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Tropical Storm Erika Bringing Heavy Rain to Hispaniola

Tropical Storm Erika is moving across Hispaniola and it is bringing heavy rain to parts of that island.  At 5:00 p.m. EDT the center of Tropical Storm Erika was located at latitude 17.9°N and longitude 71.2°W which put it about 95 miles (155 km) west-southwest of Santo Domingo, Dominican Republic and about 745 miles (1200 km) southeast of Miami, Florida.  Erika was moving toward the west at 21 m.p.h. (33 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 1009 mb.  Tropical Storm Warnings have been issued for the southeastern Bahamas, the Turks and Caicos, for the Dominican Republic, Haiti, for the Central Bahamas including Cat Island, the Exumas, Long Island, Rum Cay and San Salvador.  Tropical Storm Watches have been issued for the northwestern Bahamas including the Abacos, Andros Island, Berry Island, Bimini, Eleuthera, Grand Bahama Island and New Providence, the Cuban provinces of Ciego de Avila, Camaguey, Las Tunas, Holguin and Guantanamo.

The circulation around Tropical Storm Erika is even less well organized today.  At times a small cyclonic rotation appeared on visible satellite images, while at other times Erika looked a lot like a very strong tropical wave.  Vertical wind shear caused by an upper level trough over the western Caribbean Sea has prevented thunderstorms from forming near the core of the circulation.  The lower part of the circulation will be significantly disrupted by the mountains of Hispaniola, but the middle portion may make it across the island.  If the middle level circulation is disrupted as well, then Erika will degenerate into a disorganized area of thunderstorms.  If the middle level circulation is still intact when it emerges west of Hispaniola, then it is possible that momentum could be transferred to the surface and a new low level center spun up.  Erika would most likely be a tropical depression at most at that time.  The water north of Cuba is very warm and if Erika still exists, it could intensify somewhat as it moves toward Florida.

The winds in the lower levels continued to push Erika toward the west-northwest on Friday and that general motion is likely continue for the next 12 to 12 hours.  Erika is getting closer to the western end of the subtropical high and it could turn more toward the north during the weekend.  On its anticipated track Erika or its remnants could be approaching the Florida Keys on Sunday afternoon.  At this time it is almost impossible to know how strong it will be at that time.  If Erika does start to reorganize when it gets west of Hispaniola, watches and/or warnings could be issued for parts of Florida.

Erika Bringing Heavy Rain to the Northeastern Caribbean

Although it is not very well organized, Tropical Storm Erika brought heavy rains to parts of the northeastern Caribbean Sea on Thursday and it caused flooding on Dominica.  At 11:00 p.m. EDT on Thursday the center of Tropical Storm Erika was located at latitude 16.6°N and longitude 65.3°W which put it about 135 miles (215 km) south-southeast of San Juan, Puerto Rico and about 1145 miles (1850 km) east-southeast of Miami, Florida.  Erika was moving toward the west at 17 m.p.h. (28 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 1008 mb.  Tropical Storm Warnings have been issued for Puerto Rico, Vieques, Culebra, the U.S. Virgin Islands, the British Virgin Islands, the southeastern Bahamas, the Turks and Caicos, the coast of the Dominican Republic from Isla Saona to the border with Haiti, St. Barthelemy and St. Martin.  Tropical Storm Watches have been issued for the Central Bahamas and the coast of the Dominican Republic from Isla Saona to Punta Palenque.

The circulation around Tropical Storm Erika is poorly organized to due persistent upper level winds from the west which are causing vertical wind shear.  The low level circulation consists of a large diffuse center with at least once smaller mesoscale center rotating around inside the larger diffuse center.  The mesoscale center could have been produced by one of the thunderstorm clusters that form nightly in the southeastern portion of the circulation.  Again on Thursday the strongest thunderstorms occurred east of the center of circulation.  Those thunderstorms produced a lot of upper level divergence but the upper level westerly winds pushed that divergence east of the low level center.  As a result, the surface pressure actually rose a couple of millibars on Thursday.  Stronger thunderstorms are forming southeast of the center of Erika again tonight.  So, the pattern of the past several nights appears to be repeating itself again.

