Tag Archives: Leeward Islands

Tropical Storm Erika Approaches the Leeward Islands

Tropical Storm Erika moved steadily toward the Leeward Islands on Wednesday.  At 11:00 p.m. EDT on Wednesday the center of Tropical Storm Erika was located at latitude 16.7°N and longitude 60.2°W which put it about 110 miles (175 km) east-southeast of Antigua and about 1400 miles east-southeast of Miami, Florida.  Erika was moving toward the west at 16 m.p.h. (26 km/h).  The maximum sustained wind speed was 45 m.p.h. (70 km/h) and there were wind gusts to 60 m.p.h. (95 km/h).  The minimum surface pressure was 1006 mb.  Tropical Storm Warnings have been issued for Anguilla, Saba, St. Eustatius and St. Maarten, Puerto Rico, U.S. Virgin Islands, British Virgin Islands, Montserrat, Antigua, Barbuda, St. Kitts, Nevis, St. Martin and St. Barthelemy.  Tropical Storm Watches have been issued for the coast of the Dominican Republic from Cabo Engano to Cabo Frances Viejo, the southeastern Bahamas and the Turks and Caicos, and Guadeloupe.

Although Erika looks very impressive on infrared satellite images with a large area of cold cloud tops, it is in reality very poorly organized.  The thunderstorms generating those cold clouds are located southeast of the low level center of circulation.  There are not really any thunderstorms in other parts of Erika.  There could be a mid-level center of circulation within the cluster of thunderstorms southeast of the surface center.  Well organized tropical cyclones are vertically stacked with the mid-level center directly above the surface center.  This is certainly not the case with Erika.  Upper level winds from the west appear to be causing vertical wind shear over Erika and that is contributing to the poor organization.

Erika is moving over water where the Sea Surface temperature is almost 29°C.  So, there is sufficient energy in the upper ocean to support intensification.  However, an upper level trough over the central Caribbean Sea could continue to cause wind shear over Erika.  The intensity guidance is inconsistent.  Some guidance strengthens Erika to a hurricane during the next few days, while other guidance weakens it.  Until the circulation become more well organized and thunderstorms develop near the center of circulation, significant intensification is unlikely.  If the vertical wind shear gets any stronger it could weaken Erika to a tropical depression.

The track forecast is also challenging.  The numerical guidance shifted the track toward the east on Wednesday evening, but that was due to the fact that the models were predicting that Erika would become a stronger hurricane.  If it become a stronger storm with a taller, vertically coherent circulation, then the upper trough over the Caribbean could push Erika farther east.  However, if Erika remains weaker, then the winds in the lower levels of the atmosphere could push it farther to the west.  On its anticipated track Erika could approach southeastern Florida in about four days.

 

Danny Dissipates But Erika Forms Behind It

Vertical wind shear weakened Tropical Storm Danny as it entered the eastern Caribbean Sea and its circulation degenerated into a weak area of low pressure.  However, a new tropical storm formed about a thousand miles east of the remnants of Danny and the National Hurricane Center designated it as Tropical Storm Erika.  At 11:00 p.m. EDT on Monday the center of Tropical Storm Erika was located at latitude 14.4°N and longitude 47.7°W which put it about 955 miles (1535 km) east of the Leeward Islands and about 2240 miles (3600 km) east-southeast of Miami, Florida.  Erika was moving toward the west at 20 m.p.h. (32 km/h).  The maximum sustained wind speed was 45 m.p.h. (70 km/h) and there were gusts to 60 m.p.h. (95 km/h).  The minimum surface pressure was 1003 mb.

The circulation around Erika is a little larger than the circulation around Danny was.  There are several bands of thunderstorms near the center of circulation and the thunderstorms are generating upper level divergence in all directions.  Erika is moving over water where the Sea Surface Temperature is near 27.5°C and it will gradually move over warmer SSTs as it moves west.  Erika is moving at 20 m.p.h. and that is about the upper limit for storms that intensify over the tropical Atlantic Ocean.  Currently the low level winds and upper level winds are blowing at nearly the same speed.  So, there is not much vertical wind shear.  However, if Erika were to move toward the west faster, then vertical wind shear could be more significant.  As long as Erika does not move too fast, the environment is favorable for intensification and Danny intensified rapidly over the same region a few days ago.  When Erika enters the Caribbean Sea in two or three days, it could encounter the same upper level trough that sheared Danny apart.

