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What Goes into A Hurricane

Learn about hurricanes -- how they form, how they behave and how forecasters watch for them.

Hurricanes are products of a pre-existing weather disturbance, warm tropical waters, moistures, and heavy winds. The name hurricane is derived from the Caribbean God of Evil, Hurican.
Hurricanes, powered by heat from the seas, are steered by the easterly trade winds and the temperate westerlies as well as their own ferocious energy. Around the core, winds grow with great velocity.
Tropical cyclones are classified as follows:
Tropical Depression: An organized system of clouds and thunderstorms with a defined circulation and maximum sustained winds of 38 mph or less.
Tropical Storm: An organized system of strong thunderstorms with a defined circulation and maximum sustained winds of 39 to 73 mph.
Hurricane: An intense tropical weather system with a well-defined circulation and maximum sustained winds of 74 mph or higher.
When the winds from these storms reach 39 mph (34 kt), the cyclone is given a name.

Saffir-Simpson Hurricane Scale
The Saffir-Simpson Hurricane Scale is a 1-5 rating based on the hurricane's present intensity. This is used to give an estimate of the potential property damage and flooding expected along the coast from a hurricane landfall. Wind speed is the determining factor in the scale, as storm surge values are highly dependent on the slope of the continental shelf and the shape of the coastline, in the landfall region. Note that all winds are using the U.S. 1-minute average.

Category One Hurricane:
Winds 74-95 mph (64-82 kt or 119-153 km/hr). Storm surge generally 4-5 ft above normal. No real damage to building structures. Damage primarily to unanchored mobile homes, shrubbery, and trees. Some damage to poorly constructed signs. Also, some coastal road flooding and minor pier damage. Hurricane Lili of 2002 made landfall on the Louisiana coast as a Category One hurricane. Hurricane Gaston of 2004 was a Category One hurricane that made landfall along the central South Carolina coast.

Category Two Hurricane:
Winds 96-110 mph (83-95 kt or 154-177 km/hr). Storm surge generally 6-8 feet above normal. Some roofing material, door, and window damage of buildings. Considerable damage to shrubbery and trees with some trees blown down. Considerable damage to mobile homes, poorly constructed signs, and piers. Coastal and low-lying escape routes flood 2-4 hours before arrival of the hurricane center. Small craft in unprotected anchorages break moorings. Hurricane Frances of 2004 made landfall over the southern end of Hutchinson Island, Florida as a Category Two hurricane. Hurricane Isabel of 2003 made landfall near Drum Inlet on the Outer Banks of North Carolina as a Category 2 hurricane.

Category Three Hurricane:
Winds 111-130 mph (96-113 kt or 178-209 km/hr). Storm surge generally 9-12 ft above normal. Some structural damage to small residences and utility buildings with a minor amount of curtainwall failures. Damage to shrubbery and trees with foliage blown off trees and large trees blown down. Mobile homes and poorly constructed signs are destroyed. Low-lying escape routes are cut by rising water 3-5 hours before arrival of the center of the hurricane. Flooding near the coast destroys smaller structures with larger structures damaged by battering from floating debris. Terrain continuously lower than 5 ft above mean sea level may be flooded inland 8 miles (13 km) or more. Evacuation of low-lying residences with several blocks of the shoreline may be required. Hurricanes Jeanne and Ivan of 2004 were Category Three hurricanes when they made landfall in Florida and in Alabama, respectively.

Category Four Hurricane:
Winds 131-155 mph (114-135 kt or 210-249 km/hr). Storm surge generally 13-18 ft above normal. More extensive curtainwall failures with some complete roof structure failures on small residences. Shrubs, trees, and all signs are blown down. Complete destruction of mobile homes. Extensive damage to doors and windows. Low-lying escape routes may be cut by rising water 3-5 hours before arrival of the center of the hurricane. Major damage to lower floors of structures near the shore. Terrain lower than 10 ft above sea level may be flooded requiring massive evacuation of residential areas as far inland as 6 miles (10 km). Hurricane Charley of 2004 was a Category Four hurricane made landfall in Charlotte County, Florida with winds of 150 mph. Hurricane Dennis of 2005 struck the island of Cuba as a Category Four hurricane.

Category Five Hurricane:
Winds greater than 155 mph (135 kt or 249 km/hr). Storm surge generally greater than 18 ft above normal. Complete roof failure on many residences and industrial buildings. Some complete building failures with small utility buildings blown over or away. All shrubs, trees, and signs blown down. Complete destruction of mobile homes. Severe and extensive window and door damage. Low-lying escape routes are cut by rising water 3-5 hours before arrival of the center of the hurricane. Major damage to lower floors of all structures located less than 15 ft above sea level and within 500 yards of the shoreline. Massive evacuation of residential areas on low ground within 5-10 miles (8-16 km) of the shoreline may be required. Only 3 Category Five Hurricanes have made landfall in the United States since records began: The Labor Day Hurricane of 1935, Hurricane Camille (1969), and Hurricane Andrew in August, 1992. The 1935 Labor Day Hurricane struck the Florida Keys with a minimum pressure of 892 mb--the lowest pressure ever observed in the United States. Hurricane Camille struck the Mississippi Gulf Coast causing a 25-foot storm surge, which inundated Pass Christian. Hurricane Andrew of 1992 made landfall over southern Miami-Dade County, Florida causing 26.5 billion dollars in losses--the costliest hurricane on record. In addition, Hurricane Wilma (pdf) of 2005 was a Category Five hurricane at peak intensity and is the strongest Atlantic tropical cyclone on record with a minimum pressure of 882 mb.
Information Provided By: NOAA

