Tropical Storm Isaias developed south of Puerto Rico on Wednesday night. At 11:00 p.m. EDT on Wednesday the center of Tropical Storm Isaias was located at latitude 15.8°N and longitude 67.0°W which put it about 155 miles (250 km) south of Ponce, Puerto Rico. Isaias was moving toward the west-northwest at 20 m.p.h. (32 km/h). The maximum sustained wind speed was 50 m.p.h. (80 km/h) and there were wind gusts to 65 m.p.h. (105 km/h). The minimum surface pressure was 1004 mb.
Tropical Storm Warnings were in effect for Puerto Rico, Culebra, Vieques, the U.S. Virgin Islands, the British Virgin Islands, St. Martin, St. Barthelemy, Saba, St. Eustatius, St. Maarten, the entire coast of the Dominican Republic, Le Mole St. Nicholas, Haiti to the northern border with the Dominican Republic, the Turks and Caicos, the Acklins, Crooked Island, Long Cay, the Inaguas, Mayaguana, the Ragged Islands, Cat Island, the Exumas, Long Island, Rum Cay and San Salvador. Tropical Storm Watches were in effect for Andros Island, New Providence, Eleuthera, the Abacos, the Berry Islands, Grand Bahama Island and Bimini.
Data from a scatterometer onboard a satellite and observations from a buoy south of Puerto Rico indicated that a distinct center of circulation formed within a large low pressure system over the northeastern Caribbean Sea and the National Hurricane Center designated the system as Tropical Storm Isaias. The center of circulation formed about 150 miles (240 km) south of Puerto Rico on Wednesday night. More thunderstorms were developing near the newly formed center. Many of those thunderstorms were southeast of the center. Storms near the center were generating upper level divergence which pumped mass away from the tropical storm. Bands of showers and thunderstorms were also located on the northern periphery of the circulation and the strongest winds were occurring in those bands. Winds to tropical storm force extended out 300 miles (485 km) to the northeast of the center of circulation. Winds to tropical storm force also extended out 130 miles (215 km) to the northwest of the center. The winds south of the center of Isaias were blowing at less than tropical storm force.
Tropical Storm Isaias will move through an environment favorable for intensification during the next 12 hours. Isaias will move over water where the Sea Surface Temperature is near 29°C. It will move through a region where the upper level winds are weak and there will be little vertical wind shear. Tropical Storm Isaias will get better organized and strengthen during the first half of Thursday. A portion of the circulation around Isaias will move over Hispaniola later on Thursday. Mountains on that island will disrupt the circulation and Tropical Storm Isaias will weaken while the center moves over land.
Tropical Storm Isaias will move around the western end of a subtropical high pressure system over the North Atlantic Ocean. The high will steer Isaias toward the west-northwest on Thursday. On its anticipated track Tropical Storm Isaias will move across Hispaniola on Thursday. Isaias will cause gusty winds and it will drop locally heavy rain when it moves over Hispaniola. Heavy rain could cause flash floods.