TD 2 Expected to Become Tropical Storm Bonnie as it Crosses the Gulf Stream

Tropical Storm Warnings continue for the coast of South Carolina as Tropical Depression Two chugs northwest at 13 mph towards the Southeast U.S. coast. Radar imagery from Charleston, South Carolina showed heavy rains from TD 2 had reached the coast late Saturday morning, and heavy rains will affect most of the coast of South Carolina by late Saturday afternoon. Satellite loops on Saturday morning showed that TD 2 had a relatively meager amount of heavy thunderstorm activity, which had increased in areal coverage only slightly since Friday. Strong upper level winds from the southeast were creating a high 20 knots of wind shear over TD 2 on Saturday morning, keeping the storm's heavy thunderstorms limited to the northeast side of the center. An Air Force hurricane hunter aircraft found top sustained winds of 35 mph in a small region near the storm's center on Saturday morning. Sea surface temperatures (SSTs) near TD 2's center were about 26°C (79°F)--marginally warm enough to support a tropical storm. A large area of dry continental air lay to the west of TD 2, and this dry air was interfering with development.


Figure 1. Ocean temperatures off the Southeast U.S. coast for the period May 25 - May 27, 2016, as sensed by AVHRR satellite instruments. On Saturday afternoon, TD 2 will be traversing the warm 27 - 28°C (81 - 82°F) waters of the Gulf Stream. Image credit: Ocean Remote Sensing Group, Johns Hopkins University Applies Physics Laboratory.

A heavy rain threat for the Southeast U.S. coast
On Saturday, TD 2 will be traversing warm Gulf Stream waters of 27 - 28°C (81 - 82°F), and these warm waters should provide enough of a boost to TD 2 to strengthen it into Tropical Storm Bonnie. However, there is dry air to the west of TD 2, and the SHIPS model predicted on Saturday morning that wind shear would rise to 20 - 25 knots on Saturday evening though Sunday. These conditions will likely keep TD 2 from becoming anything stronger than a 45-mph tropical storm. In their 11 am EDT Saturday 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: 43%. Heavy rain is the main concern from TD 2, and heavy rains of 2 - 4" are likely along the coast of South Carolina through Monday, with 1 - 3" amounts likely along the southeast coast of North Carolina through Tuesday. Water temperatures off the southeast coast of North Carolina are quite cool--near 24° C (75°F), so TD 2 should weaken as it progresses to the northeast along the coast on Sunday and Monday.

Jeff Masters