MARYLAND Natural Hazards Preparedness Guide Maryland Emergency Management Agency TROPICAL CYCLONES The Eyewall: This is the region where the strongest winds TROPICAL CYCLONES get as close to the center of the storm as they can. The eyewall Tropical cyclones, a general term for tropical storms consists of a ring of tall intense thunderstorms that produce and hurricanes, are low pressure systems that form over heavy rains and usually the strongest winds. Changes in the the tropics and sub-tropics. These storms are referred to structure of the eye and eyewall can cause changes in the as “cyclones” due to their rotation. Tropical cyclones are wind speed, which is an indicator of the storm’s intensity. among the most powerful and destructive meteorological The eye can grow or shrink in size and on occasion, double systems on earth. On average it takes about fi ve days for eyewalls can form. a tropical cyclone to reach its maximum potential intensity. Their destructive elements include very high winds, heavy These are curved bands of clouds rain, lightning, tornadoes, hail, and storm surge. The Spiral Rainbands: and thunderstorms trailing away from the eyewall in a spiral There are four stages of tropical cyclone in order of fashion. These bands are capable of producing heavy bursts development: of rain and wind, as well as tornadoes. There are sometimes gaps between spiral rainbands where no rain or intense Tropical Wave: a low pressure trough of persisting wind is found. winds that blow from east to west. Tropical Depression: a closed circulation with maxi- Typical hurricane strength mum sustained surface wind speed less than 39 mph. A Tropical Cyclone’s Size: tropical cyclones are about 300 miles wide, although they Tropical Storm: a closed circulation with a maximum sustained surface wind speed from 39-73 mph. can vary considerably. Size is not necessarily an indication of Hurricane: a closed circulation maximum sustained hurricane intensity. Hurricane Andrew (1992), the second surface wind greater than 74 mph. most devastating hurricane to hit the United States, next to Katrina (2005), was a relatively small hurricane. However, the hurricane’s destructive winds and rains covered a wide The main parts of a tropical Hurricane Structure: swath. For a large system, hurricane-force winds can extend cyclone are the rainbands, the eye, and the eyewall. Air outward more than 150 miles. The area over which tropical spirals in toward the center in a counter-clockwise pattern storm-force winds occur is even greater, ranging as far out in the northern hemisphere and out the top in the opposite as 300 miles from the eye of a large hurricane. direction. In the very center of the storm, sinking air forms a mostly cloud free region called the “eye”. Hurricane Season: The National Weather Service (NWS) National Hurricane Center defi nes June 1 through November 30 as the Atlantic hurricane season. September is the most active month for tropical cyclones in Maryland. National Hurricane Center records show that on average, ten tropical cyclones develop each year over the three primary regions for development, the Atlantic Basin, the Caribbean Sea and the Gulf of Mexico. Of these, on average, six storms strengthen to become hurricanes, and of these, two reach to a major hurricane intensity of category three or higher based on the Saffi r-Simpson Hurricane Wind Scale. The Eye: The hurricane’s center is a relatively calm, generally clear area of sinking air and light winds that usually What about Maryland? Generally, Maryland is spared do not exceed 15 mph and is typically 20-40 miles across. from land falling hurricanes because of the orientation of the An eye develops when maximum sustained wind speed Mid-Atlantic coastline, as well as the State’s latitude. Because exceeds 74 mph. the coast of North Carolina extends out into the Atlantic, Maryland Natural Hazards Preparedness Guide 2 MEMA – Maryland Emergency Management Agency TROPICAL CYCLONES storms trending up the coast tend to make landfall there Active watches and warnings are contained within the or pass offshore. Most hurricanes approaching Maryland text of the public advisory and in nearly all instances provide make landfall to the south and soon are downgraded to a 48-hour lead time for watches and a 36-hour lead time tropical storm or tropical depression status. for warnings. However, a number of factors point to the potential for increased danger from severe tropical cyclones in Tropical Cyclone Impacts: Tropical cyclone hazards Maryland. Steady population growth and continuing near- come in several forms: storm surge, high winds, fl ooding, shore development are increasing the risk of human injury and tornadoes. and property loss. There is also widespread agreement among climatologists that a gradual change in climate is Storm Surge: The intensity of tropical cyclones is measured