Multi-Hazard Mitigation Plan: 04. High Wind Storms

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Chapter 4:Hazard Analysis—High Wind Storms 4.1.3 Kona Storms Kona winds occur as light and variable winds from the SW or SSW,most often during winter months when trade wind circulation diminishes, and as strong generally southerly winds when storm systems move across Hawaiian waters. Stormy Kona winds are most likely to occur when a low pressure center is located within 500 miles NW of the islands and has an unusually low central pressure, below 1000 millibars for the subtropics. Damaging Kona winds have reached velocities of 50 miles per hour for several days on end. The western or leeward sides of the islands, then, become windward in this case, as the predominant wind pattern is reversed. Kona storms generally form in the region bounded by 15° - 35°N and 175° E — 140° W and move erratically, though with a slow tendency toward the west. (Kodama 1998) These storms are persistent and can last up to two weeks. On land, effects of strong Kona winds can be very dramatic. It is not uncommon for trees to be uprooted, branches downed, and scores of roofs damaged. When reinforced by mountainous topography, downslope winds can increase and can be very destructive to land in low lying areas. During this time, considerable damage can be inflicted to boats caught in the open ocean or boats anchored in SW exposed anchorages. Coastal erosion can also result from the extended periods of heavy rain, strong surf and high winds. 4.1.4 Tropical cyclones: In the Central Pacific Ocean tropical cyclones are classified according to their strength as follows: • Hurricane: An intense tropical weather system with a well defined circulation and maximum sustained winds of 74 mph(64 knots) or higher. • Tropical Storm: An organized system of strong thunderstorms with a defined circulation and maximum sustained winds of 39 to 73 mph (34-63 knots). • Tropical Depression: An organized system of clouds and thunderstorms with defined circulation and maximum sustained winds of 38 mph(33 knots) or less. Historically, most tropical cyclones have passed the Hawaiian Islands to the south. Because they spin counter-clockwise in the Northern Hemisphere, east-facing coastlines in Hawaii receive the brunt of strong onshore winds as storms approach the islands,while the south and west coastlines feel onshore winds as the storms pass to the west. The highest wind speeds, however, may occur on the side opposite the storm approach, as downdrafts accelerate downslope as they descend over the mountainous terrain. Even so, coastlines facing the passing storms usually are adversely impacted by both wind and storm surge damage. History has shown that the islands do not have to take a direct landfall from a cyclone to sustain a high level of damage. Wind strength, storm radius of maximum winds, timing, and proximity, are important factors that control storm impact. 4.2 Significant Historical Events By far the most notable documented high wind event from a winter storm to affect Hawai'i County was that of January 1980, which caused damages of $42 million. (Disaster 4-4 Hawaii County Multi-Hazard Mitigation Plan