Tag Archives: Tropical Depression 01

TD 01 Intensifies Into Rare April Tropical Storm Arlene

Tropical Depression 01 intensified into Tropical Storm Arlene on Thursday afternoon.  Tropical storms rarely form over the Atlantic Ocean in April and Arlene is only the second storm known to do so during the satellite era.  Of course, it would have been much harder to detect tropical storms like Arlene prior to the use of geostationary satellites for meteorological monitoring.  At 5:00 p.m. EDT on Thursday the center of Tropical Storm Arlene was located at latitude 37.7°N and longitude 42.0°W which put it about 815 miles (1315 km) west of the Azores.  Tropical Storm Arlene was moving toward the west-northwest at 25 m.p.h. (41 km/h).  The maximum sustained wind speed was 45 m.p.h. (75 km/h) and there were wind gusts to 60 m.p.h. (95 km/h).  The minimum surface pressure was 936 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.  Showers and thunderstorms continued to develop around the center of circulation and Tropical Depression 01 was upgraded to Tropical Storm Arlene in the 5:00 p.m. EDT advisory on Thursday.

Tropical Storm Arlene 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 Arlene 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 Tropical Storm Arlene.  However, since the thunderstorms are not as tall, some of the stronger upper level winds are blowing over the top of Arlene’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.  Eventually, the wind shear is forecast to increase and Tropical Storm Arlene is forecast to weaken.

Tropical Storm Arlene is caught in the circulation of a large low pressure system to its west.  Arlene is forecast to make a 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 Storm Arlene.

Subtropical Depression 01 Transitions to Tropical Depression 01

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.