Jamaican Hurricane Frequency
An Individual Research Project by Alex Moffett
Supervising Professor Dr. J.B. Elsner
The Young Scholars Program
Florida State University, 1997
Explanation of Data Used
While information on the history of hurricanes affecting Jamaica goes back to the accounts of Millas in the early sixteenth century, Jamaica itself was not as well traveled as other Caribbean islands and thus contains a rather large discrepancy in its meteorological history. Millas’ account states that there was exactly one hurricane that hit Jamaica in the entire sixteenth century. The number of sixteenth and seventeenth century hurricanes is practically nonexistent when compared with the number occurring in the eighteenth, nineteenth, and twentieth centuries. Whether this data is comparatively smaller based upon a fewer number of possible witnesses, or if it is actually a fluctuation in the hurricane pattern is unknown. Because a century of hurricane free weather would have a profound effect on this study’s conclusions, information used in this study will come solely from the besttrack data set and thus begins in 1886. This assures that all information comes from the same source and that this data is credible.
Jamaica is a small island located in the middle of the Caribbean Sea, below Cuba and west of Haiti. Because of its central location, Jamaica serves as a prime target for both early season hurricanes developing off the coast of Africa, and late season hurricanes developing directly to its south. The relatively small, island nature of Jamaica assures that any hurricanes reaching landfall will have an effect on all areas of the island. Unlike other island nations such as Bermuda, Jamaica is large enough be considered a prime target for tropical hurricanes, receiving almost 400% the number of hurricane landfalls experienced by Bermuda, and is thus an excellent candidate for study.
In the 110 years of the besttrak study, Jamaica has seen the eye of a hurricane reach landfall 12 times with an average return period of 9 years. While this information does not predict a hurricane to occur every 9 years, it shows us that the people of Jamaica should be prepared to face the effects of direct landfall an average of eleven times a century. To better study the frequencies of Caribbean hurricanes that did not necessarily reach landfall, this study shall define all hurricanes that existed within 500km of Christiana as hurricane threats. In the 100 years of the study, Jamaica has experienced 80 such threats. While only 12 of these storms reached landfall, most should be considered to have brought both rain and wind to Jamaica, poor weather conditions for an island based upon both tourism and agriculture. Hurricane threats existed in 53 of the 110 years of the survey. 11 of the years saw the existence of two threats, 6 experienced three threats, and only one year, 1933, experienced 4 threats. From 1995(the last year of this study) to 1988, Jamaica has experienced only one hurricane threat. While the number of threats experienced by Jamaica has been recently dwindling, there have only been four threats in the past 15 years, the number of hits has remained constant, leaving little reason to believe that hurricanes are less of a threat to Jamaica than in previous, hurricane laden years.
Hurricane appearances should be considered independent of each other as one occurrence does not necessarily designate an increase or decrease in the probability of a second. In the 110 years of the study, single hurricane threats have appeared a total of 35 times or 32% of the years. 32% squared, or the odds of this event happening twice, equals .1024 which, when multiplied by 110 years, equals roughly 11 years. This turns out to be the exact number of years in which two hurricane threats have occurred, and demonstrates that the probability of a hurricane threat occurring is independent of any other hurricane appearances.
Breaking the hurricane threats down by month reveals that the majority of hurricanes have occurred in August, September, and October, the middle months of the Caribbean hurricane season. Hurricane appearances in May and June number only one while only four have appeared in July. None of the hurricanes to have appeared in these early months have come within 200km of the island or ended up as hits to Jamaica. While Jamaica should not ignore hurricanes appearing within this three-month interval, the chance of these hurricanes coming close enough to the island to produce any serious damage is extremely small.
August, September, and October should be considered the most dangerous months of the hurricane season, not only for their large amounts of threats, but for the high percentage of these threats developing into hits. This fact is due to the large numbers of low latitude hurricanes, developing from the intertropical convergence zone, which proceed to make direct landfall with Jamaica. The intertropical convergence zone exists primarily during August and September and is the reason that almost 40% of August hurricane threats and 10% of September hurricane threats end up hitting Jamaica. Hurricanes from during this season develop from waves off the coast of Africa. The warm temperature of Atlantic water during the summer months allows for these waves to stay together until they can reach the Caribbean where can develop into tropical storms and hurricanes. Hurricanes developing in this manner often come extremely close to Dominica, a small island located to the north of Venezuela. These hurricanes tend to develop away from Dominica and then travel in a straight path towards the island. Once the waves reach the island, they tend to increase in intensity from tropical storm to hurricanes of category 1 or 2. The hurricane then continues its straight path directly below Puerto Rico, Haiti, the Dominican Republic, into Jamaica, and below Cuba. Once past the tip of Cuba, these storms follow a path into either Mexico or Texas where they soon dissipate after reaching landfall. In predicting what hurricanes will have a high probability of making landfall, one can set up a 300km radius around Dominica and track all storms that enter the area. Of all hurricane threats to Jamaica, 43 passed through this region. While this statistic may not seem very extraordinary, when threats are broken down to be defined as all hurricanes passing within 200km of the island, 19 of the 28 hurricanes that fit this description passed within 300km of Dominica. Of the twelve hurricanes that did make landfall with Jamaica, 9 came from this area. Jamaica’s most vicious storm, the 1988 arrival of hurricane Gilbert, followed this path. When forecasting what storms will hit Jamaica, special attention must be made for these low latitude hurricanes.
Almost all hurricanes developing in late September, October, and November develop not from this intertropical convergence zone, but from areas directly south of Jamaica, primarily between Nicaragua and Colombia. While it would seem that these storms would be dangerous due to their proximity, only two of the twelve hurricane landfalls stem from this category of storm. These hurricanes usually travel extremely slowly northward, providing Jamaica time to prepare for their onset. Late season hurricanes travel in extremely unpredictable lines, often curving several times before straightening out. All hurricanes developing during October and November should be considered potential threats to Jamaica due to the relative unpredictability of both their paths and strengths.
Hurricanes exist as a serious threat to the island of Jamaica. Hurricane landfalls occur, on average, every nine years, while hurricane threats have a greater than fifty percent chance of occurring in any given year. In order to prepare for the next hurricane Gilbert, Jamaica should focus on detecting hurricanes in two prime areas, south of Jamaica between Nicaragua and Columbia, and near the island of Dominica. The south should be observed during the late months of October and November, while Dominica should be observed during the summer months of August and September. By observing these areas as sites of prospective hurricane origin, the nation of Jamaica should be afforded plenty of several days opportunity to prepare for a hurricane’s onset.