When a hurricane is expected to strike a specific coastal area within 24 hours or less. Take additional / final precautionary actions.
When a Tropical Storm reaches winds of 74 miles per hour or more, it is classified as a Hurricane. Hurricanes, or tropical cyclones, are counter-clockwise rotating wind systems that usually originate between June and November. An average hurricane has gale-force winds extending outward 200 miles and an ‘eye’, or center of circulation, of 10-15 miles in diameter. Wind velocities of 100 to 150 miles per hour are found just outside the eye, although in maximum storms (Category 5), wind velocities of over 200 miles per hour are found. Hurricane storm systems move at an average speed of 10-20 miles per hour.'
Strong winds associated with hurricanes create a dome of water often 50 miles across where the eye of the hurricane makes landfall. The tidal heights depend on the strength of the storm, the direction of landfall and whether it is normal, high or low tide. The storm surge is the most dangerous part of a hurricane. Storm surges of up to 25 feet above mean sea level have been recorded. The hammering waves of a storm surge are to blame for 9 out of 10 hurricane fatalities.
The 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). No significant 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 Lily 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). 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). Some structural damage to small residences and utility buildings with a minor amount of curtain wall 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.'
Winds 131-155 mph (114-135 kt or 210-249 km/hr). More extensive curtain wall 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). 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 Katrina, a category 5 storm over the Gulf of Mexico, was still responsible for at least 81 billion dollars of property damage when it struck the U.S. Gulf Coast as a category 3. It is by far the costliest hurricane to ever strike the United States. In addition, Hurricane Wilma 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.'