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Debby Makes a Transition to a Tropical Storm

Former Subtropical Storm Debby made a transition to a tropical storm on Wednesday.  At 11:00 a.m. EDT on Wednesday the center of Tropical Storm Debby was located at latitude 41.2°N and longitude 48.3°W which put it about 1150 miles (1855 km) west-northwest of the Azores.  Debby was moving toward the northeast at 12 m.p.h. (19 km/h).  The maximum sustained wind speed was 45 m.p.h. (75 km/h) and there were wind gusts to 60 m.p.h. (95 km/h).  The minimum surface pressure was 1003 mb.

The structure of former Subtropical Storm Debby changed on Wednesday and it exhibited the characteristics of a tropical storm.  More thunderstorms formed around the center of circulation and those thunderstorms rose higher into the atmosphere.  Additional bands of showers and thunderstorms developed and revolved around the core of the circulation.  Storms near the core began to generate upper level divergence.  The strongest winds occurred closer to the center of circulation.  The National Hurricane Center designated Debby as a tropical storm based on information from satellites.

Tropical Storm Debby will move into an environment unfavorable for a tropical storm during the next 24 to 48 hours.   Debby was over water where the Sea Surface Temperature was near 26°C, but it will soon move over much cooler water.  It will start to weaken when it moves over the cooler water.  It could take several days for the circulation around Tropical Storm Debby to spin down.

Tropical Storm Debby will be steered toward the northeast as it moves between an upper level trough to the west and an upper level ridge to the east.  On its anticipated track Tropical Storm Debby will move between the Azores and Greenland.

Subtropical Storm Debby Forms West of the Azores

Subtropical Storm Debby formed over the North Atlantic Ocean west of the Azores on Tuesday morning.  At 11:00 a.m. EDT on Tuesday the center of Subtropical Storm Debby was located at latitude 38.9°N and longitude 48.5°W which put it about 1160 miles (1870 km) west of the Azores.  Debby was moving toward the north 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 1008 mb.

A surface low pressure system revolved around the western side of an upper level low over the North Atlantic Ocean during the past few days.  More bands of showers and thunderstorms developed around the surface low on Tuesday morning and its circulation become more circular.  The National Hurricane Center designated the low pressure system as Subtropical Storm Debby based on satellite imagery.  The strongest winds were occurring in a rainband that wrapped around the northern side of the circulation.  Bands south of the center of circulation consisted primarily of showers and lower clouds.

Subtropical Storm Debby will move through an environment somewhat favorable for intensification during the next 24 hours.  Debby will move over water where the Sea Surface Temperature is between 26°C and 27°C.  Subtropical Storm Debby will move between an upper level trough to its west and an upper level ridge to its east.  The trough and ridge will produce southeasterly winds which will cause moderate vertical wind shear.  Subtropical Storm Debby could strengthen during the next 24 hours.  Debby will move over much colder water in about 36 hours and then it will weaken.

The upper level ridge and trough will steer Subtropical Storm Debby in a mainly northerly direction.  On its anticipated track Subtropical Storm Debby will remain west of the Azores.

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,

TD 4 Intensifies to Tropical Storm Danielle As It Nears Mexico

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.

Tropical Depression 4 Forms Over Bay of Campeche

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.