Long-suffering residents of the Northwest Bahamas—still coming to grips with the human and physical toll from catastrophic Hurricane Dorian—now face a tropical storm warning for a system predicted to develop into a tropical storm by Saturday. A disturbance in the Southeast Bahamas, designated Potential Tropical Cyclone 9 (PTC 9) by the National Hurricane Center on Thursday, is expected to develop into Tropical Depression 9 or Tropical Storm Humberto.

The PTC label, introduced in 2017, refers to a disturbance that is not yet a tropical cyclone (typically because it lacks a closed center of circulation) but that could bring tropical storm or hurricane conditions to land areas within 48 hours. The tropical storm warning for the Northwest Bahamas includes all areas except Andros Island. Tropical storm watches are up for a portion of the central east coast of Florida, as well.

PTC 9 was bringing heavy rain showers and gusty winds to the east coast of Florida and The Bahamas on Friday morning, including the islands of the Northwest Bahamas hardest-hit by Hurricane Dorian earlier this month. Settlement Point, on the western end of Grand Bahama Island, which was devastated by Dorian, recorded sustained winds of 28 mph, gusting to 32 mph, at 2 am EDT Friday morning. As of 11 am EDT Friday, the top surface winds found by the Hurricane Hunters in PTC 9 were just 25 mph, though the plane had not sampled the entire circulation yet.

Satellite loops and data from the Bahamas radar and the Hurricane Hunters on Friday morning showed that PTC 9 did not have a well-defined surface circulation, but heavy thunderstorm activity was steadily growing in intensity, areal coverage, and organization. The system was tangled up with an upper-level trough off the west coast of Florida, and the upper-level winds associated with this trough were bringing dry air and moderate wind shear of 15 – 20 knots to the west side of PTC 9, inhibiting development. Sea surface temperatures (SSTs) in The Bahamas, though depressed by the passage of Hurricane Dorian, were still plenty warm for development–near 29°C (84°F).

The main threat from PTC 9 to the Northwest Bahamas is heavy rain and perhaps tropical-storm-force winds, as the system will have little time to intensify further before reaching the islands. Widespread rainfall of 2” to 4” is expected, with pockets of up to 6” possible. These conditions will hamper post-Dorian relief efforts, but will provide much-needed rain to the cisterns used to provide fresh water to the islands.

Forecast for PTC 9

The forecast models for PTC 9 have come into better agreement than on Thursday. The consensus is now that PTC will not cross into the Gulf of Mexico; fewer than 10% of the 70+ members of the 0Z Friday GFS and European model ensemble forecasts now have that solution. Instead, the models now agree upon a slow motion to the northwest bringing PTC 9 very close to the northeast coast of Florida on Saturday. By Sunday, a trough of low pressure to the north of PTC 9 is expected to pull the storm to the northeast, putting it well to the southeast of North Carolina by the middle of next week. At this time, it is very uncertain how much rainfall the Carolinas might get from PTC 9.

The main factor keeping PTC 9 in check is strong southerly wind shear and dry air from an upper-level low in the eastern Gulf of Mexico. Forecast models show this low retreating toward the western Gulf over the next couple of days, with upper-level high pressure building over PTC 9. This shift will make it easier for PTC 9 to strengthen over the weekend, assuming that the disturbance can consolidate around a well-defined surface low.

The 12Z Friday SHIPS model predicted that wind shear would drop to about 10 knots on Saturday and Sunday from the current 15 – 20 knots, then rise to the high range, 15 – 30 knots, beginning on Monday afternoon. The high wind shear expected early next week is a key reason why less than 10% of the 70+ members of the 0Z GFS and European model ensemble forecasts predicted that PTC 9 would eventually become a hurricane.

Despite the passage of Hurricane Dorian, which churned up vast amounts of water around the Northwest Bahamas, PTC 9 will have ample oceanic fuel. Sea surface temperatures (SSTs) cooled by 2-3°F along Dorian’s path, but SSTs along the projected path of PTC 9 will still be near 29°C (84°F), which is very supportive of development.

Longer-range ensemble tracks from the GFS and European suggest that steering currents may collapse later next week, which could leave PTC 9 close enough to the East Coast to require continued monitoring.