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MAZATLAN

TROPICAL CYCLONES AFFECTING MAZATLAN

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 Mazatlan and are missing from the data set. Table I-1 contains a descriptive history of all the recorded 65 tropical cyclones and hurricanes passing within 180 nmi of Mazatlan during the 51-year period 1949-1999. Unless otherwise indicated, all of the tropical cyclone statistics utilized in this report for storms passing within 180 nmi of Mazatlan are based on the data set used to compile Table I-1.

The majority of tropical cyclones originate in the area to the southeast of Puerto Vallarta and south of Acapulco. These storms normally head northwest parallel to the coast or west out to sea. Storms that do approach Mazatlan from the southeast can generate winds in excess of 100 kt and produce damaging tidal surges in Mazatlan. Unlike Puerto Vallarta (refer to Puerto Vallarta port study), the surrounding topography does not protect Mazatlan from high winds and seas. It is also interesting to note that 33 of the 65 tracks did not extend back 72 hours before reaching CPA to Mazatlan. 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 Mazatlan is about 12 kt.

Although the tropical cyclone season for Mexico is considered to extend from June through October, Table I-2 shows that Mazatlan's season extends from May through November. However, only one storm was recorded during May and one during November throughout the 51-year period 1949-1999. September and October had the greatest tropical cyclone activity, with 43 of 65 storms (66.2 %) occurring during those two months. Of the 65 storms in the data set, 24 were of hurricane strength (greater than or equal to 64 kt) at CPA to Mazatlan. Table I-2 also shows the average direction towards which storms were moving when they were at CPA to Mazatlan. The average movement for all storms is 004° at 12 kt. The average speedby month varies considerably and ranges from 8-14 kt.

Figure I-4 shows in seven-day increments the annual distribution of the 65 tropical cyclones that entered the 180 nmi threat radius around Mazatlan. The period of maximum activity, from May 28 through December 2nd, is clearly shown. It is also interesting to note the bimodal distribution (June and September/October).

Figure I-5 depicts the chronology of the 65 tropical cyclones that passed within 180 nmi of Mazatlan during the 51-year period 1949-1999. As shown in the figure, there is an average occurrence rate of 1.28 storms per year for all tropical cyclones, regardless of intensity. Since 1949 there have been ten years when three or more tropical cyclones entered Mazatlan's 180 nmi threat radius. Additionally, there were twenty years in which two tropical cyclones passed within 180 nmi of Mazatlan. The mean recurrence interval for hurricanes is 2.1 years.

Figure I-6 shows the octants from which the 65 tropical cyclones were moving when they were at CPA to Mazatlan. As shown in the figure, the predominant threat direction is from the south. However a total of 61 out of the 65 tropical cyclones approached from the southeast through southwest. It must be remembered that the figure represents tropical cyclone movement at CPA, and may not represent the tropical cyclone's initial movement towards Mazatlan. Note that both the threat direction and the various speeds of movement are important considerations for evasion at sea or making storm preparations in port.

As shown in Figure I-7, the movement of the 65 tropical cyclones that entered Mazatlan's 180 nmi radius can be described as fairly consistent. The dark circles in the figure indicate the locations of the tropical cyclones when they were 72 hours from CPA to Mazatlan. Note that the dark circles are generally southeast of Mazatlan and the tropical cyclones generally follow the coastline in a northwesterly direction. However, 33 of the 65 tropical cyclones tracks did not extend back 72 hours before reaching CPA to Mazatlan.

Most of the recurvature takes place to the north of Puerto Vallarta and the expectancy of damage increases in the vicinity of Mazatlan. The highest probabilities are somewhat north of Mazatlan but during the months of September and October the chance of having storms with at least hurricane intensity at CPA are high.

Figures I-8 and I-9 show the tracks of the 65 tropical cyclones separated into two periods. During the May 1-September 15 period, 1949-1999, the tropical cyclones branch out just south of Puerto Vallarta with the majority of the tropical cyclones passing west of the port of Mazatlan. Additionally, most of the recurvature to the east takes place north of the Port. During the September 16-November 30 period, the majority of storms do recurve to the east and proceed inland either over or north of the site. These storms can pose a serious threat to Mazatlan.

Figures I-10 and I-11 are the statistical summaries of threat probability based on tropical cyclone tracks for the periods May 1 - September 15 (Figure I-10) and September 16 - November 30 (Figure I-11) 1949-1999. The data is presented with solid lines representing "percent threat" for the 180 nmi radius circle surrounding Mazatlan. The heavy dashed lines represent approximate approach times to Mazatlan based on the climatological speeds of movement. In Figure I-11 for example, a tropical cyclone located near 17.5°N 105°W has approximately a 40% probability of passing within 180 nmi of Mazatlan. In addition, this tropical cyclone would reach Mazatlan in 1.5-2 days if the speed remains close to the climatological normal for tropical cyclones passing within 180 nmi of Mazatlan. Note that the probability area is much greater in Figure I-11 when compared with Figure I-10. Additionally the speed of advance is greater during the September - November period with an average storm speed at CPA of 12-14 kt.

Wind

Examining available wind data for selected recording stations in and around Mazatlan, 14 tropical cyclones were found to contribute 22 kt and 7 tropical cyclones contributed 34 kt in the 19 year period 1980-1998. In addition, local hurricane statistics indicate, in the 43 year period from 1943-1985, winds of hurricane strength (64 kt) were observed in the Mazatlan area 6 times. On two occasions winds over 100 kt were measured (an unnamed hurricane in October 1943 and Hurricane Olivia in October 1975).

A notable weather phenomenon that occurs frequently during the summer hurricane season is a "torito." These strong southerly squalls can exist and reach 45-50 kt sustained winds for ½ to 1 hour in duration. Approximately 5 or 6 toritos occur during the months of July and August. Indicators of this event are thunderstorms showing on the horizon from the south.

The Isla de Piedra (236 ft (72 m) in elevation), across from Fiscal Wharf, normally affords some wind protection to the Navy Wharf in the event of a tropical cyclone approaching from the south.

There are no devices for wind measurements in the harbor. However, the tugboats and pilot boat have anemometers.

The closest recording station is 3.2 nmi from the Port. Depending on the wind direction, winds at the Port can differ from those at the recording station.

Waves

Ships at the anchorages and piers in Mazatlan can be easily impacted by a combination of wind and waves, particularly from the south. According to information received during the site visit in February 2000, high seas and swells from Hurricane Tico in October 1983 caused two 328 ft (100 m) long ships to drag anchor and end up aground on the breakers west of the harbor entrance.

Storm Surge

Storm surge has significantly affected the Port of Mazatlan during previous tropical storms. No statistics are maintained, but port pilots reported during the site visit in February 2000 that a 20 ft (6.1 m) storm surge occurred during Hurricane Olivia in October 1975. A ferryboat and tugboat were tossed aground on the wharf within the harbor and the city experienced extensive flooding.

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