The Saffir-Simpson scale of a hurricane's intensity is used to estimate potential property damage and coastal flooding caused by storm surge. The scale is determined by wind speed. Storm surge is an abnormal rise of water above the normal tide, generated by a storm. Flooding from storm surge depends on many factors, such as the track, intensity, size, and forward speed of the storm and the characteristics of the coastline where it comes ashore or passes nearby. Harvey was a Category 4 storm when its eye made landfall in Texas at around 10 p.m. Friday, but has since been downgraded to a tropical storm.
Winds of 74-95 mph (120-150 kph). Storm surge of 4 to 5 feet (1 to 1.5 meters) above normal. Damage primarily to unanchored mobile homes, shrubbery and trees. Some damage to poorly constructed signs and piers. Extensive damage to power lines and poles likely will result in power outages that could last a few to several days.
Winds of 96-110 mph (155-175 kph). Storm surge 6 to 8 feet (1.8-2.4 meters) above normal. Some roof, door and window damage to buildings. Considerable damage to mobile homes, small watercraft, trees, poorly constructed signs and piers. Flooding of coastal and low-lying areas. Many shallowly rooted trees will be snapped or uprooted and block numerous roads. Near-total power loss is expected with outages that could last from several days to weeks.
Winds of 111-129 mph (180-210 kph). Storm surge 9 to 12 feet (3 to 4 meters) above normal. Some structural damage to small homes. Mobile homes destroyed and large trees blown down. Coastal flooding destroys smaller structures and floating debris damages larger structures. Terrain lower than 5 feet (1.5 meters) above sea level may flood as far as 8 miles (13 kilometers) inland. Hurricane Katrina, the costliest natural disaster in U.S. history, was a Category 3 storm at landfall in 2005 after being a Category 5 in the Gulf of Mexico. At least 1,800 people died.
Winds of 130-156 mph (210-250 kph). Storm surge 13 to 18 feet (4-5 meters) above normal. Wall failures and roof collapses on small homes, and extensive damage to doors and windows. Complete destruction of some homes, especially mobile homes. Power outages will last for weeks to possibly months. Major coastal flooding damage. Most of the area will be uninhabitable for weeks or months. Two 2004 storms were Category 4: Hurricane Ivan, which made landfall near Gulf Shores, Alabama, and Hurricane Charley, which hit the Florida Gulf Coast near Fort Myers. Charley killed at least 21 people and left thousands homeless. The total U.S. damage was estimated to be near $15 billion.
Winds greater than 157 mph (250 kph). Storm surge greater than 18 feet (5 meters) above normal. Complete roof failure on many homes and industrial buildings. Smaller buildings and mobile homes blown over or completely blown away. Major damage to lower floors of all structures located less than 15 feet (4.5 meters) above sea level and within 500 yards (460 meters) of the shoreline. Massive evacuation of residential areas on low ground within 5 to 10 miles (8 to 16 kilometers) inland may be required. The last Category 5 storm to hit the United States was Hurricane Andrew in 1992. An estimated 250,000 were left homeless and the storm caused more than $20 billion in damage in the Bahamas, Florida and Louisiana. Fifty-five people were killed.
Source: National Hurricane Center