Tropical Depression 2 which was designated earlier on Thursday quickly strengthened into Tropical Storm Beryl. At 5:00 p.m. EDT on Thursday the center of Tropical Storm Beryl was located at latitude 10.3°N and longitude 42.8°W which put it about 1295 miles (2080 km) east-southeast of the Lesser Antilles. Beryl was moving toward the west at 16 m.p.h. (26 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 1004 mb.
The circulation of very small Tropical Storm Beryl organized very quickly on Thursday afternoon. There was evidence of a tiny eye on some microwave satellite images. A small tight ring of thunderstorms surrounded the center of circulation and the strongest winds were occurring in that ring of storms. Several short bands of thunderstorms were revolving around the core of the circulation. Winds to tropical storm force only extended out 35 miles (55 km) from the center of circulation. Storms near the core of Beryl were generating upper level divergence which was pumping mass away from the tropical storm.
Tropical Storm Beryl will move through an environment somewhat favorable for intensification during the next day or two. Beryl will move over water where the Sea Surface Temperature is near 26.5°C. There is cooler water north of the expected track of Beryl, which would limit intensification if the tropical storm wobbles toward the north. An upper level ridge north of Tropical Storm Beryl will generate easterly winds which will blow toward the top of the circulation. The wind speed is similar at all level and the vertical wind shear will be modest. Small tropical cyclones can intensify or weaken rapidly. Beryl is likely strengthen on Friday and it could intensify into a hurricane.
Tropical Storm Beryl was moving south of the subtropical high over the Atlantic Ocean. The high was steering Beryl toward the west. A general motion toward the west-northwest is forecast during the next two to three days. On its anticipated track Tropical Storm Beryl could be east of the Lesser Antilles on Sunday.
Tropical Depression Two formed east of the Lesser Antilles on Thursday morning. At 11:00 a.m. EDT on Thursday the center of Tropical Depression Two was located at latitude 10.2°N and longitude 41.4°W which put it about 1385 miles (2230 km) east-southeast of the Lesser Antilles. It was moving toward the west at 16 m.p.h. (26 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. (75 km/h). The minimum surface pressure was 1009 mb.
A distinct area of low pressure developed in the northern end of a tropical wave on Thursday and the National Hurricane Center designated the system as Tropical Depression Two. The circulation of the depression was quite small. Several short bands of showers and thunderstorms were revolving around the center of circulation. Storms near the center were beginning to generate upper level divergence which was pumping mass away from the depression.
Tropical Depression Two will move through an environment somewhat favorable for intensification. It will move over water where the Sea Surface Temperature is near 26.5°C. However, there is cooler water to the north of the anticipated track and the depression will not intensify if it moves over the cooler water. An upper level ridge north of the tropical depression will produce easterly winds which will blow toward the top of the circulation. The wind speeds will be similar at all levels and there will be little vertical wind shear. Tropical Depression Two is forecast to strengthen into a tropical storm during the next day or two.
Tropical Depression Two was moving around the southern side of the subtropical ridge over the Atlantic Ocean and the ridge was steering the depression toward the west. The ridge is forecast to steer the depression toward the west-northwest during the next two or three days. On its anticipated track Tropical Depression Two could be east of the Lesser Antilles on Sunday.
A low pressure system designated as Subtropical Depression 01 made a tropical transition to Tropical Depression 01 on Thursday morning. At 11:00 a.m. EDT the center of Tropical Depression 01 was located at latitude 36.1°N and longitude 40.0°W which put it about 730 miles (1170 km) west of the Azores. The tropical depression was moving toward the northwest at 14 m.p.h. (22 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. (75 km/h). The minimum surface pressure was 996 mb.
A circular ring of showers and thunderstorms developed around the center of circulation of Subtropical Depression 01. Additional narrow rainbands organized around the core of the circulation and the convection assumed a more circularly symmetrical shape. In addition data from satellites indicated that a weak warm core had formed at the top of the circulation. A more circular shape and a warm core are characteristics of a tropical cyclone and the National Hurricane Center changed the classification of Subtropical Depression 01 to Tropical Depression 01 in the 11:00 a.m. EDT advisory on Thursday.
Tropical Depression 01 is in an environment that would not normally be considered favorable for a purely tropical weather system. It is moving over water where the Sea Surface Temperature (SST) is near 19°C. However, an upper low just to the west of the depression contains colder air. Colder air in the upper levels is providing sufficient instability to generate showers and thunderstorms even though the SST is relatively cool. The cooler SST does mean that the showers and thunderstorms are not as tall as they would be if the water was warmer. The upper low west of the system is producing southerly winds which are blowing over the top of the depression. However, since the thunderstorms are not as tall, some of the stronger upper level winds are blowing over the top of the depression’s circulation and the vertical wind shear is not having as much of an effect as might be expected.
The environment is marginal for further intensification, but the circulation looks fairly intact at the current time. The cool SST is limiting the amount of energy that the tropical depression can extract from the ocean. However, since the vertical wind shear is not having as much of a negative impact on the depression, some intensification may be possible during the next 24 hours. If the depression intensifies into a tropical storm, then it would be named Arlene. Eventually, the wind shear is forecast to increase and the depression is forecast to weaken.
Tropical Depression 01 is caught in the circulation of a large low pressure system to its west. Tropical Depression is forecast to make a slow counterclockwise loop as it moves around the circulation of the larger low pressure system. It is possible that the larger low could absorb the circulation of Tropical Depression 01.
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.