Monthly Archives: May 2016

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.

Possible Development Off Southeast U.S. Coast

Even though the official start to Atlantic hurricane season is not until June 1, the atmosphere doesn’t always wait for the calendar to change to June.  Several numerical models have been predicting that a low pressure system will develop somewhere near the Bahamas later this week.  The models are also predicting that the low pressure system will move toward the southeast coast of the U.S.

A frontal boundary is over the western Atlantic Ocean.  The front runs from east of the Mid-Atlantic States to near the northern Bahamas.  Low pressure systems can develop along the southern end of frontal boundaries.  The low pressure system could start as an extratropical (non-tropical) cyclone or a subtropical (hybrid) cyclone.  The exact type of cyclone will be partly determined by the vertical wind shear later this week.  The structure of a low pressure system can change, if the environment changes and the low can be placed in a different category.

An upper level low over the western Atlantic could cause enough vertical wind shear, so that the low pressure system could be initially classified as extratropical or subtropical.  The Sea Surface Temperatures (SSTs) in the area where the low is most likely to develop are near 26° to 27°C.  There will be enough energy in the upper ocean to support the development of a tropical cyclone.  An upper level ridge could move north of the developing surface low later in the week, which could reduce the vertical wind shear.  If the shear decreases and the low pressure system extracts enough energy from the ocean to develop a warm core, it could eventually be classified as a tropical cyclone.

After the low pressure system forms, a high pressure system over the Atlantic Ocean is expected to steer the low northwest toward southeast coast of the U.S.  The low pressure system could make landfall in Georgia, South Carolina or North Carolina during the weekend or early next week.

The primary threats from the low pressure system will be coastal erosion, rip currents and potentially heavy rainfall.  The wind threat appears to be minimal, since even if the low pressure system becomes tropical, the highest probability is that it would be a tropical depression or tropical storm.

There is a great deal of uncertainty about the future of this system.  The timing of the formation of the low, the type of cyclone, the possible track of the cyclone and its intensity should become clearer later this week.  People near the southeast coast should monitor trusted sources of weather information.

Tropical Cyclone Roanu Nearing Bangladesh

Tropical Cyclone Roanu moved closer to the northern coast of the Bay of Bengal on Friday.  At 5:00 p.m. EDT on Friday the center of Tropical Cyclone Roanu was located at latitude 20.3°N and longitude 87.8°W which put it about 200 miles (320 km) south of Kolkata, India and about 355 miles (575 km) west-southwest of Chittagong, Bangladesh.  Roanu was moving toward the northeast at 11 m.p.h. (18 km/h).  The maximum sustained wind speed was 65 m.p.h. (105 km/h) and there were wind gusts to 80 m.p.h. (130 km/h).  The minimum surface pressure was 983 mb.

On Friday Tropical Cyclone Roanu went through another cycle in which most of the thunderstorms dissipated near the center of circulation and then convection redeveloped rapidly.  Thunderstorm activity is increasing at the core of the circulation and those storms are driving well developed upper level outflow.  Several well formed bands of thunderstorms are rotating around the center of circulation.  Roanu is more organized and it is a little more intense than it was 24 hours ago.

Tropical Cyclone Roanu is in an environment that could support a little more intensification before it makes landfall on the north coast of the Bay of Bengal.  It is over water where the Sea Surface Temperature is near 30.5°C.  Roanu is under an upper level ridge which means the upper level winds are light and there is not much vertical winds shear.  However, Roanu is getting closer to the north coast of the Bay of Bengal and it only has another 12 hours or so to intensify.

A ridge east of Roanu is steering the tropical cyclone toward the northeast and that general motion is expected to continue for the next several days.  On its anticipated track Tropical Cyclone Roanu will be near the coast of Bangladesh in about 12 hours.  Roanu is expected to continue moving toward the northeast after it makes landfall.

Although Tropical Cyclone Roanu could cause some minor wind damage, its main threats will be locally heavy rain and a moderate storm surge.  Tropical Cyclone Roanu could produce locally heavy rainfall, especially if the thunderstorm activity continues to pulse diurnally.  The heavy rain could produce significant fresh water flooding over parts of northeastern India, Bangladesh and Myanmar (Burma).  In addition, the winds on the eastern side of Tropical Cyclone Roanu will be blowing toward the coast of Bangladesh and those winds will push water toward the coast.  The north coast of the Bay of Bengal is particularly susceptible to storms surges and Roanu could bring a moderate storm surge to the coast of Bangladesh.  The surge will be higher in the mouths of rivers and other locations where the shape of the coast funnels the water into specific locations.

Tropical Cyclone Roanu Threatens Northern Bay of Bengal

Tropical Cyclone Roanu intensified on Thursday and it poses an increasing threat to the northern Bay of Bengal.  At 11:00 p.m. EDT on Thursday the center of Tropical Cyclone Roanu was located at latitude 17.9°N and longitude 84.9°E which put it about 125 miles (200 km) east of Visakhapatman, India and about 580 miles (935 km) west-southwest of Chittagong, Bangladesh.  Roanu was moving toward the north-northeast at 14 m.p.h. (23 km/h).  The maximum sustained wind speed was 60 m.p.h. (95 km/h) and there were wind gusts to 70 m.p.h. (115 km/h).  The minimum surface pressure was 986 mb.