A subtropical ridge is steering Erika quickly toward the west and that same general steering motion is expected to continue for the next two or three days.  Erika could be approaching the northern Leeward Islands in about 48 hours.

Tropical Storm Danny Approaching Dominica and Guadeloupe

Tropical Storm Danny weakened on Sunday but it was maintaining its status as a tropical storm as it approached Dominica and Guadeloupe.  At 11:00 p.m. EDT on Sunday the center of Tropical Storm Danny was located at latitude 15.6°N and longitude 59.8°W which put it about 115 miles (240 km) east-southeast of Guadeloupe and about 1415 miles (2280 km) east-southeast of Miami, Florida.  Danny was moving toward the west at 15 m.p.h. (24 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 1009 mb.  Tropical Storm Warnings have been issued for Antigua, Barbuda, Montserrat, St. Kitts and Nevis.  Tropical Storm Watches have been issued for Saba, St. Eustatius and St. Maarten.

Strong southwesterly winds in the upper levels created vertical wind shear which pushed the stronger thunderstorms northeast of the center of circulation.  The core of Tropical Storm Danny consisted of an area of low clouds rotating cyclonically for much of the Sunday, but satellite images and radar indicate that a few more showers and thunderstorms formed during the past few hours.  Danny still has a well defined low level circulation.  Thunderstorms northeast of the center were producing winds to tropical storm force and Danny was still classified as a Tropical Storm at 11:00 p.m. EDT on Sunday.

Upper level southwesterly winds are forecast to continue over the eastern Caribbean Sea.  The resulting vertical wind shear will make it difficult for thunderstorms to persist near the center of Tropical Storm Danny and it could weaken to a tropical depression at any time.  However, it is moving over warm water and a reduction in wind shear could allow for the circulation to maintain itself for a longer period if that happens.

A subtropical ridge north of Danny is steering the tropical storm toward the west and that general motion is expected to continue.  Danny will enter the eastern Caribbean Sea on Monday.  On its anticipated track Danny will pass south of Puerto Rico and it is unlikely to bring much rain to help alleviate the drought conditions there.  Danny could bring some wind and rain to the southern Leeward Islands as it passes over them during the next few hours.

Wind Shear Weakens Danny to a Tropical Storm

Strong upper level winds sheared the top off of Hurricane Danny on Saturday and it weakened to tropical storm intensity.  At 11:00 p.m. EDT on Saturday the center of Tropical Storm Danny was located at latitude 15.6°N and longitude 54.7°W which put it about 480 miles (775 km) east of the Leeward Islands and about 1760 miles (2840 km) east-southeast of Miami, Florida.  Danny was moving toward the west at 15 m.p.h. (24 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. (110 km/h).  The minimum surface pressure was 999 mb.  Tropical Storm Watches have been issued for Antigua, Barbuda, Montserrat, St. Kitts, Nevis, Anguilla, Saba, St. Eustatius, St. Maarten, Gudaloupe, St. Barthelemy and St. Martin.

A upper level trough over the eastern Caribbean Sea generated brisk southwesterly winds over the top of Danny and blew the upper portion of the circulation northeast of the low level center.  By late Saturday afternoon the low level center was exposed on satellite imagery.  The vertical wind shear disrupted the vertical circulation of Danny and it weakened fairly quickly.  A few more thunderstorms appear to have developed northeast of the center in recent hours and the wind shear may have decreased just slightly.  Danny will move over slightly warmer water on Sunday, which could add a little more energy to the circulation.  The wind shear is forecast to continue and Danny could weaken further.

