October 24, 2012
What is a Tropical Cyclone?
Tropical cyclones are among nature’s most powerful and destructive phenomena. If you live in an area prone to tropical cyclones, you need to be prepared. Even areas well away from the coastline can be threatened by destructive winds, tornadoes and flooding from these storms. How great is the danger? For 1970-2010, the average numbers per year were as follows:
„„Atlantic Ocean, Caribbean or Gulf of Mexico: 11 tropical storms, 6 of which became hurricanes „„East Pacific Ocean: 15 tropical storms, 8 of which became hurricanes „„Central Pacific Ocean: 4 tropical storms, 2 of which became hurricanes Over a typical 2-year period, the U.S. coastline is struck by an average of 3 hurricanes, 1 of which is classified as a major hurricane.
While hurricanes pose the greatest threat to life and property, tropical storms and depressions also can be devastating. Floods from heavy rains and severe weather, such as tornadoes, can cause extensive damage and loss of life. For example, Tropical Storm Allison produced over 40 inches of rain in the Houston area in 2001, causing about $5 billion in damage and taking the lives of 41 people.
Tropical cyclones forming between 5 and 30 degrees North latitude typically move toward the west. Sometimes the winds in the middle and upper levels of the atmosphere change and steer the cyclone toward the north and northwest. When tropical What is a Tropical Cyclone? cyclones reach latitudes near 30 degrees North, they often move northeast. Hurricane seasons and their peaks are as follows:
Atlantic and Caribbean: June 1 to November 30 with peak season mid- August to late October.
„„Central Pacific (Hawaii): June 1 to November 30 with peak season from July to September. „„East Pacific: May 15 to November 30
„„Western North Pacific: Tropical cyclones can strike year round Understanding the Terminology A tropical cyclone is a rotating, organized system of clouds and thunderstorms that originates over tropical or subtropical waters and has a closed low-level circulation.
Tropical cyclones rotate counterclockwise in the Northern Hemisphere. They are classified as follows: Tropical Depression—A tropical cyclone with maximum sustained winds of 38 mph (33 knots) or less.
„„Tropical Storm— A tropical cyclone with maximum sustained winds of 39 to 73 mph (34 to 63 knots).
„„Hurricane—A tropical cyclone with maximum sustained winds of 74 mph (64 knots) or higher. In the western North Pacific, hurricanes are called typhoons; similar storms in the Indian Ocean and South Pacific Ocean are called cyclones.
„„Major Hurricane—A tropical cyclone with maximum sustained winds of 111 mph (96 knots)
Tropical cyclone formation regions with mean tracks/NWS JetStream Online School
Storm Surge/Tide Storm surge and large waves produced by hurricanes pose the greatest threat to life and property along the coast.
STORM SURGE is an abnormal rise of water generated by a storm’s winds. Storm surge can reach heights well over 20 feet and can span hundreds of miles of coastline. In the northern hemisphere, the highest surge values typically occur in the right front quadrant of a hurricane coincident with onshore flow; in the southern hemisphere, the left front quadrant.
More intense and larger hurricanes produce higher surge. In addition, shallower offshore waters contribute to higher storm surge inundation. Storm surge is by far the greatest threat to life and property along the immediate coast. STORM TIDE is the water level rise during a storm due to the combination of storm surge and the astronomical tide.
For example, if a hurricane moves ashore at a high tide of 2 feet, a 15 foot surge would be added to the high tide, creating a storm tide of 17 feet. The combination of high winds and storm tide topped with battering waves can be deadly and cause tremendous property damage along an area of coastline hundreds of miles wide.
The destructive power of storm surge and large battering waves can result in loss of life, buildings destroyed, beach and dune erosion and road and bridge damage along the coast. Storm surge can travel several miles inland. In estuaries and bayous, salt water intrusion endangers public health and the environment.
Are You Ready? Before the Hurricane Season
>Determine safe evacuation routes inland.
>Learn locations of official shelters.
>Check emergency equipment, such as flashlights, generators and battery-powered equipment such as cell phones and your NOAA Weather Radio All Hazards receiver.
>Buy food that will keep and store drinking water.
>Buy plywood or other material to protect your home if you don’t already have it.
>Trim trees and shrubbery so branches don’t fly into your home.
>Clear clogged rain gutters and downspouts.
>Decide where to move your boat.
>Review your insurance policy.
>Find pet-friendly hotels on your evacuation route.
During the Storm
When in a Watch Area…
>Frequently listen to radio, TV or NOAA Weather Radio All Hazards for official bulletins of the storm’s progress.
>Fuel and service family vehicles.
>Inspect and secure mobile home tie downs.
>Ensure you have extra cash on hand.
>Prepare to cover all windows and doors with shutters or other shielding materials.
>Check batteries and stock up on canned food, first aid supplies, drinking water and medications.
>Bring in light-weight objects such as garbage cans, garden tools, toys and lawn furniture.
When in a Warning Area
>Closely monitor radio, TV or NOAA Weather Radio All Hazards for official bulletins.
>Close storm shutters.
>Follow instructions issued by local officials. Leave immediately if ordered!
>Stay with friends or relatives at a low-rise inland hotel or at a designated public shelter outside the flood zone.
>DO NOT stay in a mobile or manufactured home.
>Notify neighbors and a family member outside of the warned area of your evacuation plans.
>Take pets with you if possible, but remember, most public shelters do not allow pets other than those used by used by people with disabilities. Identify pet-friendly hotels along your evacuation route.
Plan to Leave if You...
>Live in a mobile home. They are unsafe in high winds no matter how well fastened to the ground.
>Live on the coastline, an offshore island or near a river or a flood plain.
>Live in a high rise building. Hurricane winds are stronger at higher elevations.
A PREPAREDNESS GUIDE U.S. DEPARTMENT OF COMMERCE National Oceanic and Atmospheric Administration National Weather Service