Tag Archives: Lesser Antilles

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.

Tropical Storm Harvey Moves Into Louisiana, Irma Strengthens Quickly

Tropical Storm Harvey finally moved into Louisiana on Wednesday after lingering for several days over the northwestern Gulf of Mexico.  Harvey weakened to a tropical depression after it moved inland.  At the same time a new tropical storm named Irma strengthened quickly over the eastern Atlantic Ocean.  At 11:00 p.m. EDT on Wednesday the center of Tropical Depression Harvey was located at latitude 31.7°N and longitude 92.3°W which put it about 30 miles (50 km) north-northeast of Alexandria, Louisiana.  Harvey was moving toward the north-northeast at 9 m.p.h. (15 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 998 mb.

After dropping record rainfall and causing destructive floods of parts of southeastern Texas and western Louisiana, Tropical Depression Harvey finally started to move steadily toward the north-northeast on Wednesday.  Harvey was still producing heavy rain over parts of western Louisiana, but the fact that it was moving should limit the total rainfall at any location.  Bands of showers and thunderstorms on the far eastern periphery of Harvey’s circulation were also dropping heavy rain.  Some of those thunderstorms were strong enough to approach severe criteria.  Tropical Depression Harvey is forecast to move toward the Ohio River Valley during the next several days and it will bring windy, wet weather to the Middle Mississippi and Ohio River Valleys.  Rain associated with the circulation of Tropical Depression Harvey could also reach the Mid-Atlantic States later this week.

At 11:00 p.m. EDT on Wednesday the center of Tropical Storm Irma was located at latitude 16.4°N and longitude 32.2°W which put it about 545 miles (875 km) west of the Cabo Verde Islands.  Irma was moving toward the west at 12 m.p.h. (19 km/h).  The maximum sustained wind speed was 65 m.p.h. (105 km/h) and there were wind gusts to 80 m.p.h. (130 km/h).  The minimum surface pressure was 999 mb.

The circulation of Tropical Storm Irma organized quickly on Wednesday.  Numerous bands of showers and thunderstorms formed and began to revolve around a well organized center of circulation.  Thunderstorms in the core of Irma generated well developed upper level divergence which pumped away mass in all direction.  There were occasional satellite images which hinted that an eye could be forming at the center of Tropical Storm Irma.

Tropical Storm Irma will be moving through an environment that will be 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.  Irma is likely to become a hurricane on Thursday and it could intensify rapidly if an eye forms.  Tropical Storm Irma is forecast to become a major hurricane and it could become one of the big classic Cape Verde hurricanes.

A strong subtropical high to the north of Irma is steering the tropical storm toward the west and a general westerly motion is forecast to continue for the next few days.  There is more divergence in the model guidance after a few days and the future track of Irma when it nears the Lesser Antilles is more uncertain.  Tropical Storm Irma has the potential to become a big dangerous hurricane and it will need to be watched carefully.

Hurricane Gert Intensifies to Cat. 2 South of Nova Scotia

Hurricane Gert intensified to Category 2 on the Saffir-Simpson Scale as it sped over the Gulf Stream south of Nova Scotia on Wednesday afternoon.  At 5:00 p.m. EDT on Wednesday the center of Hurricane Gert was located at latitude 38.7°N and longitude 62.4°W which put it about 410 miles (665 km) south of Halifax, Nova Scotia.  Gert was moving toward the northeast at 31 m.p.h. (50 km/h).  The maximum sustained wind speed was 100 mp.h. (160 km/h) and there were wind gusts to 120 m.p.h. (195 km/h).  The minimum surface pressure was 970 mb.

Although Hurricane Gert is at a fairly high latitude, it has the classic structure of a Hurricane.  There is a fairly small eye at the center of circulation.  The eye is surround by a ring of strong thunderstorm and the strongest winds are occurring in this ring of thunderstorms.  There are additional bands of showers and thunderstorms revolving around the core of the hurricane.  The circulation is symmetrical and thunderstorms in the core are producing upper level divergence which is pumping away mass to the northeast of the hurricane.  Winds to hurricane force extend out about 40 miles (65 km) primarily to the east of the center of circulation.  Winds to tropical storm force extend out about 140 miles (225 km) from the center.

Hurricane Gert is moving over the Gulf Stream which means it is moving over water where the Sea Surface Temperature (SST) is near 28.5°C.  An upper level trough west of Gert is producing southwesterly winds which are blowing toward the top of the hurricane.  However, there is not much change of wind speed with height, which means that there is little vertical wind shear.  The combination of the warm water of the Gulf Stream and little vertical shear, allowed Hurricane Gert to strengthen on Wednesday.

Hurricane Gert could intensify during the next few hours, but it will soon move into a much less favorable environment.  Gert will soon move north of the Gulf Stream where the SSTs are much cooler.  The upper level trough is moving closer to Hurricane Gert and the winds are the upper level are forecast to get stronger.  When those winds increase, there will be much more vertical wind shear.  Colder water and more wind shear will cause Hurricane Gert to weaken on Thursday.  Gert could make a transition to an extratropical cyclone in colder environment of the North Atlantic.

