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APRA HARBOR, GUAM

TROPICAL CYCLONES

For the purpose of this study, any tropical storm or typhoon approaching within 180 nmi of Guam is considered to represent a threat to the Port. Table II-2 contains a descriptive history of all tropical storms and typhoons passing within 180 nmi of Guam during the 60-year period 1945-2004. All of the tropical cyclone statistics used in this report for storms passing within 180 nmi of Guam are based on the SAIC generated data set used to compile Table II-2.

Guam lies within a broad area considered to be the normal breeding ground for tropical cyclones in the western North Pacific Ocean. Because of its location, Guam may be threatened by the passage of a tropical cyclone on a year-round basis. Although most storms affecting Guam are still in the development stage, Guam has experienced the passages of the centers of full strength typhoons, including super-typhoons (winds >129 kt). Although Guam is located within the breeding ground for tropical cyclones, it is east-southeast of the primary center of activity. The primary center is located just northeast of the Philippine island of Luzon (Naval Oceanography Command Center/Joint Typhoon Warning Center, 1990). An average of approximately 31 tropical cyclones occur annually across the western North Pacific Ocean and South China Sea. Several of these, in various stages of development, threaten Guam during an average year.

The history of tropical cyclones and the impact that they have had on Guam over the past 300+ years is summarized in the document titled Tropical Cyclones Affecting Guam (1671-1990). As indicated by the title, the original publication listed storms through 1990, but a subsequent errata sheet updated the history through 1994. A review of the document reveals that Guam has sustained considerable damage from tropical cyclone effects over the years. It also shows that Guam not only is vulnerable to damage from wind, but also from inundation due to storm surge and run-off from heavy precipitation.

As shown in Table II-3 tropical storms and typhoons have passed within 180 nmi of Apra Harbor during all months of the year. The month of October has the greatest percentage of storms of both categories, with 23.6% (49 of 208) occurring during the month. November, however, has the greatest number of typhoon strength storms, with 24.3% (18 of 74) occurring during the month. February and March have the smallest frequency of occurrence, with only one and two tropical cyclones occurring each month, respectively. Approximately one-third (74 of 208) of all tropical storms passing within 180 nmi of Apra Harbor were of at least minimum typhoon strength (greater than or equal to 64 kt) when at their CPA. Note that the ratio of tropical storms to typhoons that occurred in November is equal: 18 to 18.

Table II-3 also shows the motion history of the 208 tropical storms and typhoons that passed within 180 nmi of Apra Harbor during the period 1945-2004. The average movement for all storms when at CPA to Apra Harbor is 293° at 12 kt. It is easily seen by the arrows in the table that the predominant direction of movement is from east-southeast to west-northwest. April, with a total of only eight storms, is the exception. The table shows that April storms tend to move from south-southeast to north-northwest, with an average movement direction of 340°. Average storm speeds at CPA vary from nine to 14 kt. April and May's nine kt average speeds of movement are the slowest of all months, while November and January's 14 kt average speeds are the fastest.

Figure II-33 shows the monthly distribution of the 208 storms by 7-day periods. The period of activity extends throughout the year with a peak occurrence from August through November.

Figure II-34 shows a chronology of the 208 tropical cyclones that passed within 180 nmi of the Guam area in the 1945-2004 period. An average of 3.47 tropical cyclones per year pass within 180 nmi of Guam. Thirty-six percent of these (1.23 per-year, or 1 every 9.6 months) were at typhoon strength while within the 180 nmi radius. There have been only 4 years since 1945 when no tropical cyclone entered Guam 's 180 nmi threat radius. Most had three or more, with four years having seven storms. An atypical situation occurred during the years 1969 through 1975, when only 9 tropical storms or typhoons passed within 180 nmi of Apra Harbor during that seven-year period. 1947,1973, 1975, and 1999 had no storms passing within 180 nmi of Guam.

Figure II-35 depicts the octants from which the 208 tropical cyclones in the data set approached Apra Harbor. As the figure shows, almost 64% of the storms approached from east-southeast. Of the 208 storms in the data set, a total of 175 (84%) approached from the southeast quadrant. It should be noted that the approach direction is determined at CPA, and may not represent the initial approach direction of the tropical cyclone toward Apra Harbor.

