Tropical Storm Ericka is expected to move into the Caribbean with a possible slow strengthening as it moves north of Puerto Rico and Hispaniola. Tropical Storm Erika could become the second hurricane of the season. (NOAA GOES East)

Tropical Storm Erika formed  Monday night, the fifth named storm of the 2015 hurricane season. While Hurricane Danny struggled to maintain its intensity as it battled the hostile Atlantic environment, Tropical Storm Erika could prove to have more stamina, and is forecast to strengthen into a Category 1 hurricane by the time it reaches the Bahamas this weekend.

On Tuesday morning, Tropical Storm Erika was  about 750 miles east of the Leeward Islands in the tropical Atlantic, and had sustained winds of 45 mph, with gusts up to 60 mph. Erika’s thunderstorm activity has decreased over the past 12 hours, but the storm is expected to strengthen as it tracks west toward the Caribbean.

Erika was born from a tropical disturbance that left the coast of Africa last Thursday, and since then has swiftly moved west across the Atlantic Ocean. At 20 mph, Erika is moving at roughly twice the speed that former-Hurricane Danny moved, so effects will be seen within the next couple of days for the northeast Caribbean. A tropical storm watch is  in effect for the northern Leeward Islands.

The official forecast from the National Hurricane Center is for Erika to strengthen over the warm waters north of Hispaniola, possibly becoming a Category 1 hurricane early Saturday morning as it begins to pass over the Bahamas.

It’s far from certain what Erika’s final track will be, but Florida and the Southeast coast should be monitoring the progress of this storm over the next few days. If the storm manages to last as it tracks west through the unfavorable environment — and it’s a big if — Florida could start to feel Erika’s effects  Sunday.


Official track and intensity forecast for Tropical Storm Erika. (National Hurricane Center)

Unfortunately, forecast models disagree on Tropical Storm Erika’s track. The disagreement translates into a low-confidence forecast, which is important to keep in mind when looking at the National Hurricane Center forecast, as well.

The track forecasts have a wide range of possibilities — some models suggest the storm could track well north of the Bahamas, while others favor a track farther to the south over the Greater Antilles, including Hispaniola and Cuba. Generally, we’d expect a stronger storm if it tracks north, and a weaker storm if it stays south, especially given the mountainous terrain the storm would have to endure as it passes over the islands.

Of course, no model forecast should ever be considered a sure thing. Meteorologists look for trends and consistency among models, not necessarily a specific track.


Track guidance suite from 06Z today. (UAlbany)

The same models that forecast hurricane tracks also provide a forecast for future intensity. Looking at the Tuesday morning runs of the models, the corresponding intensity forecasts are even more spread out, also indicating   extremely low confidence in Tropical Storm Erika’s forecast.

The forecast models range from dissipating Erika all together, to strengthening the storm into a Category 4 hurricane. Even the most relied upon global forecast models — the GFS and the European ECMWF — disagree completely on Tropical Storm Erika’s future strength. The GFS dissipates the storm in a fashion similar to Danny, while the ECMWF keeps it quite strong as it recurves into the Atlantic, just east of Florida.


Forecasts from GFS (left) and ECMWF (right) both valid next Thursday evening. (tropicaltidbits.com)

The biggest factor in determining Erika’s future intensity will be  wind shear, which acts to destroy tropical storms. For now,  wind shear is moderate, so the storm should be able to slowly strengthen, or at the very least maintain its current intensity.

However, in one to three days, Tropical Storm Erika will enter the same area of high shear that ripped apart Hurricane Danny. This is not to say that Erika will respond the same way — Erika has a larger and more robust circulation which could fend off the hostile environment.

Tropical Storm Erika will reach the vicinity of the Leeward Islands on Thursday, likely as a strong tropical storm or even a Category 1 hurricane. At that point, it could dissipate; continue to head west-northwest; or it could turn to the north and away from the U.S. Much like Danny, we will have to wait and see how the storm responds to the hostile environment, since models are notoriously bad at forecasting tropical cyclones in areas of high, often detrimental wind shear.