does, added to the larger area of hurricane-force winds, can produce substantial damage.

The National Weather Service does not have an accurate way to predict exactly which storms will spawn tornadoes or where they will touch down. 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. Consequently, preparedness is critical.

Storm surge

The greatest potential for loss of life related to a hurricane is from storm surge.

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 the normal tides to create the hurricane storm tide, which can increase the mean water level to heights im-pacting roads, homes and other critical infrastructure. In addition, 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 tremendous.

The storm surge combined with wave action can cause extensive damage, severely erode beaches and coastal highways. With major storm like Katrina, Camille, and Hugo, complete devastation of coastal communities occurred. Many buildings withstand hurricane force winds until their foundations, undermined by erosion, are weakened and fail.

Inland flooding from hurricanes

While storm surge has the highest potential to cause hurricane related deaths, more people died from inland flooding associated with tropical systems from 1970 to 1999. Since the 1970s, inland flooding has been responsible for more than half of all deaths associ-ated with tropical cyclones in the United States. Flooding from hurricanes can occur hundreds of miles from the coast placing communities, which would not normally be affected by the strongest hurricane winds, in great danger.

From 1970 to 1999, 78 percent of children killed by tropical cyclones drowned in freshwater floods.

One cubic yard of water weighs 1,700 pounds. The average automobile weighs 3,400 pounds. Many automobiles will float in just 2 feet of water.

The average automobile can be swept off the road in 12 inches of moving water.

The average person can be swept off his feet in 6 inches of moving water.

At least 23 percent of U.S. tropical cyclone deaths occur to people who drown in, or attempting to abandon, their cars.

Rainfall is typically heavier with slower moving storms.

Some of the greatest rainfall amounts associated with tropical systems occurs from weaker tropical storms that have a slow for-ward speed (1-10 mph) or stall over an area. Due to the amount of rainfall a tropical storm can produce, they are capable of causing as much damage as a category 2 hurricane.

To get a generic estimate of the rainfall amount (in inches) that can be expected, divide 100 by the storm's forward motion. For example, 100/5 mph = 20 inches of rain. For specific rainfall forecasts please monitor local forecasts from the National Weather Service. Rainfall and Flooding fact: Tropical Storm Claudette (1979) brought 45 inches of rain to an area near Alvin, Texas, contrib-uting to more than $600 million in damage.

Hurricane winds

The intensity of a landfalling hurricane is expressed in terms of categories that relate wind speeds and potential damage. Ac-cording to the Saffir-Simpson Hurricane Scale, a Category 1 hurricane has lighter winds compared to storms in higher categories. A Category 4 hurricane would have winds between 131 and 155 mph and, on the average, would usually be expected to cause 100 times the damage of the Category 1 storm. Depending on circumstances, less intense storms may still be strong enough to produce dam-age, particularly in areas that have not prepared in advance.

Hurricane-force winds can easily destroy poorly constructed buildings and mobile homes. Debris such as signs, roofing material, and small items left outside become flying missiles in hurricanes. Extensive damage to trees, towers, water and underground utility lines (from uprooted trees), and fallen poles cause considerable disruption.

High-rise buildings are also vulnerable to hurricane-force winds, particularly at the higher levels since wind speed tends to in-crease with height. Recent research suggests you should stay below the 10th floor, but still above any floors at risk for flooding. It is not uncommon for high-rise buildings to suffer a great deal of damage due to windows being blown out. Consequently, the areas around these buildings can be very dangerous.

The strongest winds usually occur in the right side of the eyewall of the hurricane. Wind speed usually decreases significantly within 12 hours after landfall. Nonetheless, winds can stay above hurricane strength well inland. Hurricane Hugo (1989), for exam-ple, battered Charlotte, North Carolina (which is 175 miles inland) with wind gusts to nearly 100 mph.