Guam Hazard Mitigation Plan - Western States Seismic Policy Council

SECTIONFIVERisk AssessmentPhilippines eastward to the International Date Line. The movements of this monsoonal windflow are referred to as surges.Monsoon surges in the area are characterized as weak, medium, or strong. Weak monsoon surgeshave sustained southwesterly winds of less than 15 mph and a few episodes of heavy rain perday, which often take the form of thunderstorms, over a location. Moderate monsoon surges havemore intense southwesterly winds, ranging from 15 to 30 mph, several (three to six) episodes ofheavy rain per day, extensive periods of light rain, and heavy overcast skies containingthunderstorms. Strong monsoon surges have gale-force winds (35 to 60 mph), frequent (6 to 12)episodes of heavy rain per day, nearly continuous light rain, and little to no lightning. A strongmonsoon surge also has an eastward-moving monsoon squall line, which contains the zone ofhighest winds and the most extensive monsoon cloudiness. Strong monsoon surges can last froma few days to more than 2 weeks. Weak monsoon surges are most common and strong monsoonsurges are least common.Monsoon surges in the western North Pacific Ocean commonly occur in tandem with tropicalcyclones. Although neither weather event is necessarily associated with the other, thesuperposition of a monsoon surge on a tropical cyclone can increase the amount of rain deliveredby the tropical cyclone and extend the size of the area that the tropical cyclone affects. This typeof event is more common with weaker tropical cyclones that are more unorganized and mayextend much farther from their centers.LocationAny location on the island has virtually the same chance of experiencing the center of a tropicalstorm or typhoon and peak wind gusts up to 200 mph. For storms passing south of the island,winds on the east coast (winds blowing toward the coast) would be stronger than winds on thewest coast (winds blowing away from the coast). For storms passing north of the island, windson the west coast (blowing toward the coast) would be stronger than winds on the east coast(blowing away from the coast).The cliffs on northwest Guam have a tendency to enhance the eye wall convection and causewinds to be a little more intense than would be expected. Thus, while winds in the southsemicircleof the tropical cyclone would normally be weaker than those in the north semicircle,the orographic effects of the island seem to allow the strongest winds to exist in two semicircles:one over Guam and the other in the Rota Channel.The strongest winds will be in the eye-wall cloud near the windward coast. The winds willgenerally be weaker on the lee-side coast. Also, as the winds move inland, they weaken. Therougher the terrain, the more the sustained (1-minute average) winds diminish. The vegetation,especially large trees, helps to reduce the winds at the surface. Although the potential peak gustremains the same, the frequency with which the gust gets to the surface is greatly reduced atinland locations. Winds will be stronger along all coasts, at high elevations, along ridge lines, invalleys between hills and mountains, and on sloping terrain.A simplified wind hazard map was created for the 2011 Guam HMP (see Figure D-22), in whichsevere-wind hazard is shown to occur in all areas within 500 feet of the coast and at elevationsequal to or greater than 300 feet. All other portions of the island are assumed to be in a highwindhazard zone. (After reviewing the wind map used in earlier versions of the Guam HMP,representatives of the NWS-WFO determined that the map was insufficiently accurate.)5-39