Agwe’ Sailing Caribbean (Leeward Islands) Weather introduction and overview. Macintosh HD:Users:SF:Desktop:Caribbean Voyages:Leeward Islands Trip Binder:Caribbean Weather - Leeward Islands Overview copy.docx

1

Caribbean (Leeward Islands) Weather Basics In General • Predominant Trade Winds: NE / E / SE at 10-25Kts. • Year round temperatures range: 78° F (25°C) to 85° F (29° C). • Dry Season: February - June:

o occasional small showers. • Wet Season: July – January:

o More frequent showers. o Occasional rainy days:

! With Squalls: can be 40kts +. ! Seen from afar. ! Can’t gauge wind – assume a lot.

o Infrequently squalls or cold fronts can produce Westerly winds making usual anchorages uncomfortable.

Winter Months

• Storms and cold fronts further north create “Northerly Swells” (Like wind swells are named for where they are coming from).

o “Few are really bad” (Doyle guides). o Avoid anchorages open to the North or West! o Hurricane swells (season: June to October – ends November 30th) can

be from any direction. (Hurricane details from Doyle’s guide). ! Rarely in December. ! June; July; October - approximately one every 3 years for:

• The whole Western Atlantic; including Caribbean Sea and Gulf of Mexico.

! August; September – approximately 5 per year: • Start well out in the Atlantic. • Start at the latitude of the Windward Islands. • Usually swing north and pass through northern areas of

the Caribbean. • Very few hit the Windwards sometimes for years at a

time. Christmas Winds (Summary from Doyles Guide – Detailed description on Pg 3)

• Created by big high pressure systems to the NE. • Winds increase (25 - 30kts). • Highs can be offset by cold fronts coming down from the Northwest.

o When approaching: conditions can become calm and sunny followed by wind and rain.

o These fronts rarely make it as far south as the Windward Islands. Visibility (From Doyle’s guide)

• Varies from exceptional lows of 5 miles to a high of over 50 miles. • Extremely hazy days caused by dust from Africa.

Agwe’ Sailing Caribbean (Leeward Islands) Weather introduction and overview. Macintosh HD:Users:SF:Desktop:Caribbean Voyages:Leeward Islands Trip Binder:Caribbean Weather - Leeward Islands Overview copy.docx

2

General Terminology • Inter-tropical Convergence Zone –

o a narrow zone near the equator where northern and southern air masses converge, typically producing low atmospheric pressure. may produce squalls or cloudy weather.

• Tropical Disturbance, Tropical Wave, and Upper Level Trough – o Poorly organized weather systems associated with squalls of varying

intensity. • Tropical Depression –

o Organized weather system with sustained winds of up to 35kts and rain. o Can be very intense or very weak.

• Tropical Storm – o Sustained winds of 35 to 63kts. (Sustained winds > 64kts = a

hurricane). o Heavy rains.

• Tropical Wave – o Tropical waves, easterly waves, or tropical easterly waves, also known

as African easterly waves in the Atlantic region, are a type of atmospheric trough, an elongated area of relatively low air pressure, oriented north to south, which moves from east to west across the tropics causing areas of cloudiness and thunderstorms.

• Trough – o an elongated region of relatively low atmospheric pressure, often

associated with fronts. ... Unlike fronts, there is not a universal symbol for a trough on a weather chart. The weather charts in some countries or regions mark troughs by a line.

• Katabatic Winds – o from the Greek word κατάβᾰσις katabasis meaning "descending", is the

technical name for a drainage wind, a wind that carries high density air from a higher elevation down a slope under the force of gravity. Such winds are sometimes also called fall winds.

Agwe’ Sailing Caribbean (Leeward Islands) Weather introduction and overview. Macintosh HD:Users:SF:Desktop:Caribbean Voyages:Leeward Islands Trip Binder:Caribbean Weather - Leeward Islands Overview copy.docx

3

Christmas Winds Detailed Overview. (http://www.oceannavigator.com/January-February-2012/Christmas-winds/) Atlantic Subtropical High:.

• Located generally between 30° and 35° N. • Present in the Atlantic almost all year, but its position shifts with the seasons, and

its size and strength also varies. • Summer: larger and stronger but located at higher latitudes. • During warm to cold season transition:

o Storm track of lows shifts southward. o Creates corresponding shift southward of the subtropical high to approx.

25° to 30° N. o High becomes weaker. o Induces a stronger pressure gradient between its higher pressure and the

lower pressure farther south toward the equator creating stronger SE winds ranging 25-30 knots.

o Seas can build to 15’ + • Areas affected:

o East of the Bahamas and north of the Antilles (Cuba, Jamaica, Puerto Rico, Hispaniola, Dominican Republic, the Caymans, including the Lesser Antilles: the Leeward and Windward Islands).

o Northwestern Caribbean (south of Hispaniola and Puerto Rico) can also experience higher than normal seas.

• As the Storm track becomes more active and shifts further southward: o Strong lows move from eastern US into western Atlantic and intensify

further south – resulting in: ! A weakening of the subtropical high ! The Center of the high shifting further south and east (may even

disappear at times). ! The high moving out of the subtropical latitudes to the eastern

Atlantic. ! Interrupting the flow of the trade winds and, ! Smaller seas north of the Caribbean islands ! Possibly creating large swells from the north affecting the north

sides of the islands (from strong northerly winds generated by winter storms further north – “north easterlies”

o Ultimately causing the Christmas winds to abate.