Storm Structure
The process when a disturbance forms and strengthens into a hurricane depends on at least three conditions, warm waters, moisture, and a wind pattern near the ocean surface that spirals air inward. Bands of thunderstorms form, allowing the air to warm further and rise higher into the atmosphere. If the winds at these higher levels are relatively light, this structure can remain intact and allow for additional strengthing.
The center, or "eye," of a hurricane is relatively calm. The most violent activity takes place in the area immediately around the eye, called the eyewall. At the top of the eyewall (about 50,000 feet), most of the air is propelled outward, increasing the air's upward motion. Some of the air, however, moves inward and sinks into the eye, creating a cloud-free area.

1. Outflow. The high level clouds moving clockwise out away from the hurricane at heights of over 35,000 feet. These clouds are indicative of air spreading out over the top of the storm, which is essential to its development.
2. Feeder Bands. These are squally bands of showers characterized by strong gusty winds and heavy rains. These bands become more pronounced as the storm intensifies, and are fed by the warm ocean.
3. The Eyewall. A band of clouds, strong winds and heavy rains surrounding the eye of the storm. At the eyewall, there is rapid movement of air toward the center and upward into the cloud.
4. The Eye. What goes up must come down, so with the violent rising air converging toward the storm center at the eye, sinking air develops within. This air dries out, creating the clear, calm eye. Winds are very light here since the focus of convergence and hence strong winds are in the eyewall. Storm Surge
Low pressure in the hurricane can act as a plunger, slightly pulling up the water level. However, the components that contribute to the greatest storm surge affect are the winds blowing to the left side of the storm and the topography of the land as the storm makes land fall. The strongest surge comes ashore just to the right of the eye, where the fierce hurricane winds are blowing toward land. Winds on the left side of the storm might actually cause the water level to run slightly lower than normal. Higher water level allows waves to strike farther inland, causing massive property damage.

Forecast Process
The National Weather Service (NWS) has several tools to monitor hurricanes. While they are still far out in the ocean, indirect measurements using satellites are the main tool, although ships and buoys also provide observations. Once the storms come closer to land, more direct measurements (reconnaissance aircraft, radiosondes, and Automated Surface Observing Stations) are also used. Within about 200 miles of the coast, radar provide important indirect measurements of the storm.
Computer models used to forecast storm intensity and movement require a great deal of data about the atmosphere. Lack of observations (especially over the ocean) and errors and inconsistencies in the data are major sources of forecast errors.
In general, the National Hurricane Center provides products that have a broad view of the hurricane and its potential impacts, while the local forecast office Weather Forecast Offices takes the information from the National Hurricane Center and tailors it to their specific locale, providing the public and local emergency managers with additional information about the hazards expected in their area.
National Weather Service offices in Raleigh, Wilmington, and New Bern cover central and eastern North Carolina, and advise residents of predicted rainfall amounts and wind speeds. In addition, meteorologists in the ABC 11 Accu Weather Storm Center are on the air around the clock, keeping viewers abreast of the storm's path and the impact it will have on the area.

Hurricane Names
Experience shows that the use of short, distinctive given names in written as well as spoken communications is quicker and less subject to error than the older more cumbersome latitude-longitude identification methods. These advantages are especially important in exchanging detailed storm information between hundreds of widely scattered stations, coastal bases, and ships at sea.
Since 1953, Atlantic tropical storms have been named from lists originated by the National Hurricane Center and now maintained and updated by an international committee of the World Meteorological Organization. The lists featured only women's names until 1979, when men's and women's names were alternated. Six lists are used in rotation. Thus, the 2004 list will be used again in 2010.
The only time that there is a change in the list is if a storm is so deadly or costly that the future use of its name on a different storm would be inappropriate for reasons of sensitivity. If that occurs, then at an annual meeting by the WMO committee (called primarily to discuss many other issues) the offending name is stricken from the list and another name is selected to replace it.
Several names have been changed since the lists were last used. Four names from the 1995 list have been retired. On the 2001 list, Lorenzo has replaced Luis, Michelle has replaced Marilyn, Olga has replaced Opal, and Rebekah has replaced Roxanne. Three names from the 1996 list have been retired. On the 2002 list, Cristobal has replaced Cesar, Fay has replaced Fran, and Hanna has replaced Hortense. Two names from the 1998 list have been retired. On the 2004 list, Gaston has replaced Georges and Matthew has replaced Mitch. On the 2006 list, Kirk has replaced Keith.
Source: National Hurricane Center