Tag Archives: Tropical Depression Bonnie

Bonnie Regains Tropical Depression Status Near Cape Hatteras

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.

Bonnie Brings Heavy Rain and Flooding to South Carolina

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.