Forecasters say Tropical Storm Erika took an unexpected shift to the south, increasing the risk of a dangerous impact to the island of Hispaniola. The storm's interaction with that island could have a big impact on how much of a threat it is to Florida. (AP)

Tropical Storm Erika is battling an extremely unfavorable environment in the Caribbean, and it might get even more hostile over the next couple of days. Erika has stayed farther south than originally predicted, placing Puerto Rico and Hispaniola at higher risk, and throwing a huge amount of uncertainty into the future of the tropical storm.

At first glance, Tropical Storm Erika looks robust on satellite images this morning, with its bubbling cloud tops and wispy outflow. But in fact, the surface circulation is quite displaced from the strongest thunderstorms — a sign of strong, hostile wind shear and poor organization.

More critically, over the past 24 hours Erika’s surface circulation has been tracking due west instead of west-northwest, as it has been over the past couple of days, and as it was forecast. The tropical storm’s center of circulation reformed to the south of the previous location estimates. This motion could be a big deal for the forecasts, because if it does not get north of Puerto Rico and Hispaniola, those mountainous islands would likely shred the already-disorganized storm.

[Tropical Storm Erika brings disastrous flooding to Dominica]

To compensate for Erika’s more southerly track, the National Hurricane Center has shifted the forecast south as well, putting Florida much more in the center of the cone. In addition to this being a highly uncertain forecast in itself, there’s also a high level of uncertainty built into the forecast cone in the three to five day-range.

“In short,” the Hurricane Center wrote on Thursday morning, “potential impacts for the Bahamas and beyond are unusually uncertain.”

Additionally, the forecast cone does not serve as a catch-all for any potential outcome — instead, it is a fixed size that conveys the typical uncertainty for all National Hurricane Center forecasts, based on previous storms. By definition, there is still a 1/3 probability that the storm center could track outside of the cone, and this case may well end up in that 1/3 bin.

Tropical storm watches now extend into the eastern Bahamas, with tropical storm warnings covering Puerto Rico and the northern Leeward Islands.


Tropical Storm Erika continues to battle an incredibly unfavorable environment. The wind shear, which acts to tear apart tropical cyclones, is already quite strong — about 20 mph from the west — and it is forecast to increase on Friday to near 30 mph. That amount of shear is hard for a well-developed storm to fend off, and Erika does not have a head-start.

The latest runs of the forecast models reveal the uncertainty in the potential track as Tropical Storm Erika nears the Bahamas. Some of the models suggest Erika will pass south of Florida, some predict a direct landfall in Florida or even the Carolinas, while others forecast a track out into the open Atlantic before reaching the U.S. coast.

The corresponding intensity guidance is just as uncertain, with some models predicting Erika to stay weak through then ext five days, and others forecasting a Category 4 hurricane.


Model track forecasts (left) and intensity forecasts (right) for Tropical Storm Erika. (tropicaltidbits.com)

Given the latest trends in satellite and radar loops, it seems likely that Tropical Storm Erika will end up facing the mountains of the Greater Antilles, which should severely limit further strengthening. But, if the storm manages to make it over the mountains intact, it could quickly intensify again in the extremely warm waters around the Bahamas. For now, we’d be wise to put a bit more weight on the models that keep it further south and weaker over the next two to three days.