occurring. Potential effects include the melting of polar ice, by wind velocity, although the greatest devastation is usually expansion of the oceans, and an overall rise in sea levels. associated with storm surge and fl ooding. For example, The slow sinking of land in the Chesapeake region, due to Hurricanes Agnes (1972) and Fran (1996) produced some the combined effects of ground water withdrawal and post- of the worst fl ooding ever recorded for western and central glacial rebound, effectively doubles the global rate of sea Maryland due to heavy rains. In terms of storm surge, the level rise in Maryland’s coastal areas. These factors increase August 1933 storm was particularly destructive, generating the vulnerability of coastal areas to storm surge. a 7-foot surge and waves over 20 feet high that created the inlet now separating Ocean City from Assateague Island. NOAA/NWS Annapolis, MD, Isabel 2003 MEMA, Hurricane Isabel, Eastern Shore Flooding Advisories, Watches, and Warnings: The National Hurricane Center issues tropical cyclone advisories once a Storm surge is an abnormal local rise in sea level. storm reaches the level of tropical depression. Advisories are The storm surge is caused by the difference in wind and issued every six hours; in the eastern time zone, advisories barometric pressure between a tropical system and the are issued at 5 and 11 a.m. and 5 and 11 p.m. The Center environment outside the system. The end result is that also issues intermediate advisories every three hours when water is pushed onto a coastline. The height of the surge coastal watches or warnings have been posted and every is measured as the difference from mean sea level and can two hours when coastal watches or warnings have been reach over 25 feet in extreme circumstances. The most posted and land-based Doppler radars have identifi ed a devastating storm surges occur just to the right of the eye reliable storm center. Additionally, special public advisories of a landfalling hurricane. For coastal areas, the storm surge may be issued at any time whenever a signifi cant change has is typically the most dangerous and damaging aspect of the been noted in the storm. storm. Maryland Natural Hazards Preparedness Guide 3 MEMA – Maryland Emergency Management Agency TROPICAL CYCLONES Strong southerly winds ahead of the hurricane can debris, such as wood and metal siding, and sending them push tides well above normal levels, causing extensive tidal hurling at high speeds into other structures. fl ooding along the length of the Chesapeake Bay. Storm Tropical storm-force winds are strong enough to be surges of 5 feet or more can occur over central portions of dangerous to those caught in them. It is for this reason, the Bay, inundating sections of Dorchester and Somerset emergency managers try to have their evacuations Counties. Associated excessive rainfall amounts across the completed and their emergency response personnel lower Eastern Shore can cause signifi cant crop damage and sheltered before the onset of tropical storm force winds inland fl ooding. (39 mph). In a hurricane, the strongest winds usually occur to the right side of the hurricane. The winds associated with a hurricane usually decrease signifi cantly within 12 hours after landfall. Nonetheless, winds can stay above hurricane strength well inland. Hurricane Hugo (1989) battered High Winds: Hurricanes are known for their damaging Charlotte, North Carolina, about 175 winds and they are rated in strength by their wind speed. The miles inland, with wind gusts to nearly wind experienced from a hurricane damages and destroys 100 mph. structures in two ways. First, many homes are damaged or The intensity of a landfalling hurricane is expressed in destroyed when high winds lift the roof off of the structure. terms of categories, relating to wind speeds and potential The process is called Bernoulli’s Principle, which states the damage as described in the Saffi r-Simpson Hurricane Wind faster the air moves across a structure, the pressure lowers Scale below. on the exposed side of the roof creating higher pressure HURRICANE CATEGORIES DAMAGE in the attic. This action is similar to that on the wing of an Category Wind Speed airplane. As wind fl ow and speed increase across the wing 1 74-95 mph Some damage of an airplane, it acquires lift. In the case of a roof, the lift 2 96-110 mph Extensive damage created pulls the roof free and it becomes airborne. With 3 111-130 mph Devastating Damage the roof gone, the walls can be more easily blown down by 4 131-155 mph Catastrophic Damage the force of the wind. 5 156+ mph Catastrophic Damage Before the Hurricane Season: ✔ Know the hurricane risks in your area. ✔ Learn safe evacuation routes inland. ✔ Learn the location of designated shelters. ✔ Review needs and working conditions of emergency equipment, such as fl ashlights and battery-powered radios.