Hurricanes 101






If you want to be a hurricane expert, you need to know some hurricane basics. Read on to learn about hurricane-related hazards and how they can impact Georgia.

Hurricanes, Typhoons, and Cyclones (Oh My)

Tropical cyclones are one of natures most powerful and awe-inspiring phenomena.
Tropical cyclones are called by different names depending on where they exist: in the Atlantic Basin (the Gulf of Mexico, the Caribbean Sea, and the Atlantic Ocean) and in the eastern Pacific Ocean east of the International Date Line they are called Hurricanes; in the western Pacific they are called Typhoons; and in the Indian Ocean they are called Cyclones. Tropical cyclones have developed in the southern Atlantic Ocean, but this is extremely rare.

What Exactly is a Hurricane?

According to the Atlantic Oceanographic and Meteorological Laboratory (AOML), a tropical cyclone “…is the generic term for a non-frontal synoptic scale low-pressure system over tropical or sub-tropical waters with organized convection (i.e. thunderstorm activity) and definite cyclonic surface wind circulation.”

The National Oceanic and Atmospheric Administration’s (NOAA) National Hurricane Center (NHC) categorizes tropical cyclones in the Atlantic Basin (Atlantic Ocean, Caribbean Sea, and Gulf of Mexico) into four types based on intensity:

Tropical Disturbance: A discrete tropical weather system of apparently organized thunderstorms - generally 100 to 300 nautical miles in diameter - originating in the tropics or subtropics, and maintaining its identity for 24 hours or more.

Tropical Depression: An organized system of clouds and thunderstorms with a defined circulation and maximum sustained winds of 38 mph (33 knots) or less.

Tropical Storm: An organized system of strong thunderstorms with a defined circulation and maximum sustained winds of 39 mph to 73 mph (34-63 knots).

Hurricane: An intense tropical weather system with a well-defined circulation, producing maximum sustained winds of 74 mph (64 knots) or greater.

Hurricane intensity is classified into five categories using the Saffir-Simpson Hurricane Wind Scale (presented in Hurricane Winds below). Winds in a hurricane range from 74 to 95 mph for a Category 1 hurricane to greater than 155 mph for a Category 5 hurricane.

Did you know that hurricanes can have sustained winds of almost 200 mph? Hurricane Camille (1969) and Hurricane Allen (1980) epitomize the destructive potential of hurricanes as both had sustained winds of 190 mph and gusts well over 200 mph.
For more information about hurricane basics, click here.

Hurricane Season

Tropical cyclones can occur during most months of the year, but are most likely in the Summer and Fall. Hurricane season in the Atlantic Basin begins officially on June 1 and ends on November 30. Hurricane activity tends to peak in September, although hurricanes can develop anytime during hurricane season, or even outside of hurricane season. In 2005, Hurricane Hurricane Epsilon formed in late November and persisted through early December and Tropical Storm Zeta formed on December 30, 2005 and dissipated on January 6, 2006.

Hurricane Hazards

There are four hazards associated with tropical storms and hurricanes: storm surge, winds, inland flooding, and tornadoes. Storm surge only affects coastal areas, but the other three hazards are possible throughout Georgia.

Storm Surge

Simply put, storm surge is the water that moves inland when a hurricane moves close to or on land and is caused by the hurricane-force winds pushing the water inland. The National Hurricane Center defines storm surge as “…an abnormal rise in sea level accompanying a hurricane or other intense storm, and whose height is the difference between the observed sea surface and the level that would have occurred in the absence of the cyclone.”

The amount of storm surge generated by a hurricane is combination of storm and geographic factors. Hurricane characteristics affecting storm surge heights include: intensity of the hurricane (strength of the winds and central pressure), angle of approach, and forward speed. Geographic characteristics affecting storm surge values include: bathymetry, slope of the continental shelf, roughness of the continental shelf, shape of the coastal region, and existence of natural or manmade barriers.

When the effects of tide are included with storm surge, the phenomenon of storm surge is called storm tide. Storm tide is actually what we observe when a hurricane makes landfall.

Did you know that areas along the Georgia Coast have the second highest potential storm tide heights of area along the U.S. East Coast? The towns of Meridian and Darien, GA and the barrier islands of Blackbeard and Sapelo have the potential for 32 feet of storm tide during a Category 5 hurricane. Most areas along the Georgia Coast can have 20 feet or more of storm tide during a Category 4 hurricane. In addition, because the coastal areas of Georgia are lowlands, storm tide can travel over 20 miles inland.

