Tag Archives: Leeward Islands

Hurricane Irma Moves Toward Leeward Islands, Watches Issued

Hurricane Irma moved toward the northern Leeward Islands on Sunday and Hurricane Watches were issued for a number of those islands.  At 11:00 p.m. EDT on Sunday the center of Hurricane Irma was located at latitude 17.2°N and longitude 51.0°W which put it about 710 miles (1145 km) east of the Leeward Islands.  Irma was moving toward the west-southwest at 14 m.p.h. (22 km/h).  The maximum sustained wind speed was 115 m.p.h. (185 km/h) and there were wind gusts to 140 m.p.h. (225 km/h).  The minimum surface pressure was 961 mb.

Hurricane Watches were in effect for Antigua, Barbuda, Anguilla, Montserrat, St. Kitts, Nevis, Saba, St. Eustatius, St. Maarten, St, Martin, and St. Barthelemy.

The circulation of Hurricane Irma is very well organized.  There is a nearly circular eye with a diameter of 30 miles (48 km) at the center of circulation.  The eye is surrounded by a ring of strong thunderstorms and the strongest winds are occurring in the northeastern part of the ring.  Bands of showers and thunderstorms are revolving around the core of Hurricane Irma.  Thunderstorms around the core are generating strong upper level divergence which is pumping mass away in all directions from the hurricane.  Winds to hurricane force extend out about 35 miles (55 km) from the center.  Winds to tropical storm force extend out about 140 miles (225 km) from the center.

The Hurricane Intensity Index (HII) for Hurricane Irma is 20.6.  The Hurricane Size Index (HSI) is 11.7 and the Hurricane Wind Intensity Size Index (HWISI) is 32.3.

Hurricane Irma will move through an environment favorable for intensification.  Irma will move over water where the Sea Surface Temperature is near 29°C and there is warmer water farther west.  The upper level winds are weak and there is little vertical wind shear.  Hurricane Irma could intensify into a Category 4 hurricane on the Saffir-Simpson Scale.  There is a possibility that Irma could strengthen to Category 5 when it passes near the northern Leeward Islands and moves toward the Bahamas.  Hurricane Irma has already completed several eyewall replacement cycles and there could be more.  Each eyewall replacement cycle could result in fluctuations in the intensity of Hurricane Irma.

A strong subtropical high pressure system over the Atlantic Ocean is steering Hurricane Irma toward the west-southwest.  The high is forecast to weaken just a bit during the next several days and Hurricane Irma is expected to move more toward the west-northwest.  On its anticipated track Hurricane Irma will approach the northward Leeward Islands on Tuesday night.  It will be a strong, dangerous hurricane at that time.

Hurricane Irma Completes Quick Eyewall Replacement Cycle and Strengthens

Hurricane Irma completed a quick eyewall replacement cycle on Friday and it strengthened after the cycle ended.  At 5:00 p.m. EDT on Friday the center of Hurricane Irma was located at latitude 18.8°N and longitude 39.1°W which put it about 1495 miles (2405 km) east of the Leeward Islands.  Irma was moving toward the west at 13 m.p.h. (20 km/h).  The maximum sustained wind speed was 120 m.p.h. (195 km/h) and there were wind gusts to 150 m.p.h. (240 km/h).  The minimum surface pressure was 964 mb.

Hurricane Irma is a small circular hurricane.  There is a small eye at the center of circulation. The eye is surrounded by a ring of strong thunderstorms.  Additional bands of showers and thunderstorms are revolving around the eye.  Winds to hurricane force only extend out about 25 miles (40 km) from the center of circulation.  Winds to tropical storm force only extend out about 100 miles (160 km) from the center.  The thunderstorms around the center are generating well developed upper level divergence which is pumping mass away in all directions.

The Hurricane Intensity Index (HII) for Irma is 22.1  The Hurricane Size Index (HSI) is 7.6 and the Hurricane Wind Intensity Size Index (HWISI) is 29.7.

Hurricane Irma will move through an environment favorable for intensification.  Irma will move over water where the Sea Surface Temperature is near 28°C.  The upper level winds are weak and there is little vertical wind shear.  Hurricane Irma is likely to intensify further, but there are also likely to be additional eyewall replacement cycles which will cause fluctuations in the intensity.

Hurricane Irma is located south of the subtropical high over the Atlantic Ocean.  The high is steering Irma toward the west.  The high is forecast to strengthen over the weekend and it is likely to steer Hurricane Irma more toward the west-southwest.  On its anticipated track Hurricane Irma could approach the Leeward Islands early next week.

