A long quiet spell for tropical activity in the Atlantic has drawn to a close with the National Hurricane Center (NHC) designation of Potential Tropical Cyclone 6 at 5 p.m. EDT Monday, August 9. PTC 6 is expected to bring heavy rains and high winds, possibly as a tropical storm, as it crosses the Lesser Antilles early Tuesday and reaches the Greater Antilles by Wednesday.

A PTC refers to a system that is not yet a tropical cyclone with a closed circulation, but one that could evolve into a tropical storm and affect land areas in less than 48 hours. Ahead of PTC 6, tropical storm watches were posted late Monday for Barbados, Guadeloupe, Martinique, Dominica, the U.S. Virgin Islands, Puerto Rico, and much of the Dominican Republic coast. An Air Force reconnaissance mission is tentatively scheduled to investigate the system early Tuesday.

The elongated center of PTC 6 was located on Monday afternoon about 100 miles east-northeast of Barbados. A sizable, persistent cluster of showers and thunderstorms (convection) was focused on the northern side of the circulation, which is noteworthy for a time of day when convection is often at a minimum in an embryonic tropical cyclone. Satellite imagery showed favorable upper-air outflow toward the west and east of PTC 6, and wind shear was light (less than 10 knots). Sea surface temperatures of 28-29°C (82-84°F) are about 0.5°C above average for this time of year.

The last named system in the Atlantic was Hurricane Elsa, which became extratropical on July 9. Widespread wind shear and extensive dry air from Africa have since inhibited activity across the Gulf of Mexico, Caribbean, and Main Development Region (MDR) of the tropical Atlantic.

For the period 1991-2020, the average date for the sixth named storm in the Atlantic to develop is August 28. The next name on the Atlantic storm list is Fred.

Forecast for PTC 6

PTC 6 could take advantage of the typical nighttime peak in convection for developing tropical cyclones to reach tropical storm status Monday night or early Tuesday. As the system moves steadily west-northwest at about 15 mph, its heaviest convection can be expected to sweep across Martinique, Dominica, and Guadeloupe. Whether or not PTC 6 crosses the Lesser Antilles as a named storm, it can be expected to deliver intense squalls with heavy rain and gusty winds.

PTC 6 will continue to pass over warm water Tuesday as it crosses the northeast Caribbean, with sea surface temperatures rising to around 29°C along its path. However, it will face multiple challenges over the next several days. Northwesterly wind shear will be on the increase, reaching 10-15 knots on Tuesday and rising further to 15-20 knots by Wednesday. At that point, PTC 6 will likely be interacting with the rugged terrain of Puerto Rico and/or the eastern Dominican Republic, which will disrupt its circulation and work to prevent or limit any intensification. Mid-level relative humidity will hover around 50% over the next couple of days, which is less than optimal for development.

Despite its limited strength, PTC 6 could still bring heavy rains and some flooding as it rolls through the Greater Antilles. A flash flood watch is in effect for Puerto Rico and the U.S. Virgin Islands; rainfall totals of 2-3” are predicted, except 1-2” in northwest Puerto Rico, with locally higher amounts.

PTC 6 is projected by longer-range track models to stick near or just offshore of the northern coast of Hispaniola and Cuba into at least Thursday, which would tend to keep any intensification modest. The further north it tracks, the better its chances will be of strengthening. Assuming PTC 6 retains an identifiable circulation late this week, it could reach South Florida or the Florida Keys toward the weekend, most likely as a depression or a weak tropical storm, as implied by the NHC forecast shown above.

Beyond that point, strong ridging across the western Atlantic suggests that PTC 6 or its remnants could be steered into the eastern Gulf of Mexico, so tracking of this system may continue for some days to come.

Elsewhere, a tropical wave located several hundred miles east of PTC 6 in the central tropical Atlantic has failed to organize after several days of trying. In its Tropical Weather Outlook issued at 2 p.m. Monday, NHC gave this system a near-zero chance of development.

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