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 large area of thunderstorms developed near the center of Tropical Depression 4 during during the overnight hours. An Air Force Reserve Hurricane Hunter reconnaissance aircraft found sustained winds to tropical storm force and at 8:00 a.m. EDT on Monday the National Hurricane Center classified the system as Tropical Storm Danielle.
At 8:00 a.m. EDT on Monday the center of Tropical Storm Danielle was located at latitude 20.6°N and longitude 96.0°W which put it about 95 miles (150 km) east-southeast of Tuxpan, Mexico. Danielle was moving toward the west-northwest at 7 m.p.h. (11 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 1008 mb. The government of Mexico has issued a Tropical Storm warning for the portion of the coast from Laguna Verde to Rio Panuco.
The center of an upper level ridge moved near the center of Danielle on Sunday night, which caused the upper level winds to be weaker over the core of the circulation. As a result, a large area of thunderstorms was able to develop and persist around the center of circulation. Since Danielle was over water where the Sea Surface Temperature was near 30°C, the circulation was able to extract energy from the ocean and Danielle intensified into a tropical storm.
The thunderstorms at the core of Danielle are generating upper level divergence that is pumping mass out in all direction. The pressure at the surface is likely to decrease which should produce an additional increase in the wind speed. Danielle will be over very warm water until it makes landfall on the coast of Mexico later today. It is likely to intensify further before it reaches the coast.
An ridge of high pressure extends from the Atlantic Ocean into the Gulf of Mexico. The ridge is steering Danielle toward the west-northwest and that general motion is expected to continue. On its anticipated track the center of Danielle could be very near Tuxpan on the coast of Mexico in about 12 hours. Danielle could produce a storm surge of several feet along the coast. It could also cause some wind damage, although that should be minimal. The increase in thunderstorms near the center of circulation means that Danielle will produce locally heavy rain when it moves inland. The heavy rain will create the risk of flash floods and mudslides on steeper slopes.
Danielle was designated a tropical storm on June 20, which makes it the fourth Atlantic tropical storm of 2016. June 20th is the earliest date on record on which the fourth Atlantic tropical storm has formed. The previous record was June 23, which was the date when Tropical Storm Debbie was named in 2012. During the record setting year of 2005, the fourth Atlantic tropical storm was not named until July 5.
An Air Force Reserve Hurricane Hunter aircraft investigated an area of low pressure over the Bay of Campeche on Sunday afternoon and found that there was sufficient organization for the National Hurricane Center to designate the system Tropical Depression 4. At 11:00 p.m. EDT on Sunday the center of Tropical Depression 4 (TD4) was located at latitude 20.1°N and longitude 95.4°W which put it about 145 miles (230 km) east-southeast of Tuxpan, Mexico. TD4 was moving toward the west at 7 m.p.h. (11 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 1009 mb. The government of Mexico has issued a Tropical Storm Warning for the portion of the coast from Laguna Verde to Rio Panuco.
Although the reconnaissance aircraft found that the circulation was better organized than it was on Saturday, it still is not well organized. There is a distinct center of circulation, but the thunderstorms near the center have been developing and then weakening. The circulation appears to be somewhat elongated and stretches from the southeast toward the northwest. A persistent rain band is east and north of the center of circulation and some of the stronger winds are occurring in the rainband. A new cluster of thunderstorms appears to be developing near the center, and it could represent an improvement in the organization of the depression.
The environment is marginally favorable for intensification. Tropical Depression 4 is over water where the Sea Surface Temperature is near 30°C. An upper level ridge is producing some vertical wind shear, but the upper level ridge is also enhancing upper level divergence to the east of the depression. The depression is close to tropical storm intensity and it has another 12-18 hours to intensify before it makes landfall in Mexico.
A ridge of high pressure that extends from the Atlantic Ocean over the Gulf of Mexico is steering the depression toward the west and that general steering motion is expected to continue. On its anticipated track, Tropical Depression 4 will make landfall in Mexico on Monday. The depression could produce locally heavy rainfall and flash flooding will be the primary risk when it makes landfall.