Hurricane Irma Strengthens Rapidly Over Atlantic

Hurricane Irma strengthened rapidly as it moved over the tropical Atlantic Ocean.  At 11:00 a.m. EDT on Thursday the center of Hurricane Irma was located at latitude 16.9°N and longitude 33.8°W which put it about 1845 miles (2975 km) east of the Leeward Islands.  Irma was moving toward the west-northwest at 10 m.p.h. (16 km/h).  The maximum sustained wind speed was 100 m.p.h. (160 km/h) and there were wind gusts to 120 m.p.h. (195 km/h).  The minimum surface pressure was 979 mb.

Hurricane Irma intensified rapidly on Thursday morning.  A circular eye developed at the center of circulation and a ring of strong thunderstorms surrounded the eye.  Numerous bands of showers and thunderstorms developed outside the core of Irma.  Hurricane Irma has a very circular, symmetrical shape.  Thunderstorms in the core of Irma were generating strong upper level divergence which was pumping mass away in all directions from the hurricane.

Hurricane Irma will move through an environment favorable for intensification during the next few days.  Irma will move over water where the Sea Surface Temperature is near 28°C.  Irma is under the western end of an upper level ridge, but the upper level winds are weak and there is not much vertical wind shear.  Irma is likely to intensify during the next few days and it is forecast to become a major hurricane.  Hurricanes that intensify rapidly often develop concentric eyewalls, which cause fluctuations in their intensity.  It is highly likely that Hurricane Irma will go through one of more eyewall replacement cycles which will cause temporary fluctuations in the wind speed.  Eyewall replacement cycles often result in a larger hurricane and Hurricane Irma has the potential to develop into a big classic Cape Verde hurricane.

Hurricane Irma is nearing a weaker region in the subtropical ridge to its north.  The weakness is allowing Irma to move toward the west-northwest.  The ridge is expected to strengthen and steer Hurricane Irma more toward the west or west-southwest during the next few days.  On its anticipated track Hurricane Irma is forecast to be a major hurricane east of the Leeward Islands in five days.  The uncertainty about the future track of Hurricane Irma increases after that time.  People around the western Atlantic need to monitor the progress of Hurricane Irma.

Invest 94L Likely to Develop East of Lesser Antilles

A tropical disturbance currently designated as Invest 94L is likely to develop into a tropical cyclone east of the Lesser Antilles.  The National Hurricane Center’s probability of formation of a tropical cyclone from this system during the next five days is 70%.  At 2:00 p.m. EDT on Monday the center of Invest 94L was located at latitude 8.7°N and longitude 32.7°W which put it about 1830 miles (2950 km) east of the Lesser Antilles.  It was moving toward the west at 4 m.p.h. (6 km/h).  The maximum sustained wind speed was 30 m.p.h. (50 km/h) and there were wind gusts to 40 m.p.h. (65 km/h).  The minimum surface pressure was 1011 mb.

Invest 94L consists of a broad area of low pressure, but it has not developed a well organized inner core.  The initial area of low pressure consisted of a counterclockwise rotation associated with a tropical wave that moved over the eastern Atlantic Ocean.  The initial low slowed down as it was moving westward and a second tropical wave caught up to it.  The thunderstorms from the two waves appear to be merging into a single system.  An area of showers and thunderstorms on the northern and western sides of the surface low was part of the first tropical wave.  Other bands of showers and thunderstorms south and east of the low are associated with the second tropical wave.  If the broad low pressure system can collect the rotation from the two tropical waves, it could strengthen the surface low.

Invest 94L will move through an environment that will be favorable for the development of a tropical cyclone.  It will move over water where the Sea Surface Temperature (SST) is near 28°C.  There are easterly winds blowing at multiple levels in the atmosphere and there is little vertical winds shear.  The existence of a surface low pressure system, warm SSTs and little vertical wind shear are the primary ingredients for the development of a tropical cyclone.  That is the reason why there is strong probability that Invest 94L will develop into a tropical depression or a tropical storm.

Invest 94L will be steered by the subtropical high pressure system over the Atlantic Ocean.  A general westerly or west-northwesterly track is forecast during the next few days.  There is still significant uncertainty about the ultimate track and intensity of this system beyond that time frame.  It will need to be monitored closely as it moves west.

Tropical Storm Matthew Develops Near the Windward Islands

The National Hurricane Center determined that a surface circulation center formed within Invest 97L on Wednesday morning and it designated the system as Tropical Storm Matthew.  At 11:00 a.m. EDT on Wednesday the center of Tropical Storm Matthew as located at latitude 13.4°N and longitude 60.7°W which put it about 35 miles (55 km) southeast of St. Lucia.  Matthew was moving toward the west at 21 m.p.h. (33 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 1008 mb.

