A reconnaissance plane found that a center of circulation and winds to tropical storm force had developed in a tropical wave previously designated as Invest 91L and Potential Tropical Cyclone Nine. Based on data collected by the recon plane the National Hurricane Center named the system Tropical Storm Harvey on Thursday afternoon. At 5:00 p.m. EDT on Thursday the center of Tropical Storm Harvey was located at latitude 13.0°N and longitude 55.8°W which put it about 250 miles (400 km) east of Barbados. Harvey was moving toward the west at 18 m.p.h. (30 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 1004 mb.
A Tropical Storm Warning is in effect for Martinique, Barbados, St. Vincent and the Grenadines, and St. Lucia. A Tropical Storm Watch is in effect for Dominica.
The circulation of Tropical Storm Harvey became better organized on Thursday. A low level center of circulation developed and bands of showers and thunderstorms began to revolve around the center. Intermittent bursts of thunderstorms occurred near and to the west of the center of circulation. The circulation of Tropical Storm Harvey is asymmetrical. Most of the stronger thunderstorms are occurring in the western half of the circulation. The bands in the eastern half of the circulation consist primarily of showers and low clouds. Thunderstorms near the core of Harvey were generating some upper level divergence which was pumping mass away to the south and west of the tropical storm.
Tropical Storm Harvey will be moving through an environment that will be somewhat favorable for intensification. Harvey will move over water where the Sea Surface Temperature is near 29°C. An upper level ridge northeast of Puerto Rico is producing northeasterly winds which are blowing toward the top of Tropical Storm Harvey. Those winds are producing moderate vertical wind shear which is probably the cause of the asymmetrical distribution of thunderstorms. Tropical Storm Harvey is likely to get better organized on Friday and it could strengthen. The intensity guidance is mixed. Some guidance forecasts that Harvey will become a hurricane as it moves over the Caribbean Sea, while other guidance weaken the tropical storm back to a tropical wave. So far, the guidance has underpredicted the strength of Tropical Storm Harvey and so strengthen would seem more likely.
Tropical Storm Harvey is being steered quickly toward the west by the subtropical high over the Atlantic Ocean. A general westerly or west-northwesterly track is forecast for the next few days. On its anticipate track Tropical Storm Harvey could reach Barbados in about 12 hours. Harvey could also reach Martinique, St. Lucia, St. Vincent and the Grenadines on Friday. Tropical Storm Harvey will bring gusty winds and locally heavy rain to those islands when it moves over them.
Hurricane Gert intensified to Category 2 on the Saffir-Simpson Scale as it sped over the Gulf Stream south of Nova Scotia on Wednesday afternoon. At 5:00 p.m. EDT on Wednesday the center of Hurricane Gert was located at latitude 38.7°N and longitude 62.4°W which put it about 410 miles (665 km) south of Halifax, Nova Scotia. Gert was moving toward the northeast at 31 m.p.h. (50 km/h). The maximum sustained wind speed was 100 mp.h. (160 km/h) and there were wind gusts to 120 m.p.h. (195 km/h). The minimum surface pressure was 970 mb.
Although Hurricane Gert is at a fairly high latitude, it has the classic structure of a Hurricane. There is a fairly small eye at the center of circulation. The eye is surround by a ring of strong thunderstorm and the strongest winds are occurring in this ring of thunderstorms. There are additional bands of showers and thunderstorms revolving around the core of the hurricane. The circulation is symmetrical and thunderstorms in the core are producing upper level divergence which is pumping away mass to the northeast of the hurricane. Winds to hurricane force extend out about 40 miles (65 km) primarily to the east of the center of circulation. Winds to tropical storm force extend out about 140 miles (225 km) from the center.
Hurricane Gert is moving over the Gulf Stream which means it is moving over water where the Sea Surface Temperature (SST) is near 28.5°C. An upper level trough west of Gert is producing southwesterly winds which are blowing toward the top of the hurricane. However, there is not much change of wind speed with height, which means that there is little vertical wind shear. The combination of the warm water of the Gulf Stream and little vertical shear, allowed Hurricane Gert to strengthen on Wednesday.
Hurricane Gert could intensify during the next few hours, but it will soon move into a much less favorable environment. Gert will soon move north of the Gulf Stream where the SSTs are much cooler. The upper level trough is moving closer to Hurricane Gert and the winds are the upper level are forecast to get stronger. When those winds increase, there will be much more vertical wind shear. Colder water and more wind shear will cause Hurricane Gert to weaken on Thursday. Gert could make a transition to an extratropical cyclone in colder environment of the North Atlantic.
Southwesterly winds in the upper level trough are steering Hurricane Gert quickly toward the northeast and that general motion is expected to continue. On its anticipated track, Hurricane Gert will move south of Labrador and Greenland.
Even as Hurricane Gert speeds away over the North Atlantic three new tropical waves over the tropical Atlantic have the potential to develop into tropical cyclones. A tropical wave about 800 miles (1290 km) east of the Lesser Antilles designated as Invest 91L showed signs of organization on Wednesday. A few more thunderstorms developed closed to the center of circulation. A reconnaissance plane is scheduled to investigate this system on Thursday. A little farther to the east another tropical wave designated Invest 92L was also showing evidence or more organization. A third tropical wave just west of Africa also has the potential to develop during the next few days.