Guam's location in the trade wind belt of the western North Pacific Ocean effectively limits the directions from which storms usually affect Guam. Although there are exceptions, most of the storms passing near Apra Harbor approach from the east-southeast. There are some minor changes in approach directions from month-to-month, but the more evident differences are in speed of movement rather than in direction of approach.

Figure II-36 depicts the primary threat axis of the 208 tropical storms and typhoons that passed within 180 nmi of Guam during the period 1945-2004. The shaded arrow, pointing to the average location of CPA to the site, is a composite track zone for the 55 tropical storms and typhoons that crossed the dashed arc before entering Guam 's 180 nmi radius. The mean transit time from the arc to CPA for these 55 storms was 74 hours. However, the inset in the figure depicts a wide variation from the mean. The minimum transit time was 35 hours for Typhoon Hestor (1952) and the maximum was 209 hours for Typhoon Page (1990).

Figure II-37, Figure II-38, Figure II-39, Figure II-40, Figure II-41, and Figure II-42 show the tracks of the 208 tropical cyclones split into six periods of time: March through June, July and August, September, October, November, and December through February. Be aware that the total number of storms on the six track charts adds up to 223 because some of the tropical cyclones overlap. As shown in the figures, the movement of the 208 tropical cyclones that entered Guam 's 180 nmi radius can be erratic but for the most part present an overall consistent track throughout the entire tropical cyclone season. Note the progression to the east of Guam of the typhoon intensity (black or solid portion of tracks) as the season advances from July to December.

Figure II-43, Figure II-44, Figure II-45, Figure II-46, Figure II-47, and are the statistical summaries of threat probability based on tropical cyclone tracks for the period 1945-2004. Some probability lines extend around Guam because storms have entered the 180 nmi radius from all octants except one sinœ 1945. The summaries coincide with the tracks presented in Figure II-37, Figure II-38, Figure II-39, Figure II-40, Figure II-41, and Figure II-42 . The data is presented with solid thin lines representing "percent threat" for the 180 nmi radius circle surrounding Guam. The solid dashed lines represent approximate approach times to Guam based on the climatological speeds of movement. In Figure II-47 for example, a tropical cyclone located near 8°N 158°E has approximately a 40% probability of passing within 180 nmi of Guam during the November period. In addition, this tropical cyclone would reach Apra Harbor in 3-4 days if the speed remains close to the climatological normal for tropical cyclones passing within 180 nmi of Apra Harbor.

Figure II-49 shows the tracks of the 208 tropical cyclones that passed within 180 nmi of Guam over the years 1945-2004 (all months combined). Circular markers along the track indicates positions of tropical cyclones 72 hours before reaching the closest point of approach to Guam. Note that 118 of the tracks did not extend back 72 hours! This fact indicated that the warning lead time for a location such as Guam could be much less than other typhoon affected locations in the Western Pacific.

Figure II-50 displays symbols that represent the locations where typhoon intensity was first attained by all tropical storms and typhoons that passed within 180 nmi of Guam over the years 1945-2004. This figure illustrates the fact that storms are typically in the deepening phase when they pass Guam. Note the cluster of tropical cyclones that became typhoons just after they passed the closest point of approach to Guam. Had Guam been located a few degrees of longitude westward, its tropical cyclone climate would be much more severe. Twenty of the 208 cases never reached typhoon intensity.

Wind

The islands of the Marianas Archipelago have similar weather conditions. Under ordinary circumstances, the wind and seas in the vicinity of Guam are east due to the Northeast trades. West winds are at times experienced during the summer months as Guam is barely within the limits of the Southwest Monsoon. These winds are light as a rule. In the vicinity of Guam, northeast and east-northeast winds prevail for six months of the year. These winds blow from the northeast to east 65% of the time between December and May, and are strongest during these months. Wind speeds of 13-22 kt are very common. Between June and November, the surface winds are quite variable. Calms are rare. On some days during the rainy season, moving thunderstorm cells that bring heavy showers or steady, sometimes torrential rain, may dominate the weather.

Historically, the extreme winds on Guam have come from a single source – typhoons. Despite the existence of several anemometers on the island, these wind-measuring devices frequently fail during intense typhoons. The most common sources of failure are power outages and damage due to airborne debris. For this reason, the historical record is full of holes and incomplete data, usually during and after an eye wall passage.

Table II-4 lists the strongest tropical cyclone-induced winds observed on the island during the 1923-1997 period. The strongest measured wind gust of 148 kt occurred during Typhoon Paka (December 1997). The strongest wind gust experienced in recent history on the island is estimated to be about 173 kt during Typhoon Karen (November 1962).