Storm tide is the most dangerous hazard that a hurricane creates. Water is extremely heavy and when moving into communities, it can be very destructive. Wind-driven waves as high as 10 to 15 feet accompany storm tide adding to its destructive potential.

Did you know that one cubic yard of water (a box that measures 3 feet on all sides) weighs as much as a small car? It only takes moving water that is one foot deep to push a car off the road. Because storm surge is so destructive, coastal communities must evacuate when a hurricane threatens – we don’t evacuate coastal communities for hurricane winds. Emergency managers say “Run from the water, hide from the wind!”

If you are a coastal resident in Georgia, look at your coastal county storm surge inundation map below. One is provided for each of the six coastal counties in Georgia: Byran, Camden, Chatham, Glynn, Liberty, and McIntosh.

Click Here for Evacuation Information

For more information about storm surge, visit the National Hurricane Center’s website on hurricane preparedness by clicking here.


Hurricane Winds

The greatest cause for damage outside of coastal areas is hurricane-force winds. In fact, a hurricane is rated based upon the intensity of the winds using the famous Saffir-Simpson Hurricane Wind Scale. In the Saffir-Simpson Scale, winds are rated from a Category 1 with sustained wind speeds of 74 to 95 mph up to a Category 5 with sustained wind speeds greater then 155 mph. Tropical storm-force winds (39 to 73 mph) are dangerous enough to pose a threat to life safety.

Did you know that a Category 4 hurricane can cause 100 times the damage of a Category 1 hurricane?

When viewed from above, the winds in a hurricane near the surface move counter-clockwise around the center, and clockwise aloft. Because a hurricane moves forward, the winds on

the right side near the surface are moving faster than on the left side. After a hurricane makes landfall, it weakens rapidly. However, fast moving hurricanes can bring hurricane-force winds all the way to Atlanta and beyond.

Saffir-Simpson Hurricane Wind Scale with Georgia Storm Tide.

Saffir-Simpson Category
Wind Speeds
Expected Damage from Wind
Storm Tide on the Georgia Coast
Category 1
74 to 95 mph
Some Damage
Up to 10 feet
Category 2
96 to 110 mph
Extensive
Up to 15 feet
Category 3
110 to 130 mph
Devastating
Up to 20 feet
Category 4
131 to 155 mph
Catastrophic
Up to 25 feet
Category 5
More than 155 mph
Catastrophic
Up to 30 feet


To understand how strong winds from a hurricane can be in inland area, maps showing the Maximum Envelope of Winds (MEOW) were developed. These maps show what the winds can be at a given location if the center of the storm passes overhead.


Inland Flooding

Tropical storms and hurricanes are notorious for producing extreme amount of rainfall leading to widespread flooding. In July of 1994, Tropical Storm Alberto produced catastrophic flooding in the Southeast U.S. leading to the costliest natural disaster in Georgia’s history.

According to a Natural Disaster Survey Report produced by the NWS, the catastrophic flooding produced by Tropical Storm Alberto triggered a Presidential Major Disaster Declaration (PDD) for 78 counties in Georgia, Florida, and Alabama (55 in Georgia). It produced many impacts including $750 million dollars ($1.2 billion dollars, inflation adjusted for 2000) in total estimated damages, 33 fatalities (31 in Georgia), 50,000 people evacuated, 18,000 homes destroyed, 900,000 acres of crops destroyed, 1000 roads washed out, and over 200 small dams damaged or destroyed.

The amount of rainfall possible from a tropical cyclone is unimaginable. Tropical Storm Alberto (1994) produced a 24-hour rainfall total of 24.23 inches in Americus, Georgia (27.61 inches for a storm total). The Isla Mujeres on the Yucatan Peninsula reported that Hurricane Wilma (2005) produced 62.05 inches of precipitation in a 24-hour period. Tropical Cyclone Denise (1966) produced 45.0 inches of precipitation in 12 hours, and 71.8 inches of rain in 24 hours. Tropical Cyclone Hyacinthe (1980) produced a 10-day total of 223.5 inches of precipitation.

Tropical storms and hurricanes generally produce more rain at a given location if they are moving slowly or have stalled – tropical storms or hurricanes that are moving more quickly have less time over one location to produce rain and are less likely to produce flooding.