Invest 91L was reclassified as Subtropical Depression 01 by the National Hurricane Center (NHC) on Wednesday morning. At 11:00 a.m. EDT on Wednesday the center of Subtropical Depression 01 was located at latitude 31.9°N and longitude 40.9°W which put it about 890 miles (1435 km) west-southwest of the Azores. It was moving toward the north-northeast at 8 m.p.h. The maximum sustained wind speed was 35 m.p.h. (55 km/h) and there were wind gusts to 45 m.p.h. (75 km/h). The minimum surface pressure was 995 mb.
This system was designated Invest 91L when showers and thunderstorms began to form near the center of an old occluded extratropical cyclone. The system began to separate from the occluded front and drift toward the south-southeast. A cut off upper level low associated with the original extratropical cyclone was rotating in a similar way to the circulation in the lower levels of Invest 91L. So, there was not a lot of vertical wind shear. The lack of wind shear allowed for the circulation to develop a more circular, tropical cyclone like shape. When Invest 91L drifted toward the south-southeast it moved over slightly warmer water. Additional energy from the ocean increased the number and strength of showers and thunderstorms. A primary rainband wrapped about half way around the center of circulation and additional bands formed on the eastern side of the circulation. The system developed more convection, a more circular shape and a wind field with the strongest winds closer to the center of circulation. However, it is still under an upper low with cooler air aloft and so the system has a hybrid structure. The hybrid structure and the lack of a well defined warm core is the reason NHC classified the system as a subtropical depression instead of designating it as a tropical depression.
Subtropical Depression 01 is in an environment that would not be favorable for the intensification of a purely tropical cyclone. It is moving over water where the Sea Surface Temperature (SST) is near 21°C. However, with the upper level low limiting the vertical wind shear, the SST is warm enough to support some strengthening of a subtropical cyclone. The colder air in the upper low creates enough instability for showers and thunderstorms to develop even though they will not be as tall as they would be over warmer water in the tropics. If the maximum sustained wind speed increases to 40 m.p.h. (65 km/h), then the system would become Subtropical Storm Arlene. Showers and thunderstorms would have to convert enough latent energy to internal energy to generate a warm core in the middle and upper levels in order for the system to be classified as a tropical cyclone.
Subtropical Depression 01 is being steering by the upper level low underneath which it formed. The numerical models are forecasting a slow counterclockwise loop underneath the upper low during the next few days. Eventually, the models are forecasting that the upper low and subtropical depression will both move off toward the east.
The core of an old occluded extratropical cyclone was designated at Invest 91L on Monday. At 8:00 a.m. EDT on Monday the center of Invest 91L was located at latitude 32.7°N and longitude 47.1°W which put it about 1180 miles (1900 km) east of Bermuda. Invest 91L was moving toward the south-southeast at 7 m.p.h. (11 km/h). The maximum sustained wind speed was 60 m.p.h. (95 km/h) and there were wind gusts to 75 m.p.h. (120 km/h). The minimum surface pressure was 986 mb.
A large surface high pressure system near Europe is blocking the northeastward movement of an extratropical cyclone over the North Atlantic Ocean. The extratropical cyclone proceeded through the normal stages of development and it matured into an occluded low during the past few days. The occluded low has been nearly stationary over the Atlantic Ocean during that period. Drier air wrapped around the western and southern sides of the low and the occluded front was pushed east of the core of the circulation. More showers and thunderstorms formed closer to the center of circulation and a well defined, tightly wound circular center is apparent on the visible satellite images this morning. The appearance of the tight circular center prompted the designation of Invest 91L.
Invest 91L is in an environment that would not be favorable for the classical development of a tropical cyclone. It is over water where the Sea Surface Temperature (SST) is around 20°C. However, it is possible for a transition to a tropical or subtropical cyclone to occur over colder SSTs if the air aloft is also cold enough to allow thunderstorms to develop. It appears that there may be enough instability to allow for thunderstorms to form near the core of Invest 91L. There is currently a cold cut off low associated with the occluded surface low over the top of Invest 91L. Cold temperatures in the upper levels mean that Invest 91L is still an extratropical cyclone. However, the upper low is also rotating in the same direction as the surface low. So, there is not very much wind shear over the top of Invest 91L.
Invest 91L is forecast to move slowly southward during the next several days. As it moves south, Invest 91L will gradually move over warmer water. If more thunderstorms develop around the center of circulation and a warm core starts to develop, Invest 91L could be classified as a subtropical or tropical cyclone. Since the maximum sustained wind speed is 60 m.p.h., Invest 91L could be classified as a subtropical or tropical storm if that happens.
The large surface high pressure system is forecast to continue to block the movement of Invest 91L. Invest 91L is forecast to moved slowly toward the south or south-southeast during the next day or two. The system could meander over the central North Atlantic Ocean during much of the week.
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.
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.