As discussed in the Typhoon Vulnerability Study for Guam (see References), most storms are seen to move in from the east-southeast but exhibit wide dispersion after moving west of Guam. All else being equal, storms that pass to the south of Guam typically bring higher winds than storms passing the same distance to the north. This is due to the fact that, for storms passing to the south, the counterclockwise wind circulation around the storm is augmented by the storm's translational speed. Also, storms passing to the south tend to be somewhat more severe than those passing to the north.

Considering the above, a worst-case scenario for destructive winds at this site would be an intense storm (Typhoon Category 5) that moves slowly from south-southwest towards the north-northeast, just west of and parallel to the island. However, because the dominant steering forces for intense storms are from the east-southeast to the west-northwest, such a scenario is unlikely. However, two occurrences come to mind. Typhoon Olive (April 1963) took a northward track just west of the island and Typhoon Andy (April 1989), moving southwest of Guam, turned to the northeast and passed 90 nmi east of the island. A more realistic worst-case scenario would be for the center of an intense storm to pass just south of the center of the island, with the entire island exposed to the eye wall cloud. Typhoons Karen (November 1962), Pamela (May 1976), and Omar (August 1992) are examples, except that Karen moved across the island at 17 kt, while the other two moved across the island at less than half that speed. This scenario maximizes the worst case for winds and for west-side storm surge and inundation. The worst-case for east-side storm surge and inundation would occur where a large, intense, fast moving tropical cyclone moves south of the island. Super Typhoon Yuri (November 1991) was such a case.

Waves

Wave motion in Apra Harbor is not a common problem. The only potentially hazardous wave motion in Apra Harbor occurs during periods of strong westerly winds when swell passes through the west facing Harbor Entrance. According to a U.S. Navy Harbor Pilot, maximum wave heights in the Outer Harbor during periods of strong westerly winds are limited to ten ft. Even then, the configuration of the Harbor prevents the waves from entering the Inner Harbor.

The configuration of Apra Harbor prevents large wind waves from generating within the confines of the Harbor. The approximate one nmi north-south distance across the Outer Harbor causes waves generated by north or south winds to be fetch limited. The approximate three nmi distance east-west across the Outer Harbor would allow somewhat larger wind waves to develop, but they also would be fetch limited. In all wind situations, the Inner Harbor is well protected from waves other than wind chop.

Pilots indicated that the maximum wave height experienced inside the Harbor during a typhoon passage was 6.6 ft (2 m). Open ocean wave motion is negligible as the Port is almost surrounded by land and reef breaks. 9.8-16.4 ft (3-5 m) waves have been experienced just outside the Harbor during typhoon passages but Pilots as well as data from meteorological reports indicate that higher waves are possible.

Storm Surge

Storm surge may be visualized as a raised dome of water, moving with the storm and centered a few miles to the right of its path. Storm surge is caused by wind stress on the water surface and the effects of atmospheric pressure reduction. The piling up of water on a coast ahead of a tropical cyclone is more intense in the dangerous semicircle, the region of most intense winds.

Apra Harbor's location on the west side of Guam protects it from the worst effects of storm surge from storms moving from east to west, the most common movement. A tropical cyclone passing south of the Harbor would produce offshore flow over the Harbor that would effectively preclude any significant water rise caused by wind stress on the water. A tropical cyclone passing north of the Harbor would pose the greatest threat to Apra Harbor of storm surge due to wind stress. A water rise caused by a reduction of atmospheric pressure, the so-called barometer effect, would be approximately the same for north and south passages, and would be dependent on the distance of Apra Harbor from the center of the storm.

Low-lying areas of Guam are at risk from inundation due to storm surge during the passage of severe tropical cyclones near the island. Typhoon conditions have created water levels10-12 ft (3.1-3.7 m) above mean lower low water level in Apra Harbor (Brown, M. E. and S. Brand, 1975). During a 1994 interview with the U.S. Navy Harbor Master at Apra Harbor, it was stated that storm surges have been limited to four or five feet in the Harbor. This information was confirmed by discussions during the Site Visit in February 2006.

Port authorities stated that Typhoon Paka produced a four to five foot storm surge in the Harbor in December 1997. Typhoon Omar, a strong storm with sustained winds over 100 kt and gusts to 130 kt, passed over Guam on a west-northwesterly track in August 1992. The Harbor Master said Omar caused a rise of about three feet in the Harbor.

