Erika Floods Dominica; Major Uncertainties on Potential U.S. Impact

Heavy rains and strong gusty winds are sweeping through much of the northern Lesser Antilles Islands today as Tropical Storm Erika heads west at 16 mph. An Air Force hurricane hunter aircraft was in the storm Thursday morning, and at 8 am EDT found Erika's top surface winds were 50 mph, with a central pressure nearly unchanged at 1004 mb. The aircraft also found the center had jogged to the south by about 20 miles between 5 am and 8 am, a position which put the center of circulation closer to Erika's heaviest thunderstorms. According to the Antigua Met Service, Canefield Airport on Dominica recorded 8.86" (225 mm) of rain Wednesday night from Erika, and heavy flooding has been observed on the island. The Guadaloupe Airport has recorded 1.18" of rain today as of 9 am EDT, with a peak wind gust of 36 mph. A Personal Weather Station (PWS) on St. Thomas in the Virgin Islands at higher elevation recorded a wind gust of 47 mph Thursday morning. Erika's tropical storm-force winds were on the east side of the storm, so the strongest winds of the storm will not occur in the Virgin Islands until Thursday afternoon. Satellite loops on Thursday morning showed that Erika continued to be disorganized in the face of dry air and wind shear. There is not much heavy thunderstorm activity on the storm's north side, where there was dry air from the Saharan Air Layer, and Erika had only a modest area of heavy thunderstorms on its east side. These thunderstorms did not change much in intensity or areal coverage on Thursday morning. Wind shear due to upper-level winds out of the west was a high 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 28.5°C (83°F).


Figure 1. Heavy flooding on Dominica on Thursday morning, August 27, 2015 from Erika. Image posted via Twitter to The Weather Channel.


Figure 2. Radar image of Tropical Storm Erika at 9:45 am EDT August 27, 2015, from the Guadaloupe/Martinique radar. Image credit: Meteo France.

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--water restrictions are in place in the capital of San Juan, where hundreds of thousands of residents receive water only two days per week.


Figure 3. MODIS image of Erika from NASA's Aqua satellite taken at approximately 10 am EDT Thursday August 27, 2015. At the time, Erika had top sustained winds of 50 mph. Image credit: NASA.

Erika's potential impact on the Bahamas and U.S. East Coast
The main words to describe the forecast for Erika for its impact on the Bahamas and the U.S. East Coast: much higher uncertainty than usual. Our main models for predicting hurricane track and intensity have been showing large differences run-to-run, and with each other. Erika could dissipate before reaching the U.S., could hit as a hurricane, or could miss the coast entirely, depending upon which model you believe. In the short term, we are pretty sure Erika will struggle, though. The 8 am EDT Thursday run of the SHIPS model predicted that wind shear would 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 possible that the increasing shear and interaction with the high terrain of Hispaniola and Puerto Rico will cause Erika to dissipate. This morning's southwards jog by 20 miles may turn out to bring the center of Erika close enough to Hispaniola on Friday to significantly weaken the storm. However, dissipation of Erika is looking less likely than it did when I wrote yesterday's post, since Erika has managed to generate enough heavy thunderstorm activity to moisten its environment and help protect it from dry air and high wind shear. If Erika survives into Saturday morning, Erika could intensify into a hurricane that might affect the Bahamas and U.S. East Coast. Sea surface temperatures (SSTs) warm to near 29°C (84°F) in the Southeast Bahamas, and are 30°C (86°F) in the Northwest Bahamas, which would potentially 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, which should allow the storm to 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. If Erika stays weak, the storm is more likely to plow into Florida. To further complicate matters, steering currents may collapse next week, allowing Erika to wander offshore the Southeast U.S. coast for many days. I give a 20% chance that Erika will end up being a landfalling hurricane for the U.S. East Coast, a 20% chance storm will dissipate by Saturday, 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, and a 30% chance that Erika will miss the U.S. East Coast entirely.


Figure 4. The 00Z Thursday (8 pm EDT Wednesday) runs of the European and GFS models had similar predictions for the intensity and track of Erika for 5 pm Sunday August 30, 2015, showing a weak tropical storm off the coast of Florida. Image taken from our wundermap with the “Model Data” layer turned on.


Figure 5. Forecasts of the track of Tropical Storm Erika from the 00Z Thursday (8 pm EDT Wednesday) 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. Compared to Wednesday's runs of the GFS Ensemble, today's run has many more members showing recurvature out to sea with no impact on the U.S. The operational high-res version of the GFS is the white line.

Hurricane Ignacio a threat to Hawaii
Hawaii needs to closely watch Hurricane Ignacio, which is gathering strength in the waters about 1200 miles east-southeast of the islands. Satellite loops on Thursday 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 light to moderate wind shear, conditions that favor possible rapid intensification. The 00Z Thursday (8 pm EDT Wednesday) run of one of our top models for forecasting hurricane tracks, the European model, showed Ignacio passing within 100 miles of Hawaii on Tuesday. Hawaii should also keep an eye on Tropical Storm Jimena, which is gathering strength in the waters over 1500 miles east southeast of the islands.

Bob Henson will have another Erika post later today.

Jeff Masters