Tropical Storm Warnings are flying for the coast of South Carolina as newly-formed Tropical Depression Two
moves west-northwest at 12 mph towards the Southeast U.S. coast. An Air Force hurricane hunter aircraft found a closed circulation and top sustained winds of 35 mph in a small region near the storm's center on Friday afternoon, leading NHC to start issuing advisories on the Atlantic's second tropical cyclone of the year. Sea surface temperatures (SSTs) near TD 2's center were about 25 - 26°C (77 - 79°F), which are only marginally warm enough to support a tropical storm. Wind shear
on Friday afternoon was in the moderate range, 10 - 20 knots. A large area of dry continental air lay to the west of TD 2, and this dry air was interfering with development. Satellite loops
on Friday afternoon showed that TD 2 had only a modest amount of heavy thunderstorm activity that had increased in areal coverage only slightly since Friday morning.Figure 1.
Tropical Depression Two as seen by the MODIS instrument on NASA's Terra satellite on Friday afternoon, May 27, 2016. Image credit: NASA.A heavy rain threat for the Southeast U.S. coast
On Friday night and Saturday, TD 2 will be traversing waters of 25 - 26°C (77 - 79°F), and the 18Z (2 pm EDT) Friday run of the SHIPS model
predicted that wind shear would stay in the moderate range, 10 - 20 knots. These conditions are barely favorable enough to allow slow development of TD 2 into a 45-mph Tropical Storm Bonnie. There is dry air to the west of the storm that will interfere with development, as well. TD 2 may get a small boost when it crosses the axis of the warm Gulf Stream current on Saturday afternoon, when SSTs will be 27 - 28°C (81 - 82°F), but a 55 mph tropical storm is probably the strongest that TD 2 can get. The NHC official forecast of a top intensity of 45 mph is a more likely scenario. In their 5 pm EDT Friday Wind Probability Forecast
, NHC gave Charleston, South Carolina the highest odds of any city on the coast of seeing tropical storm-force winds of 39+ mph: 48%.
Heavy rain is the main concern from TD 2, and heavy rains should reach the coasts of South Carolina and southeastern North Carolina on Saturday afternoon or evening. In their Friday afternoon (12Z) runs, our two top two models for forecasting tropical cyclone tracks--the American GFS model and the European ECMWF model--showed TD 2 coming within 50 miles of the coast of South Carolina on Sunday morning. Steering currents will shift on Sunday evening, as TD 2 gets caught in the circulation associated with a trough of low pressure passing to the north. This shift will cause TD 2 to slow down as it reaches the coast of South Carolina, followed by a turn to the northeast. The center of TD 2 will likely track just inland along the coasts of South Carolina and North Carolina on Monday and Tuesday, spreading heavy rains of 2 - 4" along its path. It is uncertain at this time whether or not TD 2 will be able to spread heavy rains farther north into Virginia later in the week.