The precise wind directions at Apra Harbor during Omar's passage are not known. However, the track of Omar, as depicted in Figure II-51, indicates that the typhoon passed very close to NAS, Agana on a west-northwesterly track. The eye of the storm was 20 nmi in diameter as it crossed the island, which would include Apra Harbor within the eye. Such a track would have caused northerly winds at Apra Harbor, gradually shifting to northwest prior to passage of the leading wall cloud and the eye of the storm. As the eye passed Apra Harbor, wind direction would have shifted rapidly to become southwest to south and eventually south-southeast, with wind velocities gradually decreasing as the storm moved away from Guam. Winds from the west, the potentially worst direction for storm surge in Apra Harbor, would have been short lived if they existed at all during Omar's passage. It is likely that the relatively narrow entrance to Apra Harbor also mitigated the effects of storm surge within the Harbor.

Figure II-52 shows the estimated storm surge heights that occurred during Omar's passage. Heights presented are in feet above the mean high tide line. It should be noted that the eye passage coincided with one of the highest astronomical tides of August. The data are adapted from NOCC/JTWC (1992). Maximum sustained surface wind during Omar's passage was 105 kt with gusts to 130 kt.

The areas subject to storm surge inundation are largely dependent on the direction of passage. A storm passage close to and south of Guam would threaten the southeast coast of the island. A storm passage north of the island would threaten the west coast of Guam, including Apra Harbor.

The NOCC/JTWC 1990 publication Tropical Cyclones Affecting Guam (1671-1990) contains an excellent review of the storm surge threat. The following paragraphs are quoted from that document. Specific locations mentioned in the quotation are identified in Figure II-53.

"Inundation of low-lying coastal areas by the sea has occurred during the passage of severe cyclones near Guam. An inspection of the narrative accounts since 1946 would indicate that significant inundation should be expected in low-lying coastal areas with the passage of the center of a tropical cyclone of typhoon force within 60 nmi of Guam. However, specific information on Guam storm surges is quite sketchy, since little reliable documentation as to their extent and height is available. Another problem in documentation is separating the combined effect of flooding by rainfall runoff and sea inundation.

Based on narrative reports, the southern coastline of Guam is quite susceptible to flooding by the passage of typhoons as distant as 200 nmi south of Agana. Reports of the Inarajan-Merizo road being awash have been frequent. Inarajan and Talofofo Bays have suffered from inundation with the close passage of several typhoons during this century.

The village of Inarajan was swept away with the loss of 28 lives in the typhoon of November 1900. In Merizo, water was reported four to five feet deep during Lola (November 1957) and Cocos Island was completely inundated by Allyn (November 1949).

On the western coast, the village of Agat suffered severe damage by sea action during the September 1946 typhoon and Typhoon Karen (November 1962). In Agana, major inundation has occurred at least three times this century from the November 1900 typhoon, and Typhoons Karen (November 1962) and Pamela (May 1976). During the 1900 typhoon, water reached the plaza in front of the Palace near the present site of the Agana Cathedral." The cathedral is located in central Agana. "Both Karen and Pamela brought the sea in at least the same distance. During Karen and Pamela, the storm surges washed boats, docks and debris from the Agana Boat Harbor several blocks inland, leaving fishing boats weighing several tons on Marine Drive," (a major traffic route (Highway 1) on the west side of the island between Agana and Apra Harbor) "with sand deposits nearly a foot deep along the drive between Tamuning and Anigua.

During Pamela, ten small ships and tugs that had sought refuge in Apra Harbor were either sunk or run aground, and numerous other craft were sunk or damaged. Typhoon Russ (December 1990) inundated much of the low-lying coastal areas along the southeast coast of Guam. Probably the most catastrophic sea inundation on Guam was during the typhoon of November 1693 which engulfed and washed away all the existing coastal structures and vegetation."

An example of storm surge heights during a typhoon passage south of Guam is shown in Figure II-54. The figure has been adapted from NOCC/JTWC (1990), and shows estimated sea inundation heights in feet above the mean high water level occurring during the passage of Typhoon Russ in 1990. Typhoon Russ' center wind speed was 120 kt when at CPA 50 nmi south-southwest of Guam.

Source: http://www.nrlmry.navy.mil/port_studies/thh-nc/guam/apra/text/sect7.htm