Weather across the Eastern Caribbean Sea invokes thoughts of sunny skies, tradewinds and turquoise waters. However, many mariners can probably share a tale or two of getting caught in adverse weather, or being forced to stay in port due to a change in the weather pattern, or due to a tropical cyclone threaten- ing the region. Although at times the weather across the Eastern Caribbean can be rather straightforward, a basic knowledge of the general weather patterns can give mariners an edge in planning voyages for every season of the year. Out ahead of cold fronts is where the lightest weather usually develops over the northeastern Caribbean Sea During December through February mariners and vacationers alike escape winter’s icy grip for the warm- er temperatures and waters of the Caribbean Sea. However, cold, dense air under high pressure also migrates south towards the northern Caribbean waters. These highs move off the US East Coast and pass south of Bermuda before moving eastward to the central North Atlantic. Farther south, a weak trough of low pressure, or equatorial trough, extends from the equator northward to 10°N across South America. As the high passes to the north and interacts with the equatorial trough, a tight pressure gradient increases easterly winds over the Caribbean Sea. This occurs throughout the year but the highs are particu- larly strong and large in winter, generating the stron- gest tradewind surges as the cold air is slow to modify. Generally the winds will increase from the northeast to east at 20 to 25 knots with higher winds of up to 30 knots between islands. Swells will subsequently build from the northeast to east up to seven to ten feet in the southeastern open Caribbean waters and up to nine to 12 feet in open areas of the Atlantic Ocean. A ridge of high pressure can remain stationary over the area for approximately four to six days. While these highs build over the region, scattered showers and thunderstorms will develop across the eastern Caribbean Sea due to cooler air interacting with warm- er waters and moving westward with the wind. High pressure will eventually move eastward, allowing the tradewinds to diminish as cold fronts move off the southeastern US coast and track through the Bahamas. Generally the tail ends of these cold fronts will reach northern Hispaniola before stalling out, while the rest of the front sags to the north of the Virgin Islands and northern Leeward Islands (see Figure 1). This front will gradually weak- en in approximately two or three days. Out ahead of these fronts is where the lightest weather usually develops over the northeastern Caribbean Sea with SEASONAL WEATHER PATTERNS ACROSS THE EASTERN CARIBBEAN SEA by Amanda Delaney winds easing and becoming more southeasterly at five to 15 knots and swells mainly from the east low- ering to three to six feet. The showers and thunder- storms will become more isolated as air tempera- tures begin to moderate. Late December and January is when cold fronts are strongest over the western Atlantic Ocean (see Figure 2). These fronts will extend as far south as the north- eastern Caribbean before stalling across the region. Generally a strong high will build to the north over the western Atlantic behind these cold fronts and this will produce the strongest tradewinds and large northerly swells behind these cold fronts. Winds can increase from the northeast to 30 to 35 knots and swells will impact the northern shores of Puerto Rico, the Virgin Islands and northern Leeward Islands with northwest eR eee] Late December and January is when cold fronts are strongest over the western Atlantic. A strong high behind these cold fronts will produce the strongest tradewinds to north swells as high as 13 to 18 feet. Farther south over the Windward Islands, winds will increase north- east to east often reaching 25 to 30 knots and north- erly swells will build to the east of the islands and in more open waters to the west of up to nine to 13 feet. Fortunately these conditions gradually moderate in approximately two to three days. As we progress from March through May, the north- erly wind surges and large swells typically become less frequent and not as severe as their winter counter- parts. Tradewinds gradually lower to more northeast to east from 20 to 25 knots during March to 15 to 20 knots in May as highs build to the north of the islands. Swells will also diminish from the east to northeast of six to nine feet in March to four to seven feet in May in areas exposed to the Atlantic and in open areas of the Caribbean Sea. This is due to the highs to the north weakening and becoming less frequent allowing for the pressure gradient to weaken across the Caribbean Sea. The cold fronts begin to track off the southeastern US coast approximately every three to four days and generally stall over the Bahamas and remain well to the north of the northeastern Caribbean Sea. Large northerly swells are still possible during the spring, particularly when lows develop along stalled cold fronts near the Bahamas and track northeast- ward towards Bermuda. These lows can strengthen into gales and, at times, can become “cut off” from the frontal boundary when it weakens and high pressure builds to the north and east of the system (see Figure 3). These cut-off gales then meander for approximately three to four days over the western Atlantic and gener- ate northerly swells of up to nine to 13 feet that wil impact the northern islands of the Eastern Caribbean. Cut-off gales can also develop tropical or subtropical characteristics while remaining nearly stationary over warm ocean waters for a few days. Eventually these systems will resume a more northeastward track when the high to the east moves away from the system and a cold front moves offshore the US East Coast. By May, we begin to see signs of the coming summer as weak tropical waves begin to develop off western Africa and trek thousands of miles across the Atlantic Ocean to the Caribbean. It is generally early for any o: these tropical waves to develop tropical cyclones. However as these tropical waves move over the eastern Caribbean Sea every three to four days, northeast to east winds and swells will generally increase approximately 12 to 24 hours ahead of the wave. Showers and thunderstorms will accompany the stronger tropical waves and a brie lull in the winds and swells from the east to southeast will develop immediately behind the wave. June Ist is the official start of the Atlantic hurricane season. The western Caribbean Sea is more favorable for tropical cyclone development as early as May and through June. Tropical development occurs on aver- age every other year in this region and is generally not a threat to the Eastern Caribbean. Tropical waves gradually strengthen through June and July and become more frequent through the eastern Caribbean Sea, arriving in the Lesser Antilles every two or three days. Showers and thunderstorms become more prev- alent with these waves and winds will strengthen out of the east to northeast to 20 to 25 knots and swells up to seven to nine feet ahead of the tropical wave. Once the tropical wave moves through, then winds will diminish from the east to southeast at 15 to 20 knots and swells to five to seven feet. From July through August is when all eyes begin to focus to the east. The sea surface temperatures have warmed up enough to become favorable for tropical cyclone development in the eastern Atlantic Ocean. These systems generally develop west of 30°W and will track either one of two ways: if high pressure remains stationary near Bermuda (also known as the Bermuda High this time of year), then the tropical cyclone will continue on a mainly west- erly course into the Caribbean Sea. From there the tropical cyclone can continue westward toward Central America if high pressure remains strong to the north or turn more northwestward if a cold front begins to move through the United States and west- ern Atlantic Ocean. —Continued on next page SERVICES Mechanics and Electricity Boat Maintenance Engine diagnosis Breakdown service 24/7 Haulout and hull sand blasting Equipment for rent Technical shop LEAVE MARINE MECHANICS (ALL MAKES) - HAUL OUT 24h BREAKDOWN SERVICE « SALES * REPAIRS * MAINTENANCE Marina Pointe-a-Pitre 97110 Phone: +590 590 907 137 Fax: +590 590 908 651 TOHATSU E-mail: fredmarine @ wanadoo.fr GOODS Genuine parts Yanmar & Tohatsu Basic spare parts (filters, impellers, belts) Filtration FLEETGUARD Anodes, Shaft bearings Electric parts, batteries Primers and Antifouling International Various lubricants FOR RENT High pressure cleaners 150/250bars Electrical tools Diverse hand tools Vacuum cleaner for water Scaffolding YOUR BOAT IN SKILLED HANDS