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ACAPULCO

TROPICAL CYCLONES AFFECTING ACAPULCO

Specific tropical cyclone data, containing storm tracks, wind and pressure data, etc., are available starting in 1949. However, prior to 1966 satellites were not used operationally and there are likely some storms that tracked well offshore from Acapulco and are missing from the data set. Table III-1 contains a descriptive history of all the recorded 105 tropical cyclones and hurricanes passing within 180 nmi of Acapulco during the 53-year period 1949-2001. Unless otherwise indicated, all of the tropical cyclone statistics utilized in this report for storms passing within 180 nmi of Acapulco are based on the data set used to compile Table III-1.

The majority of tropical cyclones originate in the area to the south of Acapulco not far from the Port. It is interesting to note that storms cannot be tracked more than a few days back in time from the closest point of approach from the site. These storms normally head northwest parallel to the coast or west out to sea. Although these tropical cyclones can generate winds in excess of 100 kt, they historically have not produced huge tidal surges in Acapulco that do so much damage elsewhere. Acapulco is normally protected from high winds and seas because of the surrounding topography. Accordingly, ships transiting from the Port need to be aware of the rapid development of tropical cyclones as they proceed through what the pilots refer to as "hurricane alley." The average speed of advance for tropical cyclones at the latitude of Acapulco is about 10 kt.

Although the tropical cyclone season for Mexico is considered to extend from June through October, Table III-2 shows that Acapulco's season extends from May through November. However, only eight storms were recorded during May and three storms were recorded during November throughout the 53-year period 1949-2001. June and September had the greatest tropical cyclone activity, with 52 of 105 storms (49.5 %) occurring during those two months. Of the 105 storms in the data set, 31 were of hurricane strength ( greater than or equal to 64 kt) within 180 nmi of Acapulco. Table III-2 also shows the average direction towards which storms were moving when they were at CPA to Acapulco. The average movement for all storms is 300° at 10 kt. The average speed by month is variable and ranges from 7-13 kt. The average speed increases from May through July and then decreases through October.

Figure III-16 shows in seven-day increments the annual distribution of the 105 tropical cyclones that entered the 180 nmi threat radius around Acapulco. The period of maximum activity, from May 21st through November 11th, is clearly shown. It is also interesting to note the bimodal distribution (June and September).

Figure III-17 depicts the chronology of the 105 tropical cyclones that passed within 180 nmi of Acapulco during the 53-year period 1949-2001. As shown in the figure, there is an average occurrence rate of 1.98 storms per year for all tropical cyclones, regardless of intensity. The recurrence interval of all tropical cyclones is 0.5 years. There have only been nine years since 1949 when no tropical cyclone entered Acapulco's 180 nmi threat radius. 1953,1962, 1964, 1972, 1977, 1980, 1981, 1990 and 1994 had no storms passing within 180 nmi of Acapulco.

Figure III-18 shows the octants from which the 105 tropical cyclones were moving when they were at CPA to Acapulco. As shown in the figure, the predominant threat directions are from the east-southeast and south-southeast octants. A total of 70 storms (66.7 %) approached from the east-southeast. It must be remembered that the figure represents tropical cyclone movement at CPA, and may not represent the tropical cyclone's initial movement towards Acapulco. Note that both the threat direction and the slow speed of movement are important considerations for evasion at sea or making storm preparations in port.

Figure III-19, Figure III-20 and Figure III-21 show the tracks of the 105 tropical cyclones separated into three periods. As shown in all three figures, the movement of the 105 tropical cyclones that entered Acapulco's 180 nmi radius can be described as fairly consistent. It is important to note that most storms developed within 72 hours from the closest point of approach to Acapulco. During the May 1-June 30 period, 1949-2001, the tropical cyclone tracks branch out north of Acapulco and south of Puerto Vallarta with the majority of the tropical cyclones passing west of the Port. Additionally, most of the recurvature to the east takes place north of the Port of Acapulco. During the July 1-August 31 period, the majority of storms track to the northwest away from the coast of Mexico. Note the absence of storms passing directly over the Port of Acapulco. During the September 1-November 30 period, the majority of storms do recurve to the east, but normally north of Acapulco. Those closest to the Port historically have not impacted the Port. According to the information received during the site visit in August 2002, none of the tropical cyclones caused damage to ships in Acapulco. However, the climatology indicates that the highest probability of a direct threat to Acapulco would be during the May or June timeframe.

Figure III-22, Figure III-23, and Figure III-24 are the statistical summaries of threat probability based on tropical cyclone tracks for the period 1949-2001. The summaries are separated into three periods to coincide with the tracks presented in Figure III-19, Figure III-20 and Figure III-21. The data is presented with solid thin lines representing "percent threat" for the 180 nmi radius circle surrounding Acapulco. The solid thick lines represent approximate approach times to Acapulco based on the climatological speeds of movement. In Figure III-22 for example, a tropical cyclone located near 12°N 94°W has approximately a 40 % probability of passing within 180 nmi of Acapulco during the May 1 to June 30 period. In addition, this tropical cyclone would reach Acapulco in 3-4 days if the speed remains close to the climatological normal for tropical cyclones passing within 180 nmi of Acapulco. Note once again that those closest to the Port historically have not significantly impacted the Port.

Wind

Wind measuring instruments are located in the Captain of the Port (COTP) office and the data is accessible to the Pilots and Port Administration. The measured winds are representative of those experienced by ships in the Port. However, wind data is also obtained from the airport that is more representative of wind in the open ocean. None of the COTP wind data is recorded in the harbor. The Captain of the Port assured us during the site visit in August 2002 that they were not aware of any situation in which winds from tropical cyclones had impacted ships at the piers or at anchor. However, ships are normally relocated to anchorage because of the potential damage at the pier due to swell with an impending tropical cyclone.

Prevailing winds are southeasterly. In the rainy season, June-September, east and southeast winds prevail.

Waves

Ships tied up at the pier and at anchorage can encounter a large swell during storms. In anticipation of storms, ships at the pier should double up and use chaffing gear or shift to an anchorage. Pilots stated during the site visit in August 2000 that the maximum wave heights encountered during storms was 4-6 ft but that the swell can cause ships to roll at anchorage or cause a large surge at the pier. Open ocean wave motion from the southeast through southwest have the greatest impact on ships at the pier or anchorages. Swell may also come from tropical cyclones 24-48 hours after passage.

The USS Cromelin reported the following during a port visit in September 1999: "A pronounced surge alongside the pier placed a significant strain on mooring lines, requiring 24 hour attention to prevent chaffing. No Yokohama fenders or camels were available. However, the pier was fendered with large truck tires which proved adequate."

Storm Surge

Pilots indicated during the site visit in August 2002 that storm surge has not affected the Port of Acapulco during any previous tropical storms.

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