Tag Archives: 01L

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.

Subtropical Depression 01 Develops West of the Azores

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.

Tropical Storm Alex Brings Wind and Rain to the Azores

Tropical Storm Alex weakened to just below hurricane intensity as it moved across the Azores on Friday morning.  At 10:00 a.m. EST the center of Hurricane Alex was located at latitude 39.3°N and longitude 27.0°W which put it about 35 miles north of Terceira in the Azores.  Alex was moving toward the north at 28 m.p.h. (44 km/h).  The maximum sustained wind speed was 70 m.p.h. (115 km/h) and there were wind gusts to 85 m.p.h. (135 km/h).  The minimum surface pressure was 986 mb.

It appears that the center of Tropical Storm Alex made landfall on the island of Terceira.  Weather stations on Santa Maria and Sao Miguel have measured tropical storm force winds.  However, it seems like the core of Tropical Storm Alex which contains the strongest winds remained over water.  Higher wind speeds most likely occurred on the windward sides of mountains in the Azores.

In anticipation of the movement of Tropical Storm Alex away from the islands all Hurricane Warnings and Tropical Storm Warnings for the Azores have been discontinued.

Tropical Storm Alex is over water where the Sea Surface Temperature is near 16°C.  It will move over even cooler water and Alex will soon be unable to extract enough energy from the ocean to sustain the structure of a tropical cyclone.  The structure of Alex will gradually change to the structure of a cold core extratropical cyclone during the next several days.  It is likely to maintain much of its intensity as it moves through the extratropical transition.

An upper level trough is steering Tropical Storm Alex toward the north-northwest and a general northwesterly motion is expected to continue for the next two or three days.  Tropical Storm Alex could end up south of Greenland over the weekend as a strong extratropical cyclone.

Subtropical Storm Alex Transitions to Hurricane, Threatens the Azores

The structure of Subtropical Storm Alex evolved into the structure associated with a tropical cyclone and the National Hurricane Center reclassified it as Hurricane Alex on Thursday morning.  At 10:00 a.m. EST on Thursday the center of Hurricane Alex was located at latitude 31.5°N and longitude 28.4°W which put it about 490 miles (790 km) south of Faial in the Central Azores.  Alex was moving toward the north at 20 m.p.h. (32 km/h).  The maximum sustained wind speed was 85 m.p.h. (140 km/h) and there were wind gusts to 105 m.p.h. (170 km/h).  The minimum surface pressure was 981 mb.

A Hurricane Warning has been issued for Faial, Pico, Sao Jorge, Graciosa, and Terceira in the Central Azores.  A Tropical Storm Warning has been issued for Sao Miguel and Santa Maria in the Eastern Azores.

Alex has developed the structure associated with a small hurricane.  An eye is clearly visible on satellite imagery.  A ring of thunderstorms surrounds the eye and there are spiral bands rotating around the core of the circulation.  Latent energy released by convection around the core of Hurricane Alex generated a warm core in the middle and upper troposphere.  That convection is also generating upper level divergence which is pumping out mass, especially to the north and east of the center.

Hurricane Alex is over water where the Sea Surface Temperatures (SSTs) are near 20.5°C.  Those SSTs are usually considered to be too cold to supply enough energy to promote the development of a hurricane.  However, it is January and the temperatures in the middle and upper troposphere are also cold.  So, there is clearly enough instability to generate convection and create a hurricane.  The thunderstorms are in Alex are not as tall as they would be if the SSTs are warmer.  The shorter thunderstorms do not extend into the stronger winds in the upper troposphere and Hurricane Alex is not experiencing as much vertical wind shear as might be expected.   The combination of more instability and less vertical wind shear allowed Alex to transition from a subtropical storm to a hurricane.

The upper level divergence could allow Alex to intensify a little more during the next 12 hours.  Hurricane Alex will move over SSTs that are even colder and at some point the structure of Alex will change again.  It will develop fronts and transition into an extratropical cyclone.

Alex is being steered northward by an upper level trough to its west and that general motion is expected to continue for the next several days.  On its anticipated track Hurricane Alex will move over portions of the Azores on Friday.

The Hurricane Intensity Index (HII) for Alex is 12.7.  The Hurricane Size Index is (HSI) 7.0.  The Hurricane Wind Intensity Size Index (HWISI) is 19.7.  Those indices suggest that Hurricane Alex is capable of causing localized minor damage with isolated areas of serious damage.

Alex is the first Atlantic hurricane to form during the month of January since 1938.  Alex is the first tropical cyclone be a hurricane in January since Hurricane Alice in 1955.

Subtropical Storm Alex Develops over East Atlantic

A low pressure system over the eastern Atlantic Ocean has been classified as Subtropical Storm Alex by the National Hurricane Center.  At 4:00 p.m. EST on Wednesday the center of Subtropical Storm Alex was located at latitude 27.1°N and longitude 30.8°W which put it about 785 miles (1260 km) south-southwest of the Azores.  Alex was moving toward the northeast at 14 m.p.h. (23 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 990 mb.

Subtropical Storm Alex evolved out of an extratropical cyclone that formed off the southeastern coast of the U.S. late last week.  The extratropical cyclone cycled through the typical stages exhibited by low pressure systems in the middle latitudes as it moved eastward across the central Atlantic Ocean.  It eventually became an occluded extratropical cyclone over that region.  The low pressure system moved south on Tuesday, which placed it over slightly warmer Sea Surface Temperatures (SSTs).  As it began to extract more energy from the warmer ocean, the fronts attached to the low pressure system began to dissipate.  The low began to take on a more circular shape and convection began to intensify around the center of circulation.  A more circular shape with no fronts and taller clouds prompted the National Hurricane Center to classify the low as a subtropical storm.

A subtropical storm is a hybrid low with some characteristics of a tropical cyclone (circular shape and no fronts) and some characteristics of an extratropical cyclone (cooler air aloft).  Subtropical Storm Alex has a well organized low level circulation.  There is a clear area in the center that looks a little like the eye of a hurricane.  The center is surrounded by a ring of taller clouds.  Spirals bands of clouds are rotating around the core of the circulation.  However, the vertical structure is different from a tropical cyclone.  Subtropical Storm Alex is under an upper level trough and the upper level divergence is limited.  Also, the clouds in Alex are not growing as tall as the clouds in a tropical cyclone.

The environment around Subtropical Storm Alex would normally be considered to be unfavorable for intensification of a tropical cyclone.  The SSTs are near 23.5°C.  However, since Alex is near the center of an upper level trough, there is not much vertical wind shear.  So, there could be a chance for some intensification during the next 24 hours.  Subtropical Storm Alex will move over cooler SSTs and it is likely to make a transition back to an extratropical cyclone in two or three days.

The upper level trough is steering Alex toward the northeast and a general northward motion is expected to continue for the next several days.  On its anticipated track Subtropical Storm Alex could approach the Azores in about 36 hours.  It could bring strong winds and locally heavy rain when it moves over the Azores.

Subtropical Storm Alex is the first tropical or subtropical cyclone to form over the North Atlantic Ocean in January since 1978.  It is only the fourth tropical or subtropical cyclone on record to form over the Atlantic Ocean in January since 1851.