Hurricanes

What Is A Hurricane?

A hurricane is a tropical storm with winds that have reached a constant speed of 74 miles per hour or more. Hurricane winds blow in a large spiral around a relative calm center known as the "eye." The "eye" is generally 20 to 30 miles wide, and the storm may extend outward 400 miles. As a hurricane approaches, the skies will begin to darken and winds will grow in strength. As a hurricane nears land, it can bring torrential rains, high winds, and storm surges. A single hurricane can last for more than 2 weeks over open waters and can run a path across the entire length of the eastern seaboard. August and September are peak months during the hurricane season that lasts from June 1 through November 30.

Hurricane Threats

The 74 to 160 mile per hour winds of a hurricane can extend inland for hundreds of miles. Hurricanes can spawn tornadoes, which add to the destructiveness of the storm. Floods and flash floods generated by torrential rains also cause damage and loss of life. Following a hurricane, inland streams and rivers can flood and trigger landslides. Even more dangerous than the high winds of a hurricane is the storm surge-a dome of ocean water that can be 20 feet at its peak and 50 to 100 miles wide. The surge can devastate coastal communities as it sweeps ashore. Nine out of 10 hurricane fatalities are attributable to the storm surge.

  • Hurricane Winds

    Coastal communities deciding how strong their structures should be need to consider the strength of hurricane winds and the pressure they generate. As winds increase, pressure against objects is added at a disproportionate rate. Pressure against a wall mounts with the square of windspeed so that a threefold increase in windspeed gives a nine-fold increase in pressure. Thus, a 25 mph wind causes about 1.6 pounds of pressure per square foot. A four by eight sheet of plywood will be pushed by a weight of 50 pounds. In 75 mph winds, that force becomes 450 pounds, and in 125 mph winds, it becomes 1,250 pounds. For some structures, this force is enough to cause failure. These winds will weaken after landfall due to loss of warm-water energy source; and the encountering of great friction over land.

  • Rainfall and Flooding

    Heavy rains and ocean waters brought ashore by strong winds can cause flooding in excess of 50 cm (20 in) over a 24-hour period. The runoff systems in many cities are unable to handle such an increase in water because of the gentle topography in many of the coastal areas where hurricanes occur. Hurricanes are capable of producing copious amounts of flash flooding rainfall. During landfall, a hurricane rainfall of 10 to 15 inches or more is common. If the storm is large and moving slowly-less than 10 mph-the rainfall amounts from a well-organized storm are likely to be even more excessive. To get a generic estimate of the rainfall amount (in inches) that can be expected, divide the storm's forward motion by 100, i.e. Forward Speed/100 = estimated inches of rain. Rainfall and Flooding fact: Tropical Storm Claudette (1979) brought 45 inches of rain to an area near Alvin, Texas, contributing to more than $600 million in damage.

    The heaviest rain usually occurs along the coastline, but sometimes there is a secondary maximum further inland. This heavy rain usually occurs slightly to the right of the cyclone track and usually occurs between 6 hours before and 6 hours after landfall. The amount of rain depends on the size of the cyclone, the forward speed of the cyclone and whether it interacts with a cold front. Interaction with a cold front will not only produce more tornadoes but more rainfall as well.

  • Storm Surge

    Storm surge is an abnormal increase in the ocean's level, sometimes in excess of several meters high and miles wide. Storm surges can come ashore up to five hours before the storm and destroy low-elevation coastal areas. It is especially damaging when the storm surge occurs during high tide and consequently is often responsible for most hurricane-related deaths. Storm surge is a large dome of water often 50 to 100 miles wide that sweeps across the coastline near where a hurricane makes landfall. Storm surge can range from 4 to 6 feet for a minimal hurricane to greater than 20 feet for the stronger ones. The surge of high water topped by waves is devastating. The stronger the hurricane and the shallower the offshore water, the higher the surge will be. Along the immediate coast, storm surge is the greatest threat to life and property, even more so than the high winds.

    • Over 6000 people were killed in the Galveston Hurricane of 1900, most by storm surge.
    • Hurricane Camille produced a 25-foot storm surge in Mississippi.
    • Hurricane Hugo in 1989 generated a 20-foot storm tide in South Carolina.