The entire state of Georgia is vulnerable to inland flooding produced by a tropical cyclone, particularly the northern Georgia which is mountainous and prone to deadly flash flooding. According to a study conducted by the National Hurricane Center, during the years from 1970 through 1999, over half (59%) of the more than 600 U.S. direct tropical cyclone related fatalities occurred due to inland flooding; more than three-quarters of the fatalities were children under the age of 13; and most occurred in inland counties.

Do not ever drive through flood waters or a flooded road. It only takes moving water that is one foot deep to push a car off the road. Turn Around, Don’t Drown.

If you live in an inland area of Georgia and are not sure in you live in or near a flood plain, check out these Flood Insurance Rate Maps that FEMA produced by clicking here.

For more information about inland flooding, visit the National Hurricane Center’s website on hurricane preparedness by clicking here.

Also, visit the National Weather Service’s Southeast River Forecast Center for current information about potential or ongoing flooding in the Southeast U.S.

Tornadoes

Tornadoes are commonly produced by landfalling tropical cyclones. Those making landfall along the Gulf coast traditionally produce more tornadoes than those making landfall along the Atlantic coast. While some hurricanes do not produce any tornadoes, studies have shown that more than half of landfalling hurricanes produce at least one.

Did you know that a hurricane can produce over 100 tornadoes after landfall? According to the National Hurricane Center, Hurricane Ivan (2004) produced 127 tornadoes (25 in Georgia), Hurricane Beulah (1967) spawned 115 tornadoes, and Hurricane Frances (2004) spawned 106 tornadoes. Even though Hurricane Katrina (2005) made landfall in Louisiana (roughly 280 miles from Georgia), it spawned 20 tornadoes in Georgia – the highest number in history for the month of August.

Tornadoes that form within hurricanes are more common in the right front quadrant with respect to the forward direction, but can occur in the outer rainbands as well. According to the National Hurricane Center, about 10% of the tropical cyclone-related fatalities are caused by tornadoes. Tornadoes are more likely to be spawned during the 24 hours after landfall and are usually within 30 miles of the tropical cyclone’s center.

Did you know that tornadoes can produce winds in excess of 200 mph (EF5 on the Enhanced Fujita Scale and can be very expansive – some in the Great Plains have been bigger than two miles wide. However, tornadoes associated with tropical cyclones tend to be much smaller and weaker – EF0 to EF2 are most likely.


Did you know that there are six National Weather Service offices that create forecasts for different portions of Georgia? The Peachtree City, GA office covers northern and central; the Tallahassee, FL office covers the southwest; the Jacksonville, FL office covers the southeast and southern coast; the Charleston, SC office covers eastern and the northern coast; the Columbia, SC office covers a small portion of the east; and the Greenville-Spartenburg, SC office covers a small portion of the northeast.

You can each of their websites by clicking on their names here: Peachtree City (http://www.srh.noaa.gov/ffc/), Tallahassee (http://www.srh.noaa.gov/tlh/), Jacksonville (http://www.srh.noaa.gov/jax/), Charleston http://www.srh.noaa.gov/chs/), Columbia (http://www.srh.noaa.gov/cae/), Greenville-Spartenburg (http://www.srh.noaa.gov/gsp/).

Visit the National Hurricane Center’s website on hurricane preparedness (http://www.nhc.noaa.gov/HAW2/english/tornadoes.shtml) for more information about tornadoes and hurricanes.

Visit the Storm Prediction Center (http://www.spc.noaa.gov/) if you want to learn more about tornadoes, find current forecasts for them, or find out if there are any ongoing severe thunderstorm or tornado watches. Click here for Tornado Safety Tips (http://www.spc.noaa.gov/faq/tornado/safety.html).

Want to teach your kids about tornadoes? Visit FEMA for Kids (http://www.fema.gov/kids/tornado.htm) to learn more.


Other Resources

Visit the Ready Georgia website to learn more about hurricanes and how to be prepared for them. Be Prepared. Have a Plan. Stay Informed.

Visit the National Weather Service (http://www.nws.noaa.gov/) if you want to know if there is any severe weather in your area, or to obtain your current forecast.

Visit FEMA (http://www.fema.gov/areyouready/tornadoes.shtm) to learn more about how to be ready for tornadoes.

Take it to the next level! Take FEMA’s interactive web-based Independent Study (IS) 324 Community Hurricane Preparedness (http://training.fema.gov/EMIWEB/IS/is324a.asp) course.