One time Tropical Storm Bonnie regained tropical depression status near Cape Hatteras on Thursday morning. The National Hurricane Center (NHC) will resume issuing advisories on Tropical Depression Bonnie at 11:00 a.m. EDT on Thursday. At 5:00 a.m. EDT on Thursday the center of Tropical Depression Bonnie was located at latitude 35.0°N and longitude 75.7°W which put it about 15 miles (25 km) south of Hatteras,, North Carolina. Bonnie was moving toward the east-northeast at 9 m.p.h. (15 km/h). The maximum sustained wind speed was 25 m.p.h. (40 km/h) and there were wind gusts to 35 m.p.h. (55 km/h). The minimum surface pressure was 1010 mb.
After making landfall in South Carolina during the weekend, the center of Bonnie made a counterclockwise loop over land and then drifted east of Charleston. The center drifted back over water on Tuesday and more thunderstorms formed near the center of circulation. However, the vertical wind shear was still significant and the tops of the thunderstorms were periodically blown away from the lower part of the circulation. As a result the circulation of Post Tropical Depression Bonnie was relatively shallow and existed primarily in the lower troposphere below 700 mb (about 3 km above the surface). The low level circulation of Bonnie has move slowly east-northeast since that time and more thunderstorms developed in several spiral bands around the circulation.
During the past 24 hours Bonnie drifted under the axis of an upper level trough where the upper level winds are lighter. As a result thunderstorms have persisted and a well formed band wraps around the northern and western sides of the center of circulation. The circulation also extends higher into the atmosphere and a small area of upper level divergence has developed over the center of circulation. Bonnie again has the appearance of a tropical cyclone on both satellite and radar images, which is why NHC is resuming advisories on the system.
Tropical Depression Bonnie could intensify further in the short term. The Sea Surface Temperature (SST) reported by a buoy at Diamond Shoals, which is near the center of Bonnie, is near 26°C. So, there is enough energy in the upper ocean to support some strengthening. The upper level winds are light and vertical wind shear is not significant at this time. If the surface pressures start to decrease, then Bonnie has a chance to become a tropical storm again. Eventually, Bonnie will move into an environment where the SSTs are cooler and there is more vertical wind shear.
Tropical Depression Bonnie is between a subtropical high pressure system to its southeast and mid-latitude westerly flow to its north. That combination is steering Bonnie slowly toward the east-northeast. A general easterly motion is expected to continue during the next few days. On its anticipated track Bonnie will gradually move away from the east coast of the U.S.
Even though Bonnie weakened to a tropical depression before it made landfall near Charleston, it brought locally heavy rain and flooding to parts of South Carolina and eastern Georgia. A portion of Interstate 95 was closed due to high water. At 11:00 p.m. EDT on Sunday the center of Tropical Depression Bonnie was located at latitude 32.8°N and longitude 80.1°W which put it about 5 miles (10 km) west of Charleston, South Carolina. Bonnie was slowly meandering near the coast. The maximum sustained wind speed was 30 m.p.h. (50 km/h) and there were wind gusts to 40 m.p.h. (65 km/h). The minimum surface pressure was 1012 mb.
Strong southeasterly winds in the upper levels generated significant vertical wind shear that weakened Tropical Storm Bonnie to a tropical depression on Sunday morning. However, the lower level circulation remained relatively intact as Bonnie moved onto the coast near Charleston, South Carolina. Thunderstorms in a band west of the center of circulation dropped heavy rain in parts of southwestern South Carolina and eastern Georgia. The heavy rain produced some flooding.
Most of the rain has tapered off to a few narrow bands of showers, which is normal at night when weaker tropical cyclones move inland. However, daytime heating could destabilize the atmosphere and new thunderstorms could redevelop over land on Monday. Some of those storms could also produce locally heavy rainfall and cause additional flooding.
The center of Tropical Depression Bonnie has drifted back to near the Atlantic Ocean. Strong southeasterly winds are still blowing in the upper levels and the vertical wind shear should prevent significant redevelopment even if the center moves back over water. Proximity to the ocean will make it easier for the circulation to pull in more moisture, which could contribute to heavier rainfall.
A high pressure system over the Atlantic and an approaching cold front will combine to produce southwesterly winds in the lower levels of the atmosphere. Those low level winds are forecast to steer Tropical Depression Bonnie slowly toward the northeast during the next several days. On its anticipated track Tropical Depression Bonnie is expected to move slowly along the coast of South Carolina and North Carolina. The primary risks will be locally heavy rain, flooding and rip currents.