Southwesterly winds in the upper level trough are steering Hurricane  Gert quickly toward the northeast and that general motion is expected to continue.  On its anticipated track, Hurricane Gert will move south of Labrador and Greenland.

Even as Hurricane Gert speeds away over the North Atlantic three new tropical waves over the tropical Atlantic have the potential to develop into tropical cyclones.  A tropical wave about 800 miles (1290 km) east of the Lesser Antilles designated as Invest 91L showed signs of organization on Wednesday.  A few more thunderstorms developed closed to the center of circulation.  A reconnaissance plane is scheduled to investigate this system on Thursday.  A little farther to the east another tropical wave designated Invest 92L was also showing evidence or more organization.  A third tropical wave just west of Africa also has the potential to develop during the next few days.

Tropical Depression Four Develops East of the Lesser Antilles

Tropical Depression Four developed east of the Lesser Antilles on Wednesday night.  At 11:00 p.m. EDT on Wednesday the center of Tropical Depression Four was located at latitude 12.8°N and longitude 38.4°W which put it about 1545 (2485 km) east of the Lesser Antilles.  The depression was moving toward the west-northwest at 14 m.p.h. (22 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 1009 mb.

A larger area of thunderstorms formed and persisted on the western side of a tropical disturbance formerly designated as Invest 94L on Wednesday and the National Hurricane Center classified the system as Tropical Depression Four.  The distribution of thunderstorms is still asymmetrical.  Most of the stronger storms are developing in the western half of the circulation.  The thunderstorms in the western part of the circulation were beginning to generate upper level divergence.  There were few thunderstorms in the eastern half of the depression and the vertical structure of the circulation could be tilted to the west with height.

Tropical Depression Four will be moving through an environment that contains both positive and negative factors for intensification.  The depression will move over water where the Sea Surface Temperature is near 27°C.  So, there is sufficient energy in the upper ocean to support intensification.  An upper level ridge is producing easterly winds which are blowing toward the top of the circulation.  Those winds could be causing the circulation to tilt toward the west with height.  Moderate vertical wind shear could inhibit intensification.  Drier air is north of the tropical depression.  If the depression remains south of the drier air and if the vertical wind shear does not become too strong, then the depression could strengthen.  Alternatively, if the depression pulls in drier air and/or the vertical shear increases, then the depression could weaken back to a tropical wave.

The subtropical ridge over the Atlantic Ocean is steering Tropical Depression Four toward the west-northwest.  A general west-northwesterly motion is expected for the next several days.  The actual track will also have a significant effect on the future intensity of Tropical Depression Four. If the depression moves on a more southerly track, it will stay south of the drier air and it would have a greater opportunity to intensify.  If the depression moves farther to the north, it will move into the drier air and will be more likely to weaken,

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.

Two Areas to Watch for Potential Tropical Development

There are two areas to watch for the potential development of a tropical cyclone during the next week.  One location to watch for potential development is the area around the western Caribbean Sea, Yucatan peninsula and southern Gulf of Mexico.  The other area to watch is the tropical Atlantic between Africa and the Lesser Antilles.

Numerical models have been suggesting possible development of an area of low pressure somewhere over the western Caribbean Sea, Yucatan peninsula or southern Gulf of Mexico.  The scenario suggested by the models includes a surge of southerly winds across Central America, which contributes to the spinning up of an area of low pressure.  There are currently strong westerly winds in the upper levels over this area.  The strong vertical wind shear will prevent development of a tropical cyclone in the short term.  An upper level ridge could develop over the area in several days.  If that happens, the wind shear would decrease.  Some runs of the numerical models create a broad, weak area of low pressure, which would primarily be a rain threat for the Yucatan peninsula and nearby regions.  Other models develop a low pressure system that is a little stronger, although those lows also weaken when they move over the Yucatan.  The models generally move the low pressure system northwestward over the southern Gulf of Mexico.  However, there are fairly significant deviations in how far north the low moves once it is in the Gulf.  As of 8:00 p.m. EDT on June 14 the National Hurricane Center was indicating that there was a 30% probability of the development of a tropical cyclone in this area during the next five days.

Some numerical models have also been suggesting potential development of a tropical cyclone near latitude 10°N over the tropical Atlantic Ocean between Africa and the Lesser Antilles.  A tropical wave is interacting with the Intertropical Convergence Zone and there has been a persistent area of showers and thunderstorms.  The disturbance is moving slowly toward the west.  There is some vertical wind shear in this region, but the stronger upper level winds are north of the disturbance.  The disturbance is over water where the Sea Surface Temperature is near 27°C.  It would be unusual to see a tropical cyclone develop over the tropical Atlantic in June, but it has happened at least three times in the past.  As of 8:00 p.m. EDT on June 14 the National Hurricane Center is indicating that there is a 20% probability of the development of a tropical cyclone in this region during the next five days.

