With storms battering NW Europe and snow in Washington it is not surprising that the Caribbean is a popular cruising area. Currently we are in the main season which runs from December to May.
At this time the trade winds blow with a great deal of regularity from the ESE to NE, at a force 4 on the Beaufort scale, which is 11-16 knots. This is an average for 10 meters height so instruments on board will be measuring closer to 15-20 knots. Averages do not take into account where the wind is funnelled between islands or where there is shelter in their lee.
This steady direction of the wind is recorded for around 90% of the time in the southern Islands and more variable to the north (particularly north of the Virgin Islands) as depressions in the North Atlantic have more of an influence. Florida and the Bahamas are likely to experience spells of what can be described as typical trade winds broken by cold fronts from the states crossing the area. These can be intense but are usually short lived.
Driving the trade winds is the position of the Azores High, often called the Bermudan High on the west side of the Atlantic. The position and intensity varies throughout the season and also year on year. The high will usually be centred near the Azores Islands (hence the name) with a ridge extending towards Florida. Whilst this is in position the trades will blow with great regularity. At times there is a trough through the Atlantic splitting the Azores high from high pressure near Bermuda and Florida, this tends to weaken the trades in the more northerly islands. Towards the end of December and January we often get a stronger period of wind that has become known as the Christmas trades. Once the trade winds have become established they are likely to stay reasonably strong.
Inter annual variation in the trade winds can give some fairly large differences. There has been some research into the North Atlantic Oscillation (NAO) measured by the contrasting intensity of the Azores High and the Icelandic low. This has resulted in general seasonal forecasts that (when the NAO index is high) indicates a year when trade winds will be stronger than average. There is also a link, or teleconnection, between an El Ninõ event in the Pacific and stronger that average trade winds in the Atlantic. This is the opposite of that found in the Pacific where it is a La Niņa event that is associated with stronger than average trade winds.
Out at sea rainfall is almost all from large squall clouds. The number and intensity of squalls will depend on conditions in the upper atmosphere and the instability of the atmosphere. As a rule large raining clouds will give intense squalls with winds of 30-40 knots. However these will sometimes become more intense if the atmosphere is particularly unstable and squalls of 60 knots are not unheard of. For sailing boats this increase in wind is of great concern but also is the significant decrease in visibility from the heavy rain that accompanies the squall. Squalls will often be worst at night as the contrast between the cooling cloud top temperature and the sea water is at its greatest, giving early morning rainshowers.
Near Islands the topography of the land dictates the amount of rainfall. Although the rainy season does not start until May the higher islands will often have cloud cover. We do occasionally get large areas of rain and squalls working their way north from the ITCZ affecting the southern Islands with bands of cloud and rain.
Most of the time, the weather will be good, but out of season storms are possible. Weather forecasts should still be monitored as it is easy to get into a false sense of security after an extended period of NE 15-20 knots with the occasional squall.