As Danny weakened, the lower portion of the circulation was steered more by the winds closer to the surface.  Those winds pushed the center more toward the west.  The subtropical ridge that was steering Danny is expected to strengthen and Danny is forecast to move toward the west for the next day or two.  On its anticipated track Danny could be near the Leeward Islands in about 30 hours and it could be approaching Puerto Rico in 48 hours.

Danny Becomes a Major Hurricane

Hurricane Danny intensified quickly and a NOAA plane investigating it on Friday afternoon found that Danny had maximum sustained winds of 115 m.p.h.  That made Danny a Category 3 hurricane on the Saffir-Simpson Scale which also made it a Major Hurricane.  At 5:00 p.m. EDT on Friday the center of Hurricane Danny was located at latitude 14.5°N and longitude 49.1°W which put it about 860 miles (1390 km) east of the Leeward Islands and about 2140 miles (3450 km) east-southeast of Miami, Florida.  Danny was moving toward the west-northwest at 10 m.p.h. (16 km/h).  The maximum sustained wind speed was 115 m.p.h. (165 km/h) and there were wind gusts to 135 m.p.h. (195 km/h).  The minimum surface pressure was 974 mb.  Danny had a Hurricane Intensity Index (HII) of 20.6, a Hurricane Size Index (HSI) of 6.0, and a Hurricane Wind Intensity Size Index (HWISI) of 26.6

Danny remains a very small hurricane.  It is about two thirds of the size that Hurricane Charley was when it hit southwest Florida in 2004.  Danny may be starting to weaken.  The small eye is no longer visible on satellite images.  Southwesterly winds in the upper levels may be generating some vertical wind shear and those winds may also be inhibiting upper level divergence on the western side of Danny.  The small size of Danny means that it could weaken almost as fast as it intensified, if the shear increases.  Danny will move over warmer water when it passes west of longitude 55°W, which could slow the rate of weakening in about 36 hours.

Danny is being steered toward the west-northwest by a subtropical ridge located north of the hurricane.  The ridge is expected to strengthen in two or three days and steer Danny more toward the west after that time.  On its anticipated track Danny could be approaching the northern Leeward Islands in about two and a half days and it could be near Puerto Rico in about three and a half days.

Tiny Danny Quickly Intensifies Into a Hurricane

Tiny Tropical Storm Danny intensified quickly during the past 12 hours and the National Hurricane Center upgraded it to hurricane status in its 11:00 a.m. EDT advisory on Thursday.  At 11:00 a.m. EDT on Thursday the center of Hurricane Danny was located at latitude 12.5°N and longitude 44.8°W which put it about 1090 miles (1755 km) east of the Windward Islands and about 2480 miles (4000 km) east-southeast of Miami.  Danny was moving toward the west-northwest at 12 m.p.h. (19 km/h).  The maximum sustained wind speed was 75 m.p.h. (120 km/h) and there were wind gusts to 90 m.p.h. (145 km/h).  The minimum surface pressure was 992 mb.

The core of Hurricane Danny organized quickly on Thursday.  An eye formed in the center of the circulation and a ring of thunderstorms developed around the eye.  The circulation around Danny is very small.  Hurricane force winds only extend out about 12 miles (19 km) from the center and tropical storm force winds only extend 60 miles (95 km) from the center.  The Hurricane Size Index (HSI) for Danny is only 4.2.

The environment around Danny remains complex.  It is over water where the Sea Surface Temperature (SST) is near 28°C and the upper level winds are very light.  Upper level divergence is being produced by the thunderstorms around the eye.  On the other hand, slightly cooler SSTs and drier air are just to the north of Danny.  In addition, small hurricanes like Danny can be strongly affected by changes in their environment and they can intensify or weaken very quickly.  The environment would seem to support further intensification, but if Danny moves a little farther north, it could move into a more hostile environment.

Danny is being steered toward the west-northwest by the subtropical ridge to its north and that general steering pattern is expected to continue for another two or three days.  Late in the weekend the subtropical ridge is forecast to strengthen and steer Danny more toward the west.  On its anticipated track Danny could be near the Leeward Islands in about four days and near Puerto Rico in about five days.