Most of the thunderstorms in Tropical Cyclone Roanu weakened earlier today and at times it did not look much like a tropical cyclone on satellite imagery.  However, a burst of thunderstorm activity near the center of Roanu has occurred in recent hours and an eyelike feature has appeared on conventional satellite imagery.  The structure of the circulation improved as a result of the new burst of thunderstorms.  The primary rainband now coils more tightly about three quarters of the way around the apparent center of circulation.  The thunderstorms near the center of circulation are generating upper level divergence which is pumping out mass in all directions.  Several additional bands of thunderstorms have developed over the eastern half of the circulation and Tropical Cyclone Roanu is stronger than it was 24 hours ago.

Tropical Cyclone Roanu is in an environment favorable for intensification.  It is over water where the Sea Surface Temperature is near 31°C.  Roanu has moved closer to the center of an upper level ridge and the upper levels are not as strong as they were yesterday.  As a result the vertical wind shear is much less and the upper level ridge is actually helping to pump mass away from the center of Roanu.  The center of the tropical cyclone is near the east coast of India, but the core is expected to remain over water on Friday.  Roanu is likely to strengthen further and it could reach hurricane/typhoon intensity.

A ridge east of Roanu is steering the tropical cyclone toward the northeast and that general motion is expected to continue.  On its anticipated track Roanu could be south of Kolkata, India in about 18 hours.  Tropical Cyclone Roanu could be approaching Chittagong, Bangladesh in 24 to 36 hours.

Tropical Cyclone Roanu poses a significant threat to the northern Bay of Bengal.  That area is vulnerable to storm surges.  Since Roanu will approach from the southwest, the winds in its counterclockwise circulation will push water toward the north coast of the Bay of Bengal.  In addition, a strengthening Tropical Cyclone Roanu will be capable of producing locally heavy rain which could cause inland flooding.

Tropical Cyclone 01B Forms Over Bay of Bengal

The structure of a low pressure system north of Sri Lanka changed on Wednesday and it was classified as Tropical Cyclone 01B (TC01B).  At 11:00 p.m. EDT on Wednesday the center of Tropical Cyclone 01B was located at latitude 15.9°N and longitude 82.3°E which put it about 155 miles (250 km) south-southwest Visakhapatnam, India.  TC01B was moving toward the northeast at 12 m.p.h. (19 km/h).  The maximum sustained wind speed was 50 m.p.h. (80 km/h) and there were wind gusts to 65 m.p.h. (105 km/h).  The minimum surface pressure was 989 mb.

A low pressure system developed near Sri Lanka several days ago.  The low moved over the east coast of Sri Lanka, which slowed the organization of the circulation.  A more well defined center of circulation began to organize after the low moved north of Sri Lanka.  A primary rainband started to wrap around the western side of the low and several broken spiral bands formed on the eastern side.  The circulation exhibited enough characteristics associated with tropical cyclones to be classified as Tropical Cyclone 01B.

The circulation of TC01B is still not particularly well organized.  A primary rainband curls around the northern and western sides of the circulation.  However, there are not many thunderstorms in the other parts of the tropical cyclone.  An upper level ridge over the northern Bay of Bengal is generating easterly winds over the top of the tropical cyclone.  Those winds are producing moderate vertical wind shear and are contributing to the fact that most thunderstorms are west of the center of circulation.  The fact that the center of circulation is near the coast of India also means a  portion of circulation is over land, where there is more friction and less moisture.

Tropical Cyclone 01B is expected to move into a more favorable environment during the next two days.  It is over water where the Sea Surface Temperature is near 31°C.  TC01B is expected to move closer to the center of the upper level ridge where the winds are not as strong.  That would reduce the vertical wind shear, which would also be favorable for intensification.  However, the proximity of the center to the east coast of India means that a portion of the circulation will remain over land, which will slow the rate of future intensification.

A ridge is east of TC01B is expected to steer the tropical cyclone toward the northeast during the next two to three days.  On its anticipated track, TC01B could be approaching the coast of Bangladesh in 48 to 72 hours.  TC01B caused heavy rain in parts of Sri Lanka and southeast India.  It could produce more heavy rain in parts of northeast India, Bangladesh and Myanmar (Burma).  TC01B could also generate a significant storm surge along the north coast of the Bay of Bengal as it nears the coast.

Hurricane Season Officially Starts in East Pacific

May 15 is the official start of the hurricane season for the Eastern North Pacific Ocean.  The Sea Surface Temperatures (SSTs) should be cooler than they were in 2015, since the El Niño is dissipating.  That could produce a hurricane season in which the activity is nearer to normal.  In a normal year we might expect 13-17 named storms, 7-10 hurricanes and 3-4 major hurricanes.  Of course, other environmental factors including the location and strength of atmospheric systems will also determine how active the season is.

The National Hurricane Center (NHC) is currently monitoring Invest 90E which is about 950 miles (1530 km) south-southwest of the southern tip of Baja California.  There is a well defined low level circulation in Invest 90E, but there are no deep thunderstorms.  NHC is giving Invest 90E a 20 percent chance of developing into a tropical cyclone during the next five days.