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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.

TD 2 Intensifies Into Tropical Storm Bonnie

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.

Tropical Depression 2 Forms, Warning Issued for South Carolina

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.

 

Invest 91L Slightly Stronger, Moving Toward U.S.

The circulation of the low pressure system designated as Invest 91L strengthened slightly on Thursday.  At 8:00 p.m. EDT on Thursday the center of Invest 91L was located at latitude 27.4°N and longitude 70.9°W which put it about 410 miles (660 km) east-northeast of Great Abaco Island, Bahamas and about 675 miles (1090 km) south-southeast of Charleston, South Carolina.  Invest 91L was moving toward the west-northwest at 10 m.p.h. (16 km/h).  The maximum sustained wind speed was 35 m.p.h. (55 km/h) and there were wind gusts to near 45 m.p.h. (70 km/h).  The minimum surface pressure was 1013 mb.  The center of Invest 91L was near NOAA buoy 41047 which reported observations consistent with those numbers.

The organization of Invest 91L improved slightly on Thursday.  A few thunderstorms formed northeast of the center of circulation and they persisted throughout the day.  However, there are few thunderstorms in other parts of Invest 91L.  The circulation became slightly more circular although it is still elongated.  The surface pressure decreased slowly, but steadily throughout the day.

The environment around Invest 91L is only marginal for intensification.  It is over water where the Sea Surface Temperature is near 26°C to 27°C.  However, an upper level low near Cuba and an upper level ridge near the east coast of the U.S. are combining to create southerly winds over the top of Invest 91L.  Those winds are causing moderate levels over vertical wind shear which is inhibiting the development of the system.  The magnitude of the wind shear decreased during the past 24 hours and it could decrease a little more on Friday as the upper level ridge moves farther east.

If more thunderstorms develop near the center of circulation, the system could be classified as a tropical depression or tropical storm.  The National Hurricane Center (NHC) is indicating that there is an 80% probability that the Invest 91L will become a tropical cyclone or subtropical cyclone within 48 hours.  NHC has tentatively tasked a reconnaissance plane to investigate the system on Friday.

The vertical wind shear is limiting the strongest part of the circulation of Invest 91L to the lower half of the troposphere.  Thus, the system is being steered toward the west-northwest by a surface pressure system over the western Atlantic Ocean.  That general motion is expected to continue with a slow northward turn.  On its anticipated track Invest 91L could approach the southeast coast of the U.S. in 48 to 72 hours.

The primary risks with Invest 91L remain coastal erosion, rip currents and locally heavy rainfall.  Some wind damage will be possible, but it is likely to be minimal.

Low Pressure Forms East of the Bahamas

A small area of low pressure developed east of the Bahamas on Wednesday afternoon and the National Hurricane Center designated the system as Invest 91L,  At 8:00 p.m. EDT on Wednesday the center of Invest 91L was located at latitude 26.6°N and longitude 67.7°W which put it about 625 miles (1005 km) east of Great Abaco Island in the Bahamas and about 895 miles (1445 km) south-southeast of Charleston, South Carolina.  Invest 91L was moving toward the north at 8 m.p.h. (13 km/h).  The maximum sustained wind speed was he 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 1015 mb.

The circulation of Invest 91L is not well organized at the current time.  A small center of circulation was evident on late afternoon visible satellite imagery.  Several thin bands of showers are on the eastern side of the center of circulation.  However, there are very few taller thunderstorms and the circulation probably does not have a warm core at the moment.

Although Invest 91L is over water where the Sea Surface Temperature is between 26°C and 27°C, the atmospheric environment is currently unfavorable for further development.  An upper level trough near Florida is causing strong southwesterly winds to blow over the top of Invest 91L and there is significant vertical wind shear.  The wind shear is quickly dissipating any thunderstorms that start to form.  Invest 91L is forecast to move west-northwest into a more favorable environment in about a day or two.  If the system moves west, it will move into an area where the upper level winds are not as strong.  If the surface circulation is still intact at that time, less vertical wind shear could allow for the system to intensify.  The National Hurricane Center places the probability of development of a tropical cyclone or subtropical cyclone within the next five days at 60%.

Since Invest 91L primarily exists in the lower levels, it will be steered by winds in the lower troposphere.  A surface high pressure system centered off the southeast coast of the U.S. is forecast to move north of Invest 91L and the high will steer the system west-northwest toward the coast of the U.S.  On its anticipated track Invest 91L could move near the southeast coast of the U.S. during the weekend.

If Invest 91L intensifies, the highest probability is that it could become a tropical depression or tropical storm as it approaches the U.S.  It could generate coastal erosion, rip currents and locally heavy rainfall.  The threat for wind damage appears to be minimal at the current time.  People near the coast should monitor trusted sources of weather information.

Joaquin Becomes a Major Hurricane Near the Bahamas

Hurricane Joaquin intensified very rapidly on Wednesday and it reached Major Hurricane intensity.  At 11:00 p.m. EDT on Wednesday night the center of Hurricane Joaquin was located at latitude 23.8°N and longitude 73.1°W which put it about 90 miles (145 km) east of San Salvador and about 750 miles (1215 km) south-southeast of Cape Hatteras, North Carolina.  Joaquin was moving toward the southwest at 6 m.p.h. (10 km/h).  The maximum sustained wind speed was 115 m.p.h. (185 km/h) and there were wind gusts to 140 m.p.h. (220 km/h).  The wind speed made Joaquin a Category 3 hurricane on the Saffir-Simpson Scale and a Major Hurricane.  The minimum surface pressure was 951 mb.  The Hurricane Intensity Index (HII) was 20.6, the Hurricane Size Index (HSI) was 11.6, and the Hurricane Wind Intensity Size Index (HWISI) was 32.2.

A Hurricane Warning has been issued for the Central Bahamas including Cat Island, the Exumas, Long Island, Rum Cay and San Salvador.  A Hurricane Warning has been issued for the Northwestern Bahamas including the Abacos, Berry Islands, Eleuthera, Grand Bahama Island and New Providence.  A Hurricane Watch has been issued for Bimini and Andros Island.  A Tropical Storm Warning has been issued for Andros Island and the Southeastern Bahamas including the Acklins, Crooked Island, Long Cay, the Inaguas, Mayguana and the Ragged Islands.

Joaquin has a well formed structure.  The circulation is fairly symmetrical, although there are more spiral bands of thunderstorms south and east of the center.  The hurricane is over water where the Sea Surface Temperature is near 30°C.  A narrow upper level ridge west of Joaquin is causing light northerly winds over the circulation, but those winds did not inhibit rapid intensification on Wednesday.  The convection generated upper level divergence which pumped out mass and caused the pressure to decrease quickly.  Further intensification is possible during the next 24 hours, although if an eyewall replacement occurs, it would interrupt that process.  An upper level trough over the eastern U.S. will generate more vertical wind shear in a day or two and Joaquin will start to weaken.

A ridge north of Joaquin is blocking the hurricane and forcing it to move toward the southwest.  The ridge will weaken in a day or so and the upper level trough will start to steer Joaquin toward the north.  When the hurricane starts moving northward, it will also begin to move faster.  It could be approaching the Mid-Atlantic Coast by later on Saturday.