Deparment of Emergency Management - Hurricanes in Hawaii

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Deparment of Emergency Management - Hurricanes in Hawaii

Postby makahava » Wed Jun 08, 2011 2:48 pm

Emergency Management


Major hurricanes are relatively rare events at any location. Residents of the Hawaiian Islands have a good chance of living many years without experiencing one. But none of our islands is immune. "Not here! We haven't had a hurricane in years," could be the most dangerous words you'll ever hear. It's best to be prepared. This could be the year.

Tropical cyclone is the general term that describes a low pressure system that originates over the tropical oceans. By international agreement, tropical cyclones are classified according to their intensity. The terms used are:

Tropical Depression:

An area of developing counterclockwise (in the northern hemisphere, clockwise in the southern hemisphere) wind circulation that may include localized rain and thunderstorms. Maximum sustained winds up to 38 MPH (33 Knots). It is assigned a number by the National Weather Service.

Tropical Storm:

A well defined area of counterclockwise rotating wind of 39-73 MPH (34-63 Knots). Usually includes rain and thunderstorms. It is assigned a name.


A severe tropical cyclone with sustained winds of 74 MPH (64 Knots) or greater. They can move rapidly and in an erratic manner. The major hazards include high winds, heavy rainfall, flooding, storm surge and high surf. If the hurricane has developed from a tropical storm, it keeps the same name.

Hurricanes are tropical cyclones in which winds reach sustained speeds of 74 miles per hour or more, and blow around a relatively calm center--the eye of the hurricane. Every year, these violent storms bring destruction to coastlines and islands in their erratic path.

Stated very simply, hurricanes are giant whirlwinds in which air moves in a large tightening spiral around a center of extreme low pressure, reaching maximum velocity in a circular band extending outward 20 or 30 miles from the rim of the eye. This circulation is counterclockwise in the Northern Hemisphere, and clockwise in the Southern Hemisphere. Near the center, hurricane winds may gust to more than 200 miles per hour, although storms reaching Hawaii have been less powerful than this. The entire storm dominates the ocean surface and lower atmosphere over tens of thousands of square miles.

The eye, like the spiral structure of the storm, is unique to hurricanes. Here, winds are light and skies are clear or partly cloudy. But this calm is deceptive, bordered as it is by maximum force winds and torrential rains. Many persons have been killed or injured when the calm eye lured them out of shelter, only to be caught in the maximum winds at the far side of the eye, where the winds blow from a direction opposite to that in the leading half of the storm.

What makes hurricanes the dangerous storms they are is that they combine the triple hazard of violent winds, torrential rains, and abnormally high waves and storm tides. Each of these by itself can pose a serious threat to life and property. Taken together they are capable of causing widespread destruction.

Hurricanes are categorized 1 through 5, by the Saffir/Simpson Scale, according to the amount of potential damage and wind speed.

The Categories Are
Description of Damage Wind Speeds (MPH) Storm Surge (feet) Examples
1 Minimal 74 - 95 4 - 5 Iwa, 92 MPH, Nov. 1982
2 Moderate 96 - 110 6 - 8 None
3 Extensive 111 - 130 9 - 12 Uleki, 128 MPH, Sep. 1992
4 Extreme 131 - 155 13 - 18 Iniki, 145 MPH, Sep. 1992
5 Catastrophic > - 155 > - 18 Emilia & Gilma, 161 MPH, Jul 94, John, 173 MPH Aug. 1994

In Hawaii, hurricane winds, especially where augmented by local terrain, have been very damaging to trees, vegetation, and crops, as well as to lightly built dwellings and other structures. Heavy and prolonged hurricane rains falling over our steep hillsides can cause landslides and severe flash flooding. Large swell moving out ahead of the hurricane may begin to reach island shores while the storm itself is still several hundred miles away. As the hurricane nears the coastline, rapidly rising water levels from above-normal storm tides and high wind-driven waves will inundate coastal areas, erode beaches, and pound and undermine waterfront structures, highways, and other facilities.

During the last 50 years many hurricanes and tropical storms have come close to the Hawaiian Islands, but only three have had direct impact. In all three cases, Kauai was the hardest hit, although Oahu suffered significant damages as well. Hurricane Iniki was by far the most destructive storm to strike Hawaii in recorded history, with widespread wind and water damage exceeding 2.2 billion dollars. Losses in Hurricane Dot, August of 1959 were about 6 million dollars. Hurricane Iwa, in November of 1982 caused over 250 million dollars in damages.

Other hurricanes have occasionally come close enough to cause relatively minor damage, mainly in coastal areas vulnerable to high waves. Thus, Hurricane NINA, in late November 1957, brought surf of 35 feet to Kauai's southern coast, while waves from Hurricane FICO in July 1978 damaged homes and roads on the Big Island's Ka'u coast when the storm itself was more than 400 miles to the southeast.

Tropical cyclones of less than hurricane strength also have been destructive. For example, in August 1958, flooding rains and high winds from a storm that crossed Hawaii Island caused more than $500 thousand in damage.

Most Central Pacific hurricanes originate near the coasts of Central America or southern Mexico. Long before reaching the Hawaiian area, however, many of these storms die off when they move northwestward over cooler water or encounter unfavorable atmospheric conditions. Of those that survive, most remain far enough away to spare us their effects. Some hurricanes form nearer the Hawaiian Islands, while a few, like NINA and IWA, originate far to the southwest.

Hurricane season begins in June and lasts through November in the Hawaiian Islands.

In some hurricane seasons, many Central Pacific tropical cyclones occur; in others, few or none. In 1978, for example, there were 13, three of them full-fledged hurricanes, while the following year had none. There is no way of telling in advance how active a hurricane season is likely to be.

Hurricanes begin as relatively small tropical cyclones, generally off the southwest coast of Mexico or west coast of Central America. Some have, however, slowly formed south of the state of Hawaii. They then drift to the west-northwest, imbedded in the westward-blowing tradewinds of the tropics. Under certain conditions these disturbances increase in size, speed, and intensity until they become full-fledged hurricanes.

The storms move forward very slowly in the tropics, and may remain almost stationary for short periods of time. The initial forward speed is usually 15 miles per hour or less. Then, as the hurricane moves farther from the Equator, its forward speed tends to increase; at middle latitudes it may exceed 50 miles per hour in extreme cases.

The great storms are driven by the heat released by condensing water vapor, and by external mechanical forces. Once cut off from the warm ocean, the storm begins to die, starved for water and heat energy, and dragged apart by friction as it moves over the land.

Resource: A pamphlet republished by the Federal Emergency Management Agency and Hawaii State Civil Defense in cooperation with the National Oceanic and Atmospheric Administration.
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