Tropical Weather Basics

Hurricanes have the potential to cause more death and destruction than any other natural or man-made threat to Southwest Florida. A Hurricane is a tropical storm with sustained winds of 74 mph or greater. The winds blow counter-clockwise around a storm center, known as the Eye. The strongest winds are normally near the eye, but winds over 39 miles per hour (Tropical Storm Force) may extend out 150 miles from the eye.

A typical hurricane starts as a Tropical Wave. If the tropical wave becomes better organized, it can become a Tropical Depression. If conditions are favorable, the tropical depression can become a Tropical Storm, when it gets a name, and the tropical storm can become a Hurricane. Once a hurricane forms, it can last for weeks, until it encounters unfavorable conditions such as strong wind shear, cool water or it moves over land.

The Atlantic Hurricane Season runs from June 1 to November 30 with a peak in mid-August through mid-October. During June and July, hurricanes tend to form in the Caribbean and the Gulf of Mexico. By mid-August, as the tropical Atlantic warms, the focus turns to the area of the Cape Verde Islands near Africa. In October, the Atlantic begins to cool and the focus for storm development returns to the Caribbean and the Gulf of Mexico.

The National Hurricane Center monitors tropical storm development. The Center uses satellites, aircraft, radar, computer models and other tools to predict storm formation and to create track and intensity forecasts covering several days. While forecasts continue to improve, errors in both track and intensity can be large, especially at longer time periods. For this reason, one must not focus on the projected location points that comprise the Storm Track. Instead, watch the Forecast Error Cone that includes forecast errors.

The Saffir-Simpson Scale assigns hurricanes to five categories based on wind speed and damage potential. Descriptions of the categories are below:

q Category One - Winds 74-95 mph. Storm surge generally 4-5 ft above normal. Damage primarily to unanchored mobile homes, shrubbery and trees. Some damage to poorly constructed signs. Some coastal road flooding and minor pier damage.

q Category Two - Winds 96-110 mph. Storm surge generally 6-8 feet above normal. Some damage to building roofing material, doors and windows. Considerable damage to shrubbery and trees. Some trees blown down. Considerable damage to mobile homes, poorly constructed signs and piers. Coastal and low-lying escape routes flood 2-4 hours before arrival of the hurricane center. Small craft in unprotected anchorages break moorings.

q Category Three - Winds 111-130 mph. Storm surge generally 9-12 ft above normal. Some structural damage to residences and utility buildings. Minor amount of curtain wall failures. Damage to shrubbery with foliage blown off trees and large trees blown down. Mobile homes and poorly constructed signs destroyed. Low-lying escape routes cut off by rising water 3-5 hours before arrival of the hurricane center. Flooding near the coast destroys smaller structures with larger structures damaged by battering from floating debris. Terrain lower than 5 feet above mean sea level may be flooded inland 8 miles or more. Evacuation of residences within several blocks of the shoreline may be required.

q Category Four - Winds 131-155 mph. Storm surge generally 13-18 ft above normal. More extensive curtain wall failures. Some complete roof structure failures. Shrubs, trees and all signs blown down. Complete destruction of mobile homes. Extensive damage to building doors and windows. Low-lying escape routes may be cut off by rising water 3-5 hours before arrival of the hurricane center. Major damage to lower floors of structures near the shore. Terrain lower than 10 ft above sea level may flood, requiring massive evacuations as far inland as 6 miles.

q Category Five - Winds greater than 155 mph. Storm surge generally greater than 18 ft above normal. Complete roof failure on many buildings. Some complete building failures. All shrubs, trees and signs blown down. Complete destruction of mobile homes. Low-lying escape routes cut off by rising water 3-5 hours before arrival of the hurricane center. Major damage to lower floors of structures located less than 15 ft above sea level and within 500 yards of the shoreline. Massive evacuation of low areas within 10 miles of the shoreline may be required.

Information is provided to help you take appropriate protective actions. Here are a few definitions:

Advisories are issued by the National Hurricane Center for tropical depressions, storms and hurricanes every 6 hours. These increase to every 3 hours when landfall is within 24 hours.

· A Tropical Storm Watch is issued when tropical storm conditions, sustained winds of 39-73 mph, may pose a threat in the specified areas within 36 hours.

· A Tropical Storm Warning is issued when tropical storm conditions are expected in the specified area within 24 hours.

· A Hurricane Watch is issued up to 36 hours in advance of landfall and indicates that everyone in the designated area should be ready to take protective measures if a Hurricane Warning is issued.

· A Hurricane Warning is issued when hurricane conditions are expected in the designated area in 24 hours or less. When a Hurricane Warning is issued, precautions should be taken immediately. If a hurricane’s path is erratic, there may be only a few hours before the onset of hurricane conditions.

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