Hurricane Lenny. spawned by La Nina?

In the last 110 years that detailed records have been kept, the Caribbean has never experienced a tropical cyclone like Lenny. There have been two other late-season tropical storms that, like Lenny, followed a northeastward path across the Caribbean, one in 1909 and tropical storm Klaus in 1984. What made Lenny unique was the fact that not only was it a late-season hurricane that tracked from west to east, but also that it almost reached Catastrophic Category 5 status on Wednesday afternoon, November 17, when it was approximately 50 miles due south of Road Town, Tortola, and battering St. Croix. While the Caribbean Weather Center in downtown Road recorded a maximum gust of only 55 mph, a Cable & Wireless facility approximately 1,500 feet up at Chalwell recorded two maximum gusts of 185 mph and sustained winds in excess of 100 mph for more than three hours, from midday onwards.

What caused Lenny to form when it did? What caused it to track from west to east, and what allowed it to develop into a destructive Category 4 hurricane? The blame can, to some extent, be placed on La Niña. The world climate is currently in a strong La Niña, or cool period.

During La Niña years, a large part of the world experiences atmospheric pressure pattern changes. For example, in the tropical Atlantic and Caribbean during the latter part of hurricane season we have generally seen unusually low pressure. In the western Caribbean there had been unusually low atmospheric pressure some three weeks before Lenny was spawned. This allowed a large area of convection activity (thunderstorms) to develop. All that was needed was a tropical wave to come along to kick-start it, and, thus, Lenny was born on November 13 southwest of Jamaica.

However, the winter’s typical west-to-east weather patterns were already bringing cold fronts to the northwest Caribbean. Lenny, therefore, was influenced by these weather patterns and began moving east. At the same time, a persistent low pressure trough, first to the northeast of the Virgin Islands and then east of the Leewards, helped to draw Lenny east-northeastwards across the Caribbean. There was no upper wind shear, and sufficient warm, moist sea water allowed Lenny to rapidly develop into a dangerous hurricane.

With normal east-to-west-moving hurricanes, the strongest winds are usually experienced in the northwest segment of the system. With a reverse hurricane like Lenny, the strongest winds were in the southeast quadrant, and since it tracked south of the Virgin Islands, the BVI were spared the full wrath. When Lenny was some 80 miles southeast of the BVI, the trough that had been to the east of the Leewards had closed, and Lenny met instead weak high pressure, which caused it to stall and meander back and forth between St. Maarten and St. Barts. This brought destruction to St. Maarten and Anguilla.

Then, as Lenny moved from west to east, the reverse wind circulation generated an unusually large northwest swell that caused widespread damage to the western shores of the Leeward and Windward islands.

The Caribbean Weather Center’s forecasting model predicted the possibility of Lenny developing into a major hurricane, tracking from west to east and developing a large northwest swell for more than a week ahead. I began putting out warnings on the radio, on our web site and via the Internet. Perhaps because of the uniqueness of this system, sufficient heed was not given to these warnings by some islands. Thankfully, the BVI was fully prepared.

By Ocean Navigator