Hurricanes

Hurricanes

In September 2003, Hurricane Isabel caused 32 deaths and approximately $1.9 billion in damages to homes, businesses and public facilities in Virginia, affecting 75 percent of the state. Yet, this deadly storm was only a Category 1 hurricane when it reached Virginia.

Hurricanes aren’t the only tropical systems that warrant caution. Tropical Depression Gaston (2004) and Tropical Storms Jeanne (2004) and Floyd (1999) all caused major damage and prompted federal disaster declarations.

• What are tropical storms?
• What areas in Virginia are most vulnerable?
• What are the likely impacts of a hurricane?
• Warnings and advisories
• How are evacuation decisions made?

What are tropical storms?

Tropical depressions, tropical storms and hurricanes are low pressure areas that develop in the tropical regions of the ocean (between 20 degrees N latitude and the equator). These storms are masses of thunderstorms that organize and begin to rotate. These systems, in order of intensity, are called depressions (winds between 25 and 38 mph), tropical storms (winds between 39 and 73 mph) and hurricanes (winds of 74 mph or greater).

The National Weather Service tracks these storms on charts during hurricane season, June 1 through Nov. 30, using the following symbols.

A tropical depression, the lowest intensity, is given a number once it has a counterclockwise spin and winds of 38 mph or less.

When wind speeds reach 39 mph and the storm is given a name from a pre-determined list, a tropical storm is born. While a tropical storm does not produce a high storm surge, its thunderstorms can still pack a dangerous and deadly punch. In 1972, Agnes was only a tropical storm when it dropped torrential rains that led to devastating floods in Pennsylvania, Maryland and Virginia. Sixteen people died in Virginia and damage was estimated at $222 million.

A hurricane is the most intense tropical event, with five categories and winds ranging from 74 mph to 157 mph or greater. Storm surge is a major concern with hurricanes. The extremely high winds cause ocean water to pile up, creating higher than normal sea levels with waves up to 40 feet in open water. High sea levels and shallow waters can devastate a coastline and bring ocean water miles inland.

A hurricane's bands of thunderstorms produce torrential rains and sometimes tornadoes. A foot or more of rain could fall in less than a day, causing flash floods and mudslides. Large rivers in the hurricane's path might still be flooding for days after the storm has passed. The storm's driving winds can topple trees, utility poles and damage buildings. Communication and electricity might be lost for days and roads are often impassable due to fallen trees and debris.

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What areas in Virginia are most vulnerable?

Though it might seem that only the shoreline is vulnerable to the effects of hurricanes, all of Virginia can be affected by tropical systems. As we saw in 2004 with Tropical Depression Gaston and Tropical Storm Jeanne, even weakened storm systems can cause problems for inland areas. Both of these storms resulted in federal disaster declarations, loss of life and tremendous property damage.

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What are the likely impacts of a hurricane?

Hurricanes are rated on the Saffir-Simpson Hurricane Scale based on the storm's intensity. This 1 to 5 rating scale estimates potential property damage. Hurricanes reaching Category 3 and higher are considered major hurricanes because of potential for loss of life and damage, though Category 1 and 2 storms are still very dangerous and should not be taken lightly.

Category 1: Winds of 74 to 95 mph

• Potential for minimal damage.
• Expect: Trees, shrubbery, foliage and unanchored mobile home damage. Low-lying coastal roads inundated, minor pier damage. Storm surge four to five feet above normal tide level.

Category 2: Winds of 96 to 110 mph

• Potential for moderate damage.
• Expect: Considerable damage to shrubbery and trees, with some trees uprooted. Major damage to exposed mobile homes. Some damage to roofing materials. Coastal roads and low-lying escape routes inland cut by rising water two to four hours before arrival of hurricane center. Considerable damage to and flooding of piers and marinas. Evacuation of some shoreline homes in low-lying areas. Storm surge of six to eight feet above normal tide level.

Category 3: Winds of 111 to 129 mph (96-112 kt, 178-208 km/h)
 

• Potential for extensive damage.
• Expect: Foliage torn from trees with some large trees blown down. Moderate damage to roofing materials, windows and doors. Some structural damage to small buildings, and mobile homes destroyed. Serious flooding at coast and many small structures near coast destroyed. Larger structures near coast damaged by waves and floating debris. Major erosion of beaches with low-lying escape routes inland cut by rising water three to five hours before hurricane center arrives. Evacuation of all residences within 500 yards of shore and single-story homes on low ground within two miles of shore possible. Storm surge nine to 12 feet above normal tide level.

Category 4: Winds of 130 to 156 mph (113-136 kt, 209-251 km/h)
 

• Potential for extreme damage.
• Expect: Shrubs, trees and signs blown down. Complete failure of roofs on small homes. Mobile homes destroyed. Flat terrain 10 feet or less above sea level flooded inland up to six miles. Major damage to lower floors of structures near shore due to flooding and floating debris. Major erosion of beaches. Low-lying escape routes inland cut by rising water three to five hours before hurricane center arrives. Evacuation of all homes up to 500 yards from shore and single-story homes on low ground up to two miles from shore possible. Storm surge 13 to 18 feet above normal tide level.

Category 5: Winds greater than 157 mph (137 kt or higher, 252 km/h or higher)

Potential for catastrophic damage.

• Expect: Shrubs and trees blown down, considerable damage to roofs. Mobile homes destroyed. Major damage to lower floors of all structures less than 15 feet above sea level within 500 yards of shore. Low-lying escape routes inland cut by rising water three to five hours before hurricane center arrives. Massive evacuation of residential areas on low ground within five to 10 miles of shore possibly required. Storm surge greater than 18 feet above normal tide level.

The flooding and high winds associated with hurricanes may also disrupt the distribution of gasoline, kerosene, diesel fuel, fuel oils, propane and other petroleum products. This disruption could cause major problems for organizations and businesses that rely on such supplies. Additionally, such a disruption could affect backup power generation.

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Warnings and advisories

Know the difference between "watches" and "warnings."

• Tropical Storm Watch: An alert for a specific area that a tropical storm might pose a threat within 48 hours.
• Tropical Storm Warning: An alert that tropical storm conditions, including sustained winds of 39 to 73 mph, are expected in specific areas within 36 hours.
• Hurricane Watch: An alert for a specific area that hurricane conditions pose a threat within 48 hours.
• Hurricane Warning: An alert that hurricane conditions are expected in a specified coastal area within 36 hours. All precautions should be completed immediately.
• Evacuation Order: The most important instruction you will receive. If issued, leave immediately.

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How are evacuation decisions made?

Local and state officials, along with the National Weather Service, make the decision to issue an evacuation order. In making this decision, officials consider:

• Providing enough time for people in storm surge zones and mobile homes to leave before the arrival of 39 mph winds;
• Selecting an appropriate evacuation time to allow citizens to get to safety during daylight hours; and
• Providing the news media with enough time to warn the greatest number of people.

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