Tropical Storm Warnings have been issued for Guadeloupe, Martinique, Barbados, Dominica, St. Lucia, and St. Vincent and the Grenadines.

The circulation of Tropical Storm Matthew is still organizing and it appears as if the surface center is located southwest of the mid-level center.  There are many more thunderstorms north and east of the center and there are fewer thunderstorms south and west of the center.  The strongest winds are occurring in the bands of thunderstorms northeast of the center of circulation.  The winds are much weaker in the western half of Tropical Storm Matthew.  The stronger thunderstorms northeast of the center of circulation are generating upper level divergence which is pumping mass out to the north and east of Matthew.

Tropical Storm Matthew will be moving into an environment that is favorable for intensification The Sea Surface Temperatures in the eastern and central Caribbean Sea are near 30°C.  The upper level winds are weak and there is not much vertical wind shear.  Even with those favorable conditions, several factors could slow the rate at which Tropical Storm Matthew intensifies.  First, it is moving west at 21 m.p.h. (33 km/h).  Sometimes tropical cyclones generate low level vertical wind shear when they move that quickly.  Second, the asymmetrical distribution of thunderstorms around the circulation of Matthew could prevent the tropical storm form efficiently using the energy it extracts from the ocean.  In addition, if Tropical Storm Matthew moves too close to the northern coast of South America, it could pull in some drier air, which would also slow the rate of intensification.  Even with those potential inhibiting factors, Tropical Storm Matthew is likely to become a hurricane by the end of the week and it could become a major hurricane while it is over the Caribbean Sea.

A subtropical high pressure system to the north of Matthew is steering the tropical storm toward the west and that general motion is expected to continue for several more days.  Tropical Storm Matthew is likely to slow down over the weekend as it gets closer to the western end of the subtropical high.  Matthew will likely turn toward the north during the weekend.  There is a great deal of variability in how quickly and sharply the models predict the turn will be.  Some models forecast a quick sharp turn toward the north that could eventually carry Matthew near Bermuda.  Other models forecast a later slower turn that occurs over the western Caribbean Sea and could take take Matthew closer to the U.S.  It is too early to know which scenario will be the right one.

Tropical Storm Matthew will bring gusty winds and locally heavy rain to the Windward Islands and southern Leeward Islands.  Interests in other parts of the Caribbean Sea, the Bahamas, U.S. and Bermuda should monitor the progress of Tropical Storm Matthew.

Tropical Storm Karl Intensifies a Little

After several days when Tropical Storm Karl was poorly organized and barely maintained tropical storm intensity, Karl intensified a little late on Monday.  At 11:00 p.m. EDT on Monday the center of Tropical Storm Karl was located at latitude 19.7°N and longitude 50.5°W which put it about 760 miles (1225 km) east-northeast of the Leeward Islands.  Karl was moving toward the west-northwest at 14 m.p.h. (22 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 1005 mb.

Although the structure of Tropical Storm Karl improved a little during the past few hours, it is still not a well organized storm.  A cluster of thunderstorms (sometimes called a Central Dense Overcast or CDO) formed near the core of Tropical Storm Karl.  Additional thunderstorms developed in fragmented rainbands, which are primarily in the eastern half of the circulation.  The cluster of thunderstorms near the core of the circulation is beginning to generate upper level divergence.

Tropical Storm Karl is moving into a more favorable environment.  It is moving over water where the Sea Surface Temperature (SST) is near 28°C.  An upper level low to the west of Karl has been causing strong vertical wind shear over the top of the tropical storm.  However, the upper low is moving farther west of Karl and the vertical wind shear is lessening.  Warm SSTs and less vertical wind shear allowed Tropical Storm Karl to begin to get a little better organized.  Karl is likely to intensify slowly while the circulation organizes and a more well define structure develops.  Tropical Storm Karl will move over warmer SSTs during the next few days and it is forecast to become a hurricane later this week.

Tropical Storm Karl is moving around the southwestern portion of a subtropical ridge which is steering it toward the west-northwest.  Karl is likely to turn more toward the northwest during the next several days.  On its anticipated track Tropical Storm Karl is likely to move northeast of the Leeward Islands and head in the general direction of Bermuda.

Tropical Storm Karl Develops over the Eastern Atlantic

More thunderstorms developed near the center of Tropical Depression 12 and the National Hurricane Center named it Tropical Storm Karl on Thursday night.  Karl is the third tropical storm to form over the Atlantic Ocean this week.  At 11:00 p.m. EDT on Thursday the center of Tropical Storm Karl was located at latitude 17.9°N and longitude 32.5°W which put it about 1930 miles (3150 km) east of the Leeward Islands.  Karl was moving toward the west at 14 m.p.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 1005 mb.

