The Cayman Islands are a set of three small islands with a total area of 100 square miles. The three islands, Grand Cayman, Cayman Brac and Little Cayman, are located in the northwest Caribbean about 150 miles south of Cuba, 460 miles south of Miami, Florida, and 167 miles northwest of Jamaica. George Town, the capital is located on the south western shore of Grand Cayman. The unique position places the Cayman Islands far enough north to be affected by cold fronts during the winter and still within the belt that is influenced by tropical waves and hurricanes during the summer. The total population of the Cayman Islands is around 50,000 people with a fairly high per capita. Climatically the year can be divided into two seasons-the wet, summer season, generally from mid-May through October and the dry, winter season, from November to April. Occasional surges of cooler air from continental North America, the leading edge of which is called a cold front is the main winter system affecting the Cayman Islands from late October through early April. These systems are the major producers of rainfall during the dry winter months although precipitation is not quite as long lasting or of the same amounts as with summertime systems. Despite this pattern of rainfall, these fronts account for near 80% of the rainfall during the dry season. The stronger cold fronts tend to bring strong North to northwest winds and rough seas, especially along the Western and Northern Coasts of the Islands: Such fronts are locally called “Northwesters”. Spectacular wave action along the western coast and in the George Town Harbor of Grand Cayman is a side show associated with these cold air surges. Fifteen and twenty foot waves hammer the coastline leading to a halt in water-sports and diving as well as the occasional closing of port activities. Vessels, which do not heed the weather forecast to move to safe harbor, are often damaged or destroyed by the very rough seas produced by such a system. The typical synoptic pattern associated with these systems is of two forms; the first form has a fairly deep mid latitude cyclone dipping into the central Gulf of Mexico with a trailing cold front into Mexico. The second form has a rapidly intensifying mid latitude cyclone forming along a dying cold front over the southeast Gulf of Mexico. The secondary cyclone may or may not intensify fast enough to generate significant winds and seas over the northwest Caribbean. These low pressure systems tend to move fairly fast to the northeast once they form and so they must intensify fast enough to overcome this motion and be of significance. In either case the cyclone produces fresh to strong winds and high seas with a significant fetch that extends from near the northwest Gulf of Mexico across the Yucatan Channel to the Cayman Islands. On average the Cayman Islands observes the passage of some ten to fifteen cold fronts per year with around five becoming northwesters resulting in the closure of the dock and ceasing of all boating and leisure activities along the west coast. Around three such fronts actually produce seas rough enough to require the roads to be closed along the coast. A major source of revenue for the Cayman Islands is tourism. The very steady prevailing trade winds blowing from the east and northeast tends to produce rocky east coasts under the influence of rough seas and magnificent sandy beaches along the sheltered west coasts of most Caribbean countries. Because of this reason the west coasts of these islands tend to have the majority of tourist resorts and associated leisure activity. The west coast of Grand Cayman has achieved worldwide reputation for its beauty and has been named the “Seven Mile Beach.” Such beaches and associated activity is particularly sensitive to strong north and northwest wind. Additionally the protected west coast is usually a good location for a harbor such as the main harbor of the capital George Town. The affects on the George Town harbor is a major problem as the Cayman Islands are also a major stop for cruise ships. Grand Cayman is accustomed to having some 5-7 cruise ships most days during the winter months. The island does not have a docking facility that allows tourist to walk straight from the boat to the island instead tourists are brought ashore using small boats. These boats are affected by the slightest waves and in fact the port may be shut down without any sort of warning from the weather service. Several tourists have been hurt in incidents involving this form of transport. The threshold that the National Weather Service uses for rough seas and marine warnings is higher than the limit the port authority uses for the interruption of their operations. Additionally the main road runs along the coast in this area and wave action may be so extreme that wave’s splash onto the road bringing debris that blocks the road. For this reason and the danger of eager viewers attempting to get a close up view of the dramatic wave action causes the police department to close the main road. An example of a severe northwester is listed below. Weather charts indicate that in late December 1989 a deep mid-latitude cyclone dipped into the southeast Gulf of Mexico with a strong trailing cold front into the northwest Caribbean. This front moved across the Cayman area on December 24th 1989 producing little in terms of cloudiness and rainfall but supporting fresh to strong northwest winds and very rough seas, especially along the