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,
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.
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.
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 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.
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.