Warm days belie winter weather in the tropics

Jan 14, 2013 by Guest Writer

It’s early winter and another migration is coming to an end. In this case we are not talking about birds, but mariners in the northern latitudes who are heading south into ports along South Florida, the Bahamas, and the Caribbean. These regions are desirable in avoiding the cold and snowy weather, but winter is still present there and traveling can become hazardous at times.
Cold fronts track through Florida and the Northern Bahamas in November through early December about every three days. These fronts strengthen and move farther south before stalling out over Hispaniola or passing north of the Eastern Caribbean from later December through early February.
By later February, fronts begin to stall farther north over Florida and the Bahamas. Winds typically veer from southeast to southwest and ease 12-24 hours ahead of a frontal passage.
However, thunderstorms can locally enhance the southerly winds, particularly across Florida and the Bahamas. This is common when a strong low or gale is developing offshore of the southeastern U.S. coast. Thunderstorms gradually weaken once the gale moves northeast over the northwestern Atlantic.
However large northwest swells will propagate southward from the gale and reach the shores of eastern Florida, the Bahamas and the northeastern Caribbean.
In the wake of these pattern shifts, high pressure will usually build eastward from the Gulf of Mexico, across Florida and the Bahamas, then spread over the Caribbean. Initially, the cold air will produce fresh-to-strong north-to-northeast winds. (And, at times in January, up to gale force winds, especially in the Yucatan Channel and Florida Straits.)
These stronger winds usually occur about 24-48 hours behind the cold front, before gradually easing and veering to the northeast-to-east. These high winds are the leading edge of cold air that has plummeted south from southern Canada. Large swells will build from the northwest at first, and eventually originate from the northeast. The swells are usually highest in the Gulfstream, though in January it is not unusual for northwest swells as high as 12-15 feet to reach the northeastern islands of the Caribbean.
High pressure will typically move eastward from the Southeastern U.S. to the north of the Bahamas and Caribbean. The high will interact with low pressure that resides between 10N and the equator, which allows for trade winds to increase across the Caribbean and southern Bahamas.
Fresh-to-strong easterly winds are typical across this region, and stronger highs to the north will enhance these winds even more to near-gale force at times. Areas that are prone to “funneling” winds are in passages between islands, offshore capes (Cabo Beata in the southern Dominican Republic, for example), and offshore of the Colombian coast.
These conditions may last up to four days across the southern Bahamas and the northwestern Caribbean, but can last up to seven days over the southern and eastern Caribbean.
So given the frequent cold fronts to the north and strong trade winds, when would be a good time to move to a new location?
The best time to transit is usually 1-2 days prior to a frontal passage. High pressure retreats to the east and trade winds usually ease and become more southerly ahead of the cold front. This is advantageous when first departing from South Florida and heading into the Bahamas or into the Old Bahama Channel.
It is best to avoid transiting immediately after a cold front moves through the region, as strong northerly winds and building large swells will be present. If this is the only option, it is best to travel in protected waters such as in lee of islands to minimize northerly swells.
Also, when traveling between southern Central America to the northeastern Caribbean, a more northern route would be best to avoid enhanced east-to-northeast trade winds that will be offshore Colombia (where gale force winds are common). Usually aiming for a waypoint in the area of 14-15N and 76-78W will help minimize these conditions.
However, it is always best to confirm with a reliable weather source in being aware of these expected conditions before departing port.

Amanda Delaney is a Senior Meteorologist at Weather Routing Inc (www.wriwx.com). Weather Routing Inc. has been providing forecasts to mariners for over 50 years. Comments on this story are welcome at editorial@the-triton.com.