An upper level trough over the western Caribbean Sea is causing the westerly winds that are the source of vertical wind shear over Erika.  That trough is expected to drift west and slowly weaken.  If it does weaken in a couple of days, then the upper level wind pattern could be more favorable for intensification.  However, the center of Erika could pass over or very near Hispaniola.  It is likely that the mountains on that island would significantly disrupt the low level circulation.  If the low level circulation maintains its integrity, the Sea Surface Temperatures around the Bahamas and south Florida are very warm.  If Erika still has a coherent circulation at that time it could organize quickly.

A subtropical ridge is steering Erika toward the west.  Erika is nearing the western end of the ridge and a turn toward the west-northwest is possible.  If Erika gets stronger and the circulation is taller, then the upper level trough could push it more toward the north.  On the other hand, if the circulation of Erika stays weaker and shorter, then lower level flow could push it farther west.  Erika is likely to pass near or over Hispaniola on Friday and on its anticipated track it could be approaching south Florida in about 72 hours.

Even if Erika is a disorganized tropical storm it is capable of producing heavy rains and flooding on any island it crosses.

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.

 

Tropical Storm Erika Prompts Warnings for the Caribbean

The imminent approach of Tropical Storm Erika prompted the issuance of watches and warnings for locations in the northeastern Caribbean Sea.  At 11:00 p.m. EDT on Tuesday the center of Tropical Storm Erika was located at latitude 16.0°N and longitude 54.4°W which put it about 495 miles (800 km) east of Antigua and about 1780 miles (2870 km) east-southeast of Miami, Florida.  Erika 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 1006 mb.  Tropical Storm Warnings have been issued for Anguilla, Saba, St. Eustatius, and St. Maarten.  Tropical Storm Watches have been issued for Puerto Rico, the U.S. Virgin Islands, the British Virgin Islands, Montserrat, Antigua, Barbuda, St. Kitts, Nevis, Guadeloupe, St. Martin and St. Barthelemy.

For much of Monday the circulation around Tropical Storm Erika consisted of a large swirl of low level clouds and a few thunderstorms well to the southeast of the center.  The minimum surface pressure rose several millibars which was indicative of a weakening storm.  In the past several hours satellite imagery suggests that a few new thunderstorms could be forming closer to the center of circulation.  Erika is moving over water where the Sea Surface Temperature (SST) is between 28°C and 29°C.  So there is plenty of energy in the upper ocean.  The circulation could be pulling in some drier air from farther north of the storm.  There is also some vertical wind shear which may also be inhibiting intensification.    The combination of positive environmental factors like SST and negative environmental factors like drier air and wind shear make the intensity forecast challenging.  Guidance from numerical models is divergent.  Some models predict intensification while others predict that Erika will dissipate like Danny did.  If more thunderstorms continue to develop around the center of circulation, then intensification would be more likely.  On the other hand, if the recently formed thunderstorms dissipate in a few hours, the Erika could weaken to a tropical depression.

A subtropical ridge is steering Erika a little north of due west and that general motion is expected to continue during the next several days.  On its anticipated track Erika would approach the northern Leeward Islands in about 24 hours and it could be near Puerto Rico in less than two 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.

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.

Ana Transitions to a Tropical Storm

As Subtropical Storm Ana sat over the relatively warm Sea Surface Temperatures of the Gulf Stream, it drew energy from the upper ocean.  More thunderstorms developed near the center of circulation and condensation released latent energy which warmed the middle and upper levels.  As a result, the circulation became more circularly symmetrical and eventually some upper level outflow developed at the top of the system. As a result of the structural changes, Ana made a transition from a subtropical storm to a tropical storm.

At 11:00 a.m. EDT on Saturday the center of Tropical Storm Ana was located at latitude 32.7°N and longitude 77.9°W which put it about 90 miles southeast of Myrtle Beach, South Carolina and about 105 miles south of Wilmington, North Carolina.  Ana was moving toward the northwest at 3 m.p.h.  The maximum sustained wind speed was 60 m.p.h. and there were gusts to 70 m.p.h.  The minimum surface pressure was 1001 mb.

