|FORECASTING EXTREME WEATHER CONDITIONS
ASSOCIATED WITH THE CAYMAN ISLANDS
COLD FRONT (NORTHWESTER)
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
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.
Compendium of Meteorology Professor T.N. Krishnamurti
Part 4 – Tropical Meteorology
Mid-Latitude Weather Systems T.N. Carlson