By Jeff Masters From Weather Underground

Tropical storm watches are up for much of the northern Lesser Antilles Islands as Tropical Storm Erika speeds westwards at 20 mph. Erika formed in the waters a few hundred miles east of the northern Lesser Antilles Islands overnight, and is likely to take a west to west-northwest course over the next few days, a path that should concern residents of the Bahamas and the U.S. East Coast. However, Erika’s survival over the next few days is not a sure thing. Satellite loops on Tuesday morning showed that Erika was struggling, with the low-level center exposed to view and only a small and decreasing area of heavy thunderstorms, limited to the storm’s south side. Wind shear due to upper-level winds out of the northeast was moderate, 10 – 15 knots, and this shear was driving plenty of dry air on the north side of Erika into its core, disrupting the storm. Sea surface temperatures (SSTs) are favorable for development, though—near 27.5°C—and will warm to 28.5°C by Thursday.

Figure 1. Latest satellite image of Tropical Storm Erika.

Forecast for Erika
The 8 am EDT Tuesday run of the SHIPS model predicted that wind shear would remain in the moderate range through the remainder of the week. Steering currents for Erika are very similar to what Danny experienced, and the 8 pm EDT Monday (00Z Tuesday) run of the GFS and European models showed Erika taking a track into or just north of the northern Lesser Antilles Islands on Wednesday night, and into the Southeast Bahamas by Saturday. A trough of low pressure capable of turning Erika to the north will set up shop along the U.S. East Coast late this week, but it is uncertain at this time whether or not Erika will be strong enough to get picked up by this trough. If Erika manages to fight off the dry air and wind shear besetting it and grow into a strong tropical storm by Thursday, it would likely take a more northerly course and be a long-range threat to the U.S. East Coast, Bermuda, or Canada early next week. If Erika stays weak, it will track more to the south, and take a path close to Hispaniola and into the Bahamas by this weekend. A path close to Hispaniola would potentially disrupt a weak Erika and cause it to dissipate; this was the solution of both the 00Z and 06Z Tuesday morning runs of the GFS model. Two of our other top three models from last year for predicting hurricane tracks–the HWRF and UKMET models–showed Erika staying stronger and taking a more northerly track with their 00Z Tuesday runs; the European model was in between, and showed Erika coming very close to the Florida coast by Monday (Figure 2). I’m not ready to ring the alarm bells on this storm yet, as a bet against significant strengthening is usually called for during a strong El Niño season. Still, El Niño years do get major hurricanes–the big hurricane of the last strong El Niño year of 1997 was also named Erika, and peaked at 946 mb /125 mph winds (thanks go to wunderground member BaltimoreBrian for this reminder.) The models should do a better job with Erika in their Tuesday night runs, since they will have data from the Hurricane Hunters to chew on. The NOAA jet is in the air today releasing dropsondes in the vicinity of Erika to characterize the upper-air steering currents, and there will also be low-level penetrations of Erika this afternoon from the Air Force Hurricane Hunters.

Figure 2. The 00Z Tuesday (8 pm EDT Monday) runs of the European and GFS models had two very different predictions of the intensity of Erika for 8 am Monday August 31, 2015. The European model showed Erika as a strong tropical storm just off the coast of Florida (purple colors = winds of at least 58 mph), while the GFS model showed Erika merely as a strong tropical wave with no closed circulation. Image taken from our wundermap with the “Model Data” layer turned on.

Figure 3. Forecasts of the track of Tropical Storm Erika from the 00Z Tuesday (8 pm EDT Monday) run of the GFS model from the twenty members of the GFS Ensemble model. The GFS Ensemble takes the operational version of the GFS model and makes twenty different runs of it at lower resolution with slightly different initial conditions to generate an ensemble of possible forecasts. As we can see, there are a wide variety of possible solutions. The operational high-res version of the GFS (white line) shows Erika dissipating near Hispaniola.

For more on this story and updates go to: http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=3086