Tropical Storm Erika a Potential Threat to the Bahamas and U.S. East Coast

Tropical storm warnings are flying for Puerto Rico, the Virgin Islands, and much of the northern Lesser Antilles Islands as Tropical Storm Erika speeds westwards at 17 mph. An Air Force hurricane hunter aircraft was in the storm Wednesday morning, and found Erika's winds had increased slightly, with top surface winds up to 45 mph from their previous 40 mph. Erika's tropical storm-force winds were all on the east and northeast sides of the storm. Satellite loops on Wednesday morning showed that Erika continued to be disorganized in the face of dry air and wind shear. The low-level center was partially exposed to view on the north side, the location of plenty of dry air from the Saharan Air Layer. Erika had only a modest area of heavy thunderstorms on its east side, and these thunderstorms did not change much in intensity or areal coverage on Wednesday morning. Wind shear due to upper-level winds out of the west was a moderate 10 - 20 knots, and this shear was driving dry air on the northwest side of Erika into its core, disrupting the storm. Sea surface temperatures (SSTs) were favorable for development, though—near 28°C (82°F).


Figure 1. Latest satellite image of Tropical Storm Erika.


Figure 2. NOAA Hurricane Hunter aircraft N43RF flew into Tropical Storm Erika on Tuesday evening, and the crew captured this rainbow SE of the center. Image credit: @NOAA_HurrHunter.

Erika's impact on the Caribbean islands
Erika's expected rainfall amounts of 3 -5" in the islands may cause some isolated flash flooding and mudslides, but should help alleviate severe to extreme drought conditions some of the islands are experiencing. Puerto Rico, for example, desperately needs the rain--the remnants of Danny brought 0.48" of rain to the capital of San Juan on Tuesday, but the city is still 10" below the normal 33" of rain it should have received by this point in the year. Water restrictions are in place in the city, whereby hundreds of thousands of residents receive water only two days per week.

Erika's potential impact on the Bahamas and U.S. East Coast
The 8 am EDT Wednesday run of the SHIPS model predicted that wind shear would remain in the moderate range through Thursday, then increase to a high 25 - 30 knots Thursday night though Friday afternoon, due to an upper-level low that is expected to remain near eastern Cuba through Friday. At that time, Erika will be passing just north of Puerto Rico and the Dominican Republic, and it is quite possible that the increasing shear and interaction with the high terrain of Hispaniola and Puerto Rico will cause Erika to dissipate, as predicted by the Wednesday morning runs of the GFS model. If Erika survives into Friday afternoon, the potential for a dangerous storm that will affect the Bahamas and U.S. East Coast increases substantially. Sea surface temperatures (SSTs) warm to near 29°C (84°F) in the Southeast Bahamas, and 30°C (86°F) in the Northwest Bahamas. This will provide plenty of extra fuel for intensification. The upper low over Cuba is forecast to weaken on Saturday, which should cause wind shear to drop to the moderate range, 10 - 15 knots, as Erika traverses the Bahamas, allowing the storm to take advantage of the warmer ocean temperatures are grow more organized. A weak trough of low pressure capable of turning Erika to the north will set up shop along the U.S. East Coast late this week, and it is possible that Erika will be strong enough to get picked up by this trough and turn to the north just before reaching the Florida coast on Sunday evening or Monday morning, as suggested by the Wednesday morning runs of two of our top models for predicting hurricane tracks, the UKMET and HWRF models. If Erika stays weak, the storm is more likely to plow into Florida, as predicted by the Wednesday morning run of the European model. I give a 20% chance that Erika will end up being a landfalling hurricane for the U.S. East Coast, a 40% chance storm will dissipate by Saturday, and a 30% chance the storm will be too weak and disorganized to have time to organize into a hurricane before hitting the U.S. East Coast. There is also a small chance (10%) that Erika could miss the U.S. East Coast--a situation that would most likely arise if Erika quickly organizes into a hurricane by Saturday, and thus "feels" the steering influence of winds higher up in the atmosphere, forcing the storm to recurve out to sea.


Figure 3. The 00Z Wednesday (8 pm EDT Tuesday) runs of the European and GFS models had two very different predictions of the intensity of Erika for 5 pm Sunday August 30, 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 4. Forecasts of the track of Tropical Storm Erika from the 00Z Wednesday (8 pm EDT Tuesday) 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 moving over South Florida. Most of the GFS ensemble members keep Erika weak, resulting in a more southwards track for the storm than our other top models are showing.

Tropical Storm Ignacio a threat to Hawaii
Hawaii needs to closely watch Tropical Storm Ignacio, which is gathering strength in the waters about 1400 miles east-southeast of the islands. Satellite loops on Wednesday morning showed that Ignacio had an impressive area of heavy thunderstorms that were growing in intensity and areal extent, and Ignacio is over warm waters with moderate wind shear, conditions that favor possible rapid intensification. Our two top models for forecasting hurricane tracks, the European and GFS models, both showed Ignacio passing within 200 miles of Hawaii on Tuesday. Hawaii should also watch Invest 96E, which is close to tropical depression status over 1500 miles east-southeast of the islands. 96E will track towards the Hawaiian Islands over the next seven days, and could be a long-range threat late next week.

Bob Henson will have another Erika post later today. Hope you caught his retrospective look back at Hurricane Katrina on #WUTV last night!

Jeff Masters