The circulation of Tropical Storm Karl is asymmetrical.  Karl does have a large circular low level wind fields.  However the thunderstorms are northeast of the center of circulation, although some thunderstorms have developed closer to the northeast side of the center in recent hours.  Southwesterly winds in the upper levels are generating moderate vertical wind shear, which has contributed to the asymmetrical distribution of thunderstorms.  In addition, the upper level winds are inhibiting upper level divergence to the west of Tropical Storm Karl.

Tropical Storm Karl is moving through an environment that is not very favorable for intensification.  It is moving over water where the Sea Surface Temperature is near 27°C.  There is moderate vertical wind shear, which will make it hard for thunderstorms to persist near the center of the tropical storm.  Karl could move into an area where the upper level winds are not as strong in a day or two.  The environment could be more favorable for intensification at that time.

A subtropical ridge north of Karl is steering the tropical storm toward the west.  The ridge is expected to get stronger and it should steer Tropical Storm Karl toward the west or west-southwest during the next few days.

Tropical Depression Six Intensifies Into Tropical Storm Fiona

Tropical Depression Six intensified on Wednesday and the National Hurricane Center named it Tropical Storm Fiona.  At 5:00 p.m. EDT on Wednesday the center of Tropical Storm Fiona was located at latitude 15.1°N and longitude 37.8°W which put it about 1545 miles (2490 km) east of the Leeward Islands.  Fiona was moving toward the northwest at 16 m.p.h. (26 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 Fiona has a small circulation and winds to tropical storm force only extend out about 25 miles (40 km) from the center circulation.  Most of the thunderstorms are occurring close to the center of circulation.  Those thunderstorms are generating some upper level divergence, but it does not extend very far from the core of Tropical Storm Fiona.  There are bands of lower clouds and showers rotating around the core of Fiona.

Tropical Storm Fiona is moving through an environment that is mostly favorable for intensification.  It is moving over water where the Sea Surface Temperature is near 27°C.  An upper level ridge north of Fiona is generating northeasterly winds which are blowing across the top of the tropical storm.  Those winds are causing some vertical wind shear, but it should only inhibit, but not stop, intensification.  There is drier air northwest of Fiona and that is the main negative factor in the environment.  If the drier air gets pulled into the core of Tropical Storm Fiona, it will reduce the supply of energy and weaken Fiona.  Since the core of Fiona is so small, the tropical storm could intensify or weaken very quickly as the environmental conditions change.

The subtropical high that extends from Africa across the Atlantic Ocean is splitting into an eastern ridge and a western ridge.  Tropical Storm Fiona is moving toward the weakness where the split is occurring.  This is producing a motion toward the northwest and that general motion is expected to continue during the next several days.  On its anticipated track Tropical Storm Fiona could be northeast of the Leeward Islands in a few days.

Tropical Depression Six Forms Over Eastern Atlantic

A center of circulation developed within a tropical wave designated as Invest 98L on Tuesday and the National Hurricane Center classified the system as Tropical Depression Six.  At 11:00 p.m. EDT on Tuesday the center of Tropical Depression Six was located at latitude 12.6°N and longitude 34.1°W which put it about 1800 miles (2900 km) east of the Leeward Islands.  The 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. (70 km/h).  The minimum surface pressure was 1008 mb.

The circulation of Tropical Depression Six is still organizing.  A center of circulation exists near the surface and thunderstorms are developing near the center.  Several partial spiral bands are beginning to form.  The thunderstorms near the center are generating a small region of upper level divergence, but the divergence is occurring mainly to the southwest of the depression.

Tropical Depression Six is moving through an environment that favors intensification.  It is moving over water where the Sea Surface Temperature is near 27°C.  An upper level ridge to the north is causing northeasterly winds to blow across the top of the depression.  There is some vertical wind shear, but the shear will only slow the rate of intensification.  The shear is not strong enough to prevent intensification and Tropical Depression Six is expected to become a tropical storm during the next day or two.

The subtropical ridge over Africa and the Atlantic Ocean is splitting into two parts and Tropical Depression Six is moving toward the northwest into the split that is developing.  The depression is expected to continue to move toward the northwest in the short term.  The longer term motion will depend on how strong Tropical Depression Six becomes.  If it intensifies more and develops a taller circulation, then the depression will be steered by the winds higher in the atmosphere.  The winds at those levels are more likely to carry it into the central Atlantic Ocean.  However, if vertical wind shear weakens the depression and the circulation is shallower, then the winds in the lower levels of the atmosphere could carry the depression farther to the west.  In either case Tropical Depression Six is no immediate threat to any land area.

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.