Tropical Storm Grace Forms Southwest of Cape Verde

Enough organization developed in the center of an area of low pressure that moved off the coast of Africa several days ago for the National Hurricane Center to classify it as Tropical Storm Grace.  At 5:00 p.m. EDT on Saturday the center of Tropical Storm Grace was located at latitude 12.6°N and longitude 26.4°W which put it about 285 miles (460 km) southwest of the Cape Verde Islands.  Grace was moving toward the west at 14 m.p.h. (22 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 1007 mb.

The area of low pressure that became Tropical Storm Grace had a large area of thunderstorms when it moved off the coast of West Africa.  A few thunderstorms developed closer to the center of circulation and there are signs that a couple of spiral bands could be forming near the core of Grace.  The strongest thunderstorms are occurring in the southwestern quadrant of the circulation.  Some of the thunderstorms appear to be getting taller, but the core of the circulation is still organizing.

Tropical Storm Grace is moving over water where the Sea Surface Temperature is near 29°C.  Although there are stronger upper level westerly winds north of the tropical storm, the vertical wind shear over Tropical Storm Grace is relatively modest.  It has a day or two for the core of the circulation to organize and to intensify.  When Grace moves farther west it could move into a region where there is drier air.  An upper level trough could produce more vertical wind shear when the tropical storm gets closer to the Caribbean Sea.

A subtropical ridge is steering Grace toward the west and that steering pattern is expected to continue for the next few days.  If Grace gets stronger and the convection taller, it would take a track a little farther to the north.  If it remains weaker and shallower, lower level winds will steer it more to the west.  On its anticipated track, Grace could approach the Lesser Antilles in five or six days.

Tropical Storm Danny Heading West

Tropical Storm Danny has moved steadily westward during the past 24 hours.  At 11:00 a.m. EDT on Wednesday the center of Tropical Storm Danny was located at latitude 11.2°N and longitude 41.1°W which put it about 1385 miles (2235 km) east of the Lesser Antilles and about 2740 miles (4410 km) east-southeast of Miami, Florida.  Danny was moving toward the west at 12 m.p.h. (19 km/h).  The maximum sustained wind speed was 50 m.p.h. (80 km/h) and there were wind gusts to 60 m.p.h. (95 km/h).  The minimum surface pressure was 1000 mb.

The organization of the circulation of Danny has varied during the past day.  It has a visible tight core at the center, but earlier today there was no convection around the core.  Recent satellite images show new thunderstorms developing near the core.  The environment around Danny is complex and it contains both positive and negative factors.  Danny is currently over water where the Sea Surface Temperature (SST) is near 28°C.  However, there is slightly cooler water just to the north of the tropical storm.  Satellite imagery indicates that there could also be drier air north of the circulation and some of that drier air could be pulled into Danny.  The upper level winds are light and upper level divergence is occurring.  However, a surge in the low level trade winds is about 250 miles (400 km) east of Danny.   If the surge in the trade winds reaches the core of Danny, it could push the lower part of the circulation out ahead (to the west) of the upper part of the circulation.  In that case Danny would weaken.

The complexity of the environment around Tropical Storm Danny makes the intensity forecast challenging.  If the trade wind surge does not reach the core of Danny and it stays over warm SSTs, then gradual intensification is possible.  On the other hand, if Danny moves more northward over cooler SSTs and into drier air, or if the trade wind surge creates more vertical wind shear, then Danny could weaken.

A subtropical ridge to the north of Danny is steering Danny toward the west and a generally west or west-northwest motion is forecast for the next few days.  If Danny is a little stronger, it could move a little farther north, and if it is weaker, it could stay farther south.  On its anticipated track, Danny could approach the Lesser Antilles in four or five days.  Interests in those areas should monitor Danny for future developments.

Update on Invest 96L

A tropical disturbance approaching the Lesser Antilles was designated Invest 96L by the National Hurricane Center (NHC).  The disturbance appears to consist of a tropical wave, a broad surface low centered east of Guadaloupe and a small cyclonic meso-vortex rotating around the northeastern portion of the broader low pressure system.  The overall system is moving toward the west-northwest at 21 m.p.h.  There appears to be a broad area of light winds within the surface low and stronger winds on the north side of the small meso-vortex.  A reconnaissance aircraft did find winds to tropical storm force on the north side of the system, but it also reported that the overall circulation was poorly defined.

This disturbance has a complicated origin which is linked to its slow development.  The disturbance originally consisted of two tropical waves moving north of a broad but weak low pressure system located within the Intertropical Convergence Zone/monsoon trough.  The complex structure inhibited the development of a dominant center of circulation and several clusters of thunderstorms have produced small meso-vortices like the one mentioned previously.  It appears that there has been a slight increase in organization today as the broad area of low pressures appears to have a more symmetrical shape.  It is unclear if an upper low to the northwest of the system is creating wind shear over the top of it.

NHC is giving a 70% chance that a tropical cyclone will form out of this system within the next five days.  As broad low pressure system moves west-northwest it will affect the weather over the northeastern Caribbean Sea during the next several days.  Another reconnaissance aircraft is scheduled to investigate the disturbance tomorrow afternoon, if necessary.