Hurricane Threats

Hurricane Winds

Hurricane winds are a force to be reckoned with by coastal communities deciding how strong their structures should be. As winds increase, pressure against objects is added at a disproportionate rate. Pressure against a wall mounts with the square of windspeed so that a threefold increase in windspeed gives a nine-fold increase in pressure. Thus, a 25 mph wind causes about 1.6 pounds of pressure per square foot. A four by eight sheet of plywood will be pushed by a weight of 50 pounds. In 75 mph winds, that force becomes 450 pounds, and in 125 mph winds, it becomes 1250 pounds. For some structures, this force is enough to cause failure.

These winds will weaken after landfall due to: 1) loss of warm-water energy source; and 2) the encountering of great friction over land.

Rainfall and Flooding

Heavy rains and ocean waters brought ashore by strong winds can cause flooding in excess of 50 cm (20 in) over a 24 hour period. The runoff systems in many cities are unable to handle such an increase in water because of the gentle topography in many of the coastal areas where hurricanes occur

Hurricanes are capable of producing copious amounts of flash flooding rainfall. During landfall, a hurricane rainfall of 10 to 15 inches or more is common. Obviously, if the storm is large and moving slowly--less than 10 mph--the rainfall amounts from an well-organized storm are likely to be even more excessive. To get a generic estimate of the rainfall amount (in inches) that can be expected, divide the storm's forward motion into 100, i.e. Forward Speed/100 = estimated inches of rain.

Rainfall and Flooding fact: Tropical Storm Claudette (1979) brought 45 inches of rain to an area near Alvin, Texas, contributing to more than 600 million in damage.

The heaviest rain usually occurs along the coastline, but sometimes there is a secondary maximum further inland. This heavy rain usually occurs slightly to the right of the cyclone track and usually occurs between 6 hours before and 6 hours after landfall. The amount of rain depends on the size of the cyclone, the forward speed of the cyclone and whether it interacts with a cold front. Interaction with a cold front will not only produce more tornadoes but more rainfall as well.

Storm Surge

An abnormal increase in the ocean's level, sometimes in excess of several meters high and miles wide. Storm surges can come ashore up to five hours before the storm and destroy low-elevation coastal areas. It is especially damaging when the storm surge occurs during high tide and consequently, is often responsible for most hurricane-related deaths

Storm surge is a large dome of water often 50 to 100 miles wide that sweeps across the coastline near where a hurricane makes landfall. Storm surge can range from 4 to 6 feet for a minimal hurricane to greater than 20 feet for the stronger ones. The surge of high water topped by waves is devastating. The stronger the hurricane and the shallower the offshore water, the higher the surge will be. Along the immediate coast, storm surge is the greatest threat to life and property, even more so than the high winds!

• Over 6000 people were killed in the Galveston Hurricane of 1900, most by storm surge.
• Hurricane Camille produced a 25-foot storm surge in Mississippi.
• Hurricane Hugo in 1989 generated a 20-foot storm tide in South Carolina.

Tornadoes

A number theories exist about their origin, but in the case of Hurricane Andrew, severe damage was inflicted by small spin-up vortices that developed in regions of strong wind-shear found in the hurricane's the eye wall. The strong damaging winds of the hurricane frequently cover the smaller tornado paths, making the separation of their damaging effects very difficult.

Hurricanes also produce tornadoes, which add to the hurricanes destructive power. Typically, the more intense a hurricane is, the greater the tornado threat. When a hurricane brings its winds inland, the fast-moving air hits terrain and structures, causing a frictional convergence which enhances lifting. Frictional convergence may be at least a contributing factor to tornado formation in hurricanes. The greatest concentration of tornadoes occur in right front quadrant of the hurricane.

Tropical Cyclone Spawned Tornadoes Facts

• 10% of deaths in the United States are associated with hurricanes are a result of tornadoes;
• Most tornadoes occur within 24 hours after hurricane landfall. The exception is when there is interaction with a cold front after landfall. Then more tornadoes will occur two or three days after landfall, well inland;
• Most tornadoes occur within 150 miles of the coastline;
• More tornadoes occur during the morning and afternoon rather than evening or night due to the need for a tornado to have a heat source;
• The Gulf of Mexico hurricanes produce more tornadoes than Atlantic storms;
• The majority of tornadoes occur within 30 miles of the center of the cyclone, but there is a secondary maximum further away in the outer rain bands (100-150 miles away from the center);

Tornado winds can reach up to 300 mph at a forward speed of 60 mph and are usually 100-300 yards wide.

This Homepage was prepared by Kenneth F. Hambrick, Polk County Coordinator. Contact at E-Mail address: webmaster@pcoem.org  for additional information about our program. The information contained in this Homepage is considered public domain and the Office of Emergency Management encourages interested persons to utilize any portions of it that might be of educational benefit or enhance their local programs.

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