The northern swells that sometimes appear in the islands are created by big storms off the US East Coast
. The cold fronts coming off the coast in the winter intensify and can reach hurricane
strength. The swells churned up by these big storms take three or four days to reach the eastern Caribbean
islands. Bays and anchorages open to the north can become untenable.
winds in the Caribbean
are a result of enhanced Tradewinds driven by a strong mid-Atlantic High, and arrive from the east. They can be daunting, but are puny compared to a big northerly swell event.
During a north swell event in 2008, the north shore of Puerto experienced 40 foot breakers. Surfers in normally placid Cane Garden Bay on Tortola had a field day.
Here's a photo
of a north swell event on Nevis
in March, 2008. The swells were 12-14 feet and lasted a couple of days. My boat made it through with no problems.