west coast. The waves associated with this front were so extreme that the main road was closed due to damage. Although the Cayman Islands had been hit by numerous northwesters over the years, for the first time in the history of the Cayman Islands, the historic landmark and popular tourist visitation stop, the Cayman Islands Turtle farm was extremely damaged and shut down. Many of the turtles were washed out of the facility and holding pens either severely damaged or destroyed. It took several weeks before the roads and dock could be fully repaired and the Turtle Farm a few months. Other associated forecasting elements include early swells, rough seas being produced from systems with northeast winds and rainfall. The forecasting of swells in advance of an approaching front is critical especially for the port operations and fishermen. As the cold front moves into the extreme northwest Caribbean waves build up in advance of the system and propagate outward reaching the Cayman Islands as a turbulent swell. As a result of this swell, the sea will become very muddy instead of the crystal clear waters the country typically experiences a day in advance of the major cold front reaching the area. Generally the sea does not become rough until the increase in northerly winds after the passage of the front. The waves associated with the swells are limited by the lighter but contrary winds in advance of the front but increase rapidly after the passage. Numerous fronts every year become stationary west of Cayman but close enough to produce significant swells to propagate into our area. While these fronts remain west of the area they still produce some cloudiness, shower activity and swells that may be significant enough to limit activity along the west coast. Now most front passing through the Cayman Islands produce fresh northeast winds and rough seas limited to the north coast. However there are times when the northeast winds may in fact be stronger but still produce a rough sea along the west coast. These are very difficult to forecast as it is dependent upon the angle and height of the waves with the coast. The amount of shelter is overcome by the stronger winds and resultant higher waves. These larger waves propagate south along the west coast but with the same resultant action of a northwester. In terms of rainfall, as frontal systems move over the northwest Caribbean there is a clash of cooler polar air north of the front and warm, moist tropical air over the Caribbean. This results in a very unstable situation resulting in cloudiness, heavy showers and thundershowers. Rainfall amounts and cloud cover vary significantly from system to system with some systems producing little to no clouds much less rainfall and others producing deluges and flooding. Below is an extreme example of precipitation that these systems can produce. On Saturday January 18 2003 the Cayman Islands experienced an evening of torrential rainfall that resulted in widespread flooding of the Capital, George Town. The National Weather Service recorded a record 6-hour record 9.06 inches between 1 and 7 p.m. local time. This total was so excessive that the 6 hr total was greater than any 24-hr total since records commenced in 1957. This resulted in flooding of the capital and southwest Grand Cayman. An evaluation of the situation indicates that a cold front moved rapidly into the northwest Caribbean during the night of Friday 17 January 2003, then slowed over the Cayman area as it weakened. As the system moved across the Cayman area the front became stationary. An upper trough moving across the continental USA caused a strong jet stream to become more favorably oriented resulting further enhancement of weather along the cold front. Consequently a line of heavy showers developed from Honduras to Grand Cayman. The prevailing flow caused these heavy showers to move northward across the Cayman area Saturday afternoon leading to the torrential downpours and extreme rainfall totals. This process of heavy showers and thundershowers moving along a line is called training. Two more important points involve the accuracy of most weather models and the size of the event. Weather models for the most part are designed with formulae that operate fairly well in the subtropics but are erratic at best in the tropics. A major problem here is the lack of data for an analysis for the models to work with. Data is fairly well spread out in the subtropics and equipment is maintained fairly well. This does not hold true in the tropics where the data is mostly missing due to transmitting problems, unevenly spaced or in some cases inaccurate. With regards to the second major point, we realize that most marine and weather charts have to be interpreted for this localized effect. While these systems produce significant localized conditions we must realize in the overall scheme of things they are meso-scale and we are attempting to forecast there occurrence from a synoptic level chart. What this means is that the forecaster must correctly interpret the wind and wave charts to predict a significant weather event. In conclusion we see once more that the job of a weather forecaster is not as easy as most would think. Every location produces its own set of problems in forecasting as the forecaster attempts to satisfy the needs of his customers. References Compendium of Meteorology Professor T.N. Krishnamurti Volume II Part 4 – Tropical Meteorology Mid-Latitude Weather Systems T.N. Carlson |