Tropical Depression 2 intensified into Tropical Storm Bonnie as it passed over the warm waters of the Gulf Stream on Saturday. At 5:00 p.m. EDT on Saturday the center of Tropical Storm Bonnie was located at latitude 31.1°N and longitude 79.4°W which put it about 120 miles (195 km) south-southeast of Charleston, South Carolina. Bonnie was moving toward the northwest at 10 m.p.h. (16 km/h). The maximum sustained wind speed was 40 m.p.h. (65 km/h) and there were wind gusts to 50 m.p.h. (80 km/h). The minimum surface pressure was 1008 mb. A Tropical Storm Warning remains in effect for the coast of South Carolina from the Savannah River to Little River Inlet.
Although the convection in Tropical Depression 2 dissipated when it passed over a region of slightly cooler Sea Surface Temperatures (SSTs) on Friday night, thunderstorms redeveloped and persisted when the system crossed the warmer SSTs in the Gulf Stream on Saturday. Persistent thunderstorms produced enough increase in the wind speed to intensify Tropical Depression 2 into Tropical Storm Bonnie.
The circulation around Tropical Storm Bonnie is very asymmetrical. Almost all of the thunderstorms are occurring in the northwestern quadrant of the storm and the strongest winds are being generated in that part of the circulation. There are mainly thin bands of showers in the rest of the storm. Winds to tropical storm force extend out about 60 miles (95 km) from the center in the northwest quadrant of Bonnie.
An upper level low over Florida and an upper level ridge over the western Atlantic Ocean are combining to generate southeasterly winds which are blowing over the top of Tropical Storm Bonnie. Moderate vertical wind shear is contributing to the asymmetrical structure by tilting the circulation toward the northwest. The wind shear is strong enough to inhibit intensification, but Tropical Storm Bonnie is extracting more energy as it passes over the Gulf Stream. Bonnie could possibly intensify a little more while it is over the Gulf Stream.
The ridge east of Bonnie is steering the tropical storm toward the northwest and that general motion is expected to continue for another 12 to 24 hours. An upper level trough approaching the ridge from west and the trough is expected to cause the ridge to weaken. When the ridge weakens, the steering currents will also weaken. Tropical Storm Bonnie could stall or meander for several days when that happens.
On its anticipated track the center of Tropical Storm Bonnie will approach the coast of South Carolina on Sunday. The primary threats are coastal erosion, rip currents and locally heavy rainfall. Persistent rain could create the potential for fresh water flooding. Wind damage is likely to be minimal. However, where the winds blow onshore, they could push water toward the coast and contribute to some coastal flooding.
A reconnaissance plane investigated the system that was formerly designated Invest 91L on Friday afternoon and the plane found that the system had enough tropical characteristics to be classified as Tropical Depression 2 by the National Hurricane Center (NHC). At 5:00 p.m. EDT on Friday the center of Tropical Depression 2 (TD2) was located at latitude 28.5°N and longitude 74.7°W which put it about 435 miles (695 km) southeast of Charleston, South Carolina. TD2 was moving toward the west-northwest at 13 m.p.h. (20 km/h). The maximum sustained wind speed was 35 m.p.h. (55 km/h) and there were wind gusts to 45 m.p.h. (70 km/h). The minimum surface pressure was 1009 mb. A Tropical Storm Warning has been issued for the coast of South Carolina from the Savannah River to Little River Inlet.
The circulation in Tropical Depression 2 became better organized on Friday, but it would not yet be considered well organized. More thunderstorms formed near the center of circulation and a primary rainband now curls around the the northern and western sides of the center. However, there are only thin bands of showers in much of the eastern half of the circulation. With persistent thunderstorms near the core of TD2, it possess enough tropical characteristics to be considered a tropical cyclone.
An upper level low near Cuba and an upper level ridge northeast of TD2 are combining to generate northerly winds over the top of the circulation. Those winds are causing moderate vertical wind shear. TD2 will also be moving over slightly cooler Sear Surface Temperatures (SSTs) during the next 12 to 18 hours. The combination of the vertical wind shear and cooler SSTs could keep TD2 from intensifying much during that time period. TD2 will move over the warmer SSTs of the Gulf Stream later on Saturday. The vertical wind shear could also decrease somewhat. Warmer SSTs and less wind shear could allow TD2 to intensify into Tropical Storm Bonnie later on Saturday.
A ridge northeast of TD2 is steering the tropical depression toward the west-northwest and a general motion in that direction is expected for another 36 to 48 hours. After about two days the steering currents could weaken. On its anticipated track TD2/Tropical Storm Bonnie is expected to approach the coast of South Carolina on Sunday. Coastal erosion, rip currents and locally heavy rain are the primary risks. Wind damage is likely to be minimal.