Guam Hazard Mitigation Plan - Western States Seismic Policy Council

SECTIONFIVERisk AssessmentCloud-to-ground lightning can kill or injure people by direct or indirect means. The lightningcurrent can branch off to strike a person from a tree, fence, pole, or other tall object. Not allpeople struck by lightning are killed. However, those that survive usually suffer from someeffects if the strike. Lightning current can also be conducted through the ground to a person afterlightning strikes a nearby tree, antenna, or other tall object. The current can travel through poweror telephone lines or plumbing pipes to a person who is in contact with an electric appliance, atelephone, or a plumbing fixture. Lightning can damage property or cause fires through similarprocesses.LocationOn Guam, lightning typically occurs in association with thunderstorm events that are caused byafternoon island warming, large clusters of thunderstorm cells, embedded thunderstorms ofmoderate and weak monsoon surges, and thunderstorms of tropical cyclones. Storms that resultfrom island heating occur with the greatest concentration near the western coast, from TumonBay to Orote Point. These storm events largely depend on the strength and direction of dailywind patterns. Individually isolated lightning and thunderstorm events primarily occur in theearly morning hours before sunrise, reflecting the nighttime maxima over the oceans. Largeclusters of thunderstorms generally cover an area of 3,475 square miles. These thunderstormclusters are known as Mesoscale Convective Systems.Previous OccurrencesIn recent history, lightning strikes on Guam have resulted in one fatality and several fires andpower outages. In 2002, residents around the island reported several lightning strikes inassociation with Super Typhoon Pongsona. In August 2003, an 18-year female tourist wasswimming 150-feet offshore of Tumon Bay when she was electrocuted by lightning. No otherinjuries are reported from lightning events on Guam. Two other lightning events have beenreported; in August 2005 lightning damaged a water booster pump station and caused atemporary outage in Yigo and in June 2010 power was lost for some residents in the Yigo areaafter lightning strikes.Probability of Future EventsAccording to the NWS-WFO, Guam experiences more lightning activity than any other place inMicronesia. Thunderstorms generally occur during the wet season, which begins in June andgoes through December. During the wet season on Guam, isolated thunderstorms that occur dueto island heating are most likely to occur near the western coast in the afternoon hours. Inaddition, isolated thunderstorm events that occur due to the maxima over the ocean are mostlikely to occur over the ocean or along the coastline before sunrise and this event has thelikelihood to occur on any day during the wet season. Table 5-13 displays the average monthlyfrequency of cloud-to-ground lightning on Guam.Lightning and thunderstorms are also known to occur on Guam, though less often, in the dryseason during tropical cyclones, during rare northward spreads of clusters of thunderstorms thatoccur during breaks in the trade winds, and during “shear line” weather patterns. These areclimatic events involving a band of moisture in the tropics that extends from an extratropical(north of the tropics in the western North Pacific Ocean) cold front storm system that traversesthe mid-latitudes of the North Pacific. A shear line event leads to a strengthening of the tradewinds.5-35