     

  • Tornadoes

    Hurricanes also produce tornadoes, which add to the hurricane's destructive power. Typically, the more intense a hurricane is, the greater the tornado threat. When a hurricane brings its winds inland, the fast-moving air hits terrain and structures, causing a frictional convergence which enhances lifting. Frictional convergence may be at least a contributing factor to tornado formation in hurricanes. The greatest concentration of tornadoes occurs in the right front quadrant of the hurricane. A number of theories exist about their origin, but in the case of Hurricane Andrew, severe damage was inflicted by small spin-up vortices that developed in regions of strong wind-shear found in the hurricane's the eye wall. The strong damaging winds of the hurricane frequently cover the smaller tornado paths, making the separation of their damaging effects very difficult.

    Tropical Cyclone Spawned Tornadoes Facts

    • 10% of deaths in the United States are associated with hurricanes are a result of tornadoes.
    • Most tornadoes occur within 24 hours after hurricane landfall. The exception is when there is interaction with a cold front after landfall. Then more tornadoes will occur two or three days after landfall, well inland.
    • Most tornadoes occur within 150 miles of the coastline.
    • More tornadoes occur during the morning and afternoon rather than evening or night due to the need for a tornado to have a heat source.
    • The Gulf of Mexico hurricanes produce more tornadoes than Atlantic storms.
    • The majority of tornadoes occur within 30 miles of the center of the cyclone, but there is a secondary maximum further away in the outer rain bands (100-150 miles away from the center).
    • Tornado winds can reach up to 300 mph at a forward speed of 60 mph and are usually 100-300 yards wide.

     

  • Inland Flooding From Hurricanes

    The next time you hear hurricane -- think inland flooding!

    While storm surge has been the number one cause of hurricane related deaths in the past, more people have died from inland flooding associated with tropical systems in the last 30 years. Since the 1970's, inland flooding has been responsible for more than half of all deaths associated with tropical cyclones in the United States. Flooding from hurricanes can occur hundreds of miles from the coast placing communities, which would not normally be affected by the strongest hurricane winds, in great danger.

    Some of the greatest rainfall amounts associated with tropical systems occur from weaker Tropical Storms that have a slow forward speed (1 to 10mph) or stall over an area. Due to the amount of rainfall a Tropical Storm can produce, they are capable of causing as much damage as a category 2 hurricane.

    Facts About Inland Flooding From Hurricanes

    • Freshwater floods accounted for more than half (59%) of U.S. tropical cyclone deaths over the past 30 years. These floods are why 63% of U.S. tropical cyclone deaths during that period occurred in inland counties.
    • Over the past 30 years, 78% of children killed by tropical cyclones drowned in freshwater floods.
    • One cubic yard of water weighs 1700lbs. The average automobile weighs 3400lbs. Many automobiles will float in just 2 feet of water.
    • The average person can be swept off their feet in 6 inches of moving water.
    • The average automobile can be swept off the road in 12 inches of moving water.
    • The average automobile can be swept off the road in 12 inches of moving water. At least 23% of U.S. tropical cyclone deaths occur to people who drown in, or attempting to abandon, their cars.
    • Rainfall is typically heavier with slower moving storms.
    • Rainfall Rule of Thumb: to estimate the total amount of rainfall that can be expected from a tropical system, divide 100 by the forward speed of the storm in miles per hour. Example (100/Forward Speed = estimated inches of rain). Your local NWS forecast office will have a more accurate estimation method to predict rainfall.

What Should I Do?

Before Hurricane Season Starts

  • Plan an evacuation route.
    • Contact the local emergency management office or American Red Cross chapter, and ask for the community hurricane preparedness plan. This plan should include information on the safest evacuation routes and nearby shelters.
  • Learn safe routes inland.
    •   Be ready to drive 20 to 50 miles inland to locate a safe place.
    • Have disaster supplies on hand.
      • Flashlight and extra batteries
      • Portable, battery-operated radio and extra batteries
      • First aid kit and manual
      • Emergency food and water
      • Non-electric can opener
      • Essential medicines
      • Cash and credit cards
      • Sturdy shoes
  • Make arrangements for pets.
    • Pets may not be allowed into emergency shelters for health and space reasons.
    • Contact your local humane society for information on local animal shelters.
  • Make sure that all family members know how to respond after a hurricane.
    • Teach family members how and when to turn off gas, electricity, and water.
    • Teach children how and when to call 9-1-1, police, or fire department and which radio station to tune to for emergency information.
  • Protect your windows.
    • Permanent shutters are the best protection. A lower-cost approach is to put up plywood panels. Use 1/2 inch plywood - marine plywood is best - cut to fit each window. Remember to mark which board fits which window.
    • Pre-drill holes every 18 inches for screws. Do this long before the storm.
  • Trim back dead or weak branches from trees.
  • Check into the purchase of Flood Insurance.
    • You can find out about the National Flood Insurance Program through your local insurance agent or emergency management office. There is normally a 30-day waiting period before a new policy becomes effective.
    • Homeowners polices do not cover damage from the flooding that accompanies a hurricane.
  • Develop an emergency communication plan.
    • In case family members are separated from one another during a disaster (a real possibility during the day when adults are at work and children are at school), have a plan for getting back together.
    • Ask an out-of-state relative or friend to serve as the "family contact." After a disaster, it's often easier to call long distance. Make sure everyone in the family knows the name, address, and phone number of the contact person.