A Tropical Storm Warning remains in effect from South Santee River, South Carolina to Cape Lookout, North Carolina.  A Tropical Storm Watch is in effect from Edisto Beach to South Santee River, South Carolina.

As the high pressure system that has been blocking the forward progress of Ana moves eastward, the tropical storm will move toward the coast.  It is likely to make landfall near the border of South Carolina and North Carolina on Sunday morning.  An upper level trough approaching from the west will turn Ana toward the northeast and sweep it out to sea after it makes landfall.

Ana is still over the relatively warm water of the Gulf Stream and it is likely to maintain its intensity on Saturday.  When Ana moves north of the Gulf Stream, it will cross over cooler water.  In addition, the approaching upper level trough will begin to generate some vertical wind shear over Ana.  The effects of cooler water and more wind shear mean that Ana will likely be weakening as it approaches the coast.  It will bring some wind, locally heavy rain and beach erosion, but the impacts should be minor in most locations.

Tropical Storm Warning Issued for Ana

The National Hurricane Center has issued a Tropical Storm Warning for the portion of the coast from South Santee River, South Carolina to Surf City, North Carolina.  A Tropical Storm Watch remains in effect from Edisto Beach to South Santee River, South Carolina and from Surf City to Cape Lookout, North Carolina.

At 11:00 a.m. EDT on Friday the center of Subtropical Storm Ana was located at latitude 31.5°N north and longitude 77.3°W which put it about 250 miles east of Savannah, Georgia, about 190 miles east-southeast of Charleston, South Carolina and about 180 miles south-southeast of Wilmington, North Carolina.  Ana was nearly stationary.  The maximum sustained wind speed was 45 m.p.h. and there were gusts to 60 m.p.h.  The minimum surface pressure was 1001 mb.

Thunderstorms are forming on the southern and eastern sides of the circulation and Ana is beginning to look more like a tropical cyclone on satellite images.  It could slowly make a transition to a tropical storm during the next several days.

A high pressure system north of Ana is expected to move slowly eastward and steer Ana toward the coast during the next two or three days.  After that time an upper level trough approaching from the west is expected to steer Ana to the northeast and out to sea.

A combination of marginal Sea Surface Temperatures, drier air and some vertical wind shear is expected to limit the potential for intensification.  However, some modest intensification is possible and the surface pressure has decreased slightly in recent hours.

 

Subtropical Storm Ana Forms Off Southeast U.S. Coast

Thunderstorms developed closer to a broad area of low pressure off the southeast U.S. coast and the National Hurricane Center (NHC) classified it as Subtropical Storm Ana.  At 11:00 p.m. EDT on Thursday the center of Subtropical Storm Ana was located at latitude 31.5°N and longitude 77.6°E which put it about 250 miles east of Savannah, Georgia, about 180 miles southeast of Charleston, South Carolina and about 200 miles south of Wilmington, North Carolina.  Ana was moving toward the north at 2 m.p.h.  The maximum sustained wind speed was 45 m.p.h. and there were gusts to 60 m.p.h.  The minimum surface pressure was 1004 mb.  A Tropical Storm Watch has been issued for the portion of the coast from Edisto Beach, South Carolina to Cape Lookout, North Carolina.

More thunderstorms are being generated near the center of circulation, but the strongest winds are about 90 miles from the center.  So, the structure does not match a classical tropical cyclone where the strongest winds are closer to the center and NHC classified it as a subtropical storm.  The center of circulation is over Sea Surface Temperatures (SSTs) near 26°C which are warm enough to support the development of a tropical cyclone.  So, Ana could make a transition to a tropical storm during the next 24 to 48 hours.  The upper level winds are lighter and vertical wind shear has decreased.  However, because of the proximity to the coast some drier air is being pulled around the western side of the circulation.  Only modest intensification is expected due to marginal SSTs, some vertical wind shear and drier air.

A high pressure system north of Ana is likely to limit the motion of Ana for the next day or two.  Eventually, the high will shift eastward and an upper level trough approaching from the west will accelerate Ana toward the northeast.  Persistent northeasterly winds along the coast could create problems with beach erosion.  The slow movement also means some locations could receive locally heavy rainfall.