Tammy on Track to Affect Parts of the Leeward Islands as a Hurricane this Weekend

Tropical storm conditions possible by as early as this evening for the lower Leeward Islands, including Dominica and Guadeloupe

Friday morning satellite of Tropical Storm Tammy approaching the Lesser Antilles. Credit: NOAA

Tammy – now a 65 mph tropical storm – appears to be gradually strengthening and is forecast to become a hurricane before reaching the northeastern Caribbean islands on Saturday.

Hurricane Watches and Tropical Storm Warnings are in effect for most of the Leeward Islands, excluding the U.S. and British Virgin Islands, for the likelihood of tropical storm conditions (winds above 38 mph) and the possibility of hurricane conditions (winds 74 mph or stronger) within the next day or two. Puerto Rico and the U.S. Virgin Islands are not currently under any watches or warnings as Tammy’s worst weather should remain to the east of these areas.

Although the storm is more symmetrical today than yesterday, stronger winds aloft continue to push the mid-level center away from the low-level center. Without a stacked circulation, Tammy has been unable to clear the atmospheric hurdle to hurricane status. The expectation from our forecast models is that this modest wind shear will relax over the coming hours, allowing gradual strengthening of the storm into a low-grade hurricane by the time it makes its closest approach to the islands on Saturday.

It's worth noting how unusual a hurricane would be this far east in the deep tropical Atlantic (south of 20°N) this late in the season. Only two hurricanes since 1900 – Tomas in 2010 and Jose in 1999 – have been recorded this far southeast in the Atlantic this deep into the season.

It's still unclear whether the center of Tammy will track squarely over parts of the Leeward Islands or just to the east tomorrow. Forecast models have overall trended slower and east of the islands since yesterday but regardless the core of the system will be close enough that areas under the hurricane watch will be directly affected.

Global model forecast trends since Wednesday, with darker blue lines indicating more recent model runs. The darkest blue line farthest east shows the most recent track guidance through Saturday afternoon based on a blend of our most reliable computer models. Models have trended east since Wednesday and come into better agreement (indicated by the smaller ellipse) on Tammy’s center moving east of Puerto Rico and the U.S. Virgin Islands this weekend. Credit: Tomer Burg/University of Oklahoma

In addition to the likelihood of strong, gusty winds up to hurricane strength, heavy rainfall is expected throughout the Leeward Islands which could lead to areas of flash flooding and mudslides. Up to a foot of rain locally is possible in some areas, with lower amounts on Tammy’s periphery. Tammy will drag its tail of heavy rains across the eastern Caribbean into Sunday as it turns northward, extending the flood threat through the weekend.

The storm is expected to get picked up by a jet stream dip next week and curve northeastward into the open Atlantic. Although Tammy will get a boost from jet stream winds, likely growing into a stronger hurricane, it won’t pose any direct threat to land.

What’s behind Tammy in the Atlantic?

For now things looks mostly mundane into early next week across the tropical Atlantic. Long range models are beginning to hint at lower pressures left in the wake of Tammy, extending from the eastern or central Caribbean into the western Atlantic. Right now, there isn’t much to hang our hat on, but it’s a part of the Atlantic we’ll keep an eye to for the latter part of next week.