During A Hurricane Watch

A Hurricane Watch is issued when there is a threat of hurricane conditions within 24-36 hours.

  • Listen to a battery-operated radio or television for hurricane progress reports.
  • Check emergency supplies.
  • Fuel car.
  • Bring in outdoor objects such as lawn furniture, toys, and garden tools and anchor objects that cannot be brought inside.
  • Secure buildings by closing and boarding up windows. Remove outside antennas.
  • Turn refrigerator and freezer to coldest settings. Open only when absolutely necessary and close quickly.
  • Store drinking water in clean bathtubs, jugs, bottles, and cooking utensils.
  • Store valuables and personal papers in a waterproof container on the highest level of your home.
  • Review evacuation plan.
  • Moor boat securely or move it to a designated safe place. Use rope or chain to secure boat to trailer. Use tiedowns to anchor trailer to the ground or house.

During A Hurricane Warning

A Hurricane Warning is issued when hurricane conditions (winds of 74 miles per hour or greater, or dangerously high water and rough seas) are expected in 24 hours or less.

  • Listen constantly to a battery-operated radio or television for official instructions.
  • If in a mobile home, check tiedowns and evacuate immediately.
  • Avoid elevators.
  • If at home:
    • Stay inside, away from windows, skylights, and glass doors.
    • Keep a supply of flashlights and extra batteries handy. Avoid open flames, such as candles and kerosene lamps, as a source of light.
    • If power is lost, turn off major appliances to reduce power "surge" when electricity is restored.
  • If officials indicate evacuation is necessary:
    • Leave as soon as possible. Avoid flooded roads and watch for washed-out bridges.
    • Secure your home by unplugging appliances and turning off electricity and the main water valve.
    • Tell someone outside of the storm area where you are going.
    • If time permits, and you live in an identified surge zone, elevate furniture to protect it from flooding or better yet, move it to a higher floor.
    • Take pre-assembled emergency supplies, warm protective clothing, blankets and sleeping bags to shelter.
    • Lock up home and leave.

After The Storm: Wind & Flood - Rebuilding Techniques

  • Stay tuned to local radio for information.
  • Help injured or trapped persons.
  • Give first aid where appropriate.
  • Do not move seriously injured persons unless they are in immediate danger of further injury. Call for help.
  • Return home only after authorities advise that it is safe to do so.
  • Avoid loose or dangling power lines and report them immediately to the power company, police, or fire department.
  • Enter your home with caution. Beware of snakes, insects, and animals driven to higher ground by flood water.
  • Open windows and doors to ventilate and dry your home.
  • Check refrigerated foods for spoilage.
  • Take pictures of the damage, both to the house and its contents for insurance claims.
  • Drive only if absolutely necessary and avoid flooded roads and washed-out bridges.
  • Use telephone only for emergency calls.

Inspecting Utilities In A Damaged Home

  • Check for gas leaks--If you smell gas or hear blowing or hissing noise, open a window and quickly leave the building. Turn off the gas at the outside main valve if you can and call the gas company from a neighbor's home. If you turn off the gas for any reason, it must be turned back on by a professional.
  • Look for electrical system damage--If you see sparks or broken or frayed wires, or if you smell hot insulation, turn off the electricity at the main fuse box or circuit breaker. If you have to step in water to get to the fuse box or circuit breaker, call an electrician first for advice.
  • Check for sewage and water lines damage--If you suspect sewage lines are damaged avoid using the toilets and call a plumber. If water pipes are damaged, contact the water company and avoid the water from the tap. You can obtain safe water by melting ice cubes.