Tropical Storm Isaias (pronounced ees-ah-EE-ahs) formed on Wednesday night over the eastern Caribbean, bringing torrential rains and tropical storm-force winds to Puerto Rico and the Dominican Republic on Thursday. Isaias’s formation date of July 30 (Greenwich time) comes nearly a week earlier than the previous earliest appearance of the Atlantic’s ninth storm of the year, Irene, on August 5, 2005.

Tropical storm warnings and watches were in effect Thursday from Puerto Rico and the Virgin Islands into the Bahamas. A warning means tropical storm conditions are expected in the next 36 hours, and a watch means tropical storm conditions are possible within 48 hours. A tropical storm watch will likely be issued for portions of Florida later Thursday, since Isaias is likely to spread heavy rains into southeast Florida beginning on Saturday morning.

Long-range radar out of Puerto Rico on Thursday afternoon showed very heavy rainbands from Isaias were affecting Puerto Rico and the Dominican Republic. Numerous flash flood warnings and severe thunderstorm warnings were issued on Thursday for Puerto Rico. As of 9:30 a.m. EDT Thursday, the National Weather Service in San Juan, Puerto Rico, listed seven rivers on the island that were above flood stage, and 15 others near flood stage. Rainfall was heaviest on the western end of the island, with six Weather Underground personal weather stations reporting over 5 inches of rain. The peak rainfall amount was measured at Haciendas del Club on the west coast of the island, where 19.05″ had fallen as of 1:05 p.m. EDT Thursday.

At 11 a.m. EDT Thursday, Isaias was located near the southeastern coast of the Dominican Republic, with top sustained winds near 60 mph and a central pressure of 1003 mb. Punta Cana, on the eastern tip of the Dominican Republic, reported a wind gust of 60 mph late Thursday morning. Satellite images showed that Isaias was steadily growing more organized, with a large area of intense thunderstorms building near the surface circulation center, with additional low-level spiral bands forming. Isaias had favorable conditions for development, with sea surface temperatures (SSTs) near 29 degrees Celsius (84°F) and moderate wind shear of 10 – 15 knots. The system was embedded in a moderately dry atmosphere with a mid-level relative humidity of 60%, but the absence of strong wind shear to a large extent was allowing the storm to wall off the dry air of the Saharan Air Layer to its northwest.

Forecast for Isaias

Isaias will pass over Hispaniola on Thursday, and the mountainous terrain on the island will likely significantly disrupt Isaias’s circulation. The uncertainty in the future of the storm remains very high until it becomes apparent how the encounter with the island affects Isaias’s circulation and center location.

Isaias will also have to contend with dry air from the Saharan Air Layer, which surrounds it on three sides. The 12Z Thursday run of the SHIPS model predicted that the atmosphere surrounding Isaias would remain dry during the next five days, with a mid-level relative humidity of 50 – 55%. The model also predicted that wind shear would rise to a moderately high 15 – 25 knots on Friday through Monday, which should interfere with intensification by driving the dry air surrounding the storm into its core.

However, SSTs for Isaias will warm to 30 degrees Celsius (86°F) by Saturday, which will aid the intensification process. Also favoring intensification will be a large-scale region of ascending air over the Atlantic, caused by passage of an atmospheric disturbance called a Convectively Coupled Kelvin Wave.

The 11 a.m. EDT Thursday National Hurricane Center (NHC) intensity forecast called for Isaias to remain just below hurricane strength, peaking with 70 mph winds this weekend. Most of the top intensity models predicted that Isaias would peak as a strong tropical storm or low-end category 1 hurricane. A good uncertainty range for Isaias’s peak intensity this weekend off the Southeast U.S. coast is 60 – 85 mph, and I expect that we will see Isaias peak as a category 1 hurricane.

Invest 93L off the coast of Africa close to tropical depression status

A strong and well-organized tropical wave off the coast of Africa was designated Invest 93L by NHC on Thursday morning. A Thursday morning pass by the ASCAT satellite showed that 93L had a well-defined surface circulation, and satellite images revealed a well-organized system with plenty of heavy thunderstorms. In an 8 a.m. EDT Thursday Tropical Weather Outlook, NHC gave 93L two-day and five-day odds of formation of 20%. I think that these odds are too low, and should be 60%.

The system is expected to head north-northwest into a stable air mass and over cooler waters this weekend, and it likely will dissipate early next week.

The next name on the Atlantic list of storms is Josephine. The current record for earliest 10th Atlantic named storm formation is August 22, when Jose formed in 2005.

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