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Tropical Depression Four Forms, Tropical Storm Warning for South Carolina

Tropical Depression Four formed on Monday morning and the National Hurricane Center issued a Tropical Storm Warning for a portion of the coast of South Carolina. At 11:00 a.m. EDT on Monday the center of Tropical Depression Four was located at latitude 31.9°N and longitude 78.3°W which put it about 110 miles (175 km) east-southeast of Charleston, South Carolina. Tropical Depression Four was moving toward the west-northwest at 16 m.p.h. (26 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 1013 mb.

A Tropical Storm Warning was issued for the portion of the coast from Edisto Beach to South Santee River, South Carolina. The Tropical Storm Warning included Charleston.

The National Hurricane Center designated a low pressure system off the coast of South Carolina as Tropical Depression Four on Monday morning. Bands of showers and lower clouds were revolving around the center of Tropical Depression Four. Thunderstorms were occurring in bands on the far western side of the circulation around the depression. An upper low was centered over northern Florida on Monday morning. The upper low was producing easterly winds that were blowing across the top of the tropical depression. The easterly winds were causing moderate vertical wind shear and the wind shear was blowing the tops off clouds in many of the bands closer to the center of the depression

Tropical Depression Four will move through an environment that is only slightly favorable for intensification during the next 12 hours. The depression will move over water where the Sea Surface Temperatures are near 27°C. The upper level low will continue to cause vertical wind shear that will inhibit the formation of thunderstorms near the center of Tropical Depression Four. If the wind shear decreases, then there is a chance Tropical Depression Four could strengthen to a tropical storm later on Monday.

Tropical Depression Four will move south of a high pressure system over the western Atlantic Ocean during the next 24 hours. The high will steer the depression toward the west-northwest. On its anticipated track the center of Tropical Depression Four could make landfall near Edisto Beach, South Carolina on Monday night. The depression will produce gusty winds along the coast of South Carolina. It could also drop locally heavy rain over parts of southern South Carolina and extreme eastern Georgia.

Tropical Storm Dolly Develops Southwest of Newfoundland

Tropical Storm Dolly developed southwest of Newfoundland on Tuesday.  At 1:00 p.m. EDT on Tuesday the center of Tropical Storm Dolly was located at latitude 39.4°N and longitude 61.9°W which put it about 660 miles (1065 km) southwest of Cape Race, Newfoundland.  Dolly was moving toward the east-northeast at 13 m.p.h. (20 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. (5 km/h).  The minimum surface pressure was 1002 mb.

Former Subtropical Depression Four strengthened on Tuesday and its structure continued to make a transition to a more tropical cyclone like appearance.  A band of showers and thunderstorms wrapped closely around the southern side of the center of circulation.  The strongest winds were occurring in that band.  Other bands of showers and thunderstorms were revolving around the center of circulation.  A scatterometer on board a satellite estimated that there were winds above tropical storm force and the National Hurricane Center designated the system as Tropical Storm Dolly.  The winds to tropical storm force extended out 65 miles (105 km) on the southern side of Dolly.  The winds in the other parts of the circulation were generally blowing at less than tropical storm force.

Tropical Storm Dolly will move through an environment that could be favorable for intensification during the next 6 to 12 hours.  Dolly will move near the northern part of the Gulf Stream where the water is near 26°C.  It will move around the southeastern part of a weakening upper level low.  The upper low will produce some vertical wind shear, but the shear will not be strong enough to prevent intensification during the next 6 to 12 hours.  Tropical Storm Dolly is likely to move north of the Gulf Stream on Wednesday.  When Dolly moves over colder water it will start to weaken.

Tropical Storm Dolly will move around the southeastern side of the upper low during the next few hours.  The upper low will weaken on Wednesday and both the upper low and Dolly will be steered toward the northeast by a larger upper level trough over eastern North America.  On its anticipated track Tropical Storm Dolly could bring gusty winds and rain to southeastern Newfoundland on Wednesday night and Thursday.

Subtropical Depression Four Forms Southeast of Cape Cod

Subtropical Depression Four developed southeast of Cape Cod, Massachusetts on Monday afternoon.  At 5:00 p.m. EDT on Monday the center of Subtropical Depression Four was located at latitude 38.2°N and longitude 65.7°W which put it about 310 miles (495 km) southeast of Cape Cod, Massachusetts.  The depression was moving toward the east-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 1009 mb.

The structure of a previously non-tropical low pressure system southeast of Cape Cod evolved into a more symmetrical circular shape on Monday.  Bands of showers and thunderstorms revolved more closely around the center of the low.  The low pressure system exhibited a more tropical like structure on visible satellite imagery.  However, the National Hurricane Center (NHC) observed that the surface low was under an upper low which meant the system had a cold core in the upper troposphere.  So, NHC designated the system as Subtropical Depression Four when it initiated advisories on Monday afternoon.

The circulation around Subtropical Depression Four had a hybrid structure with a tropical like appearance in the lower levels and a non-tropical cold core in the upper levels.  The depression will move through an environment favorable for intensification during the next 12 to 18 hours.  The core of the depression will move near the northern part of the Gulf Stream where the Sea Surface Temperature is near 26°C.  It will move near the south side of the core of the upper low where the winds are weaker.  Those winds will cause some vertical wind shear, but the shear will not be too strong during the next 12 hours.  The depression could strengthen into a subtropical storm.  There is a chance that the circulation could develop a warm core in the upper levels if the depression stays near the Gulf Stream.  The warm water in the Gulf Stream and the cold air in the upper troposphere will enhance instability which will contribute to the development of thunderstorms.  If thunderstorms release enough latent energy in the upper troposphere, then a warm core could form and the system could make an additional transition into a tropical storm.

Subtropical Depression Four is likely to move in tandem with the upper low during the next day or two.  The upper low will move toward the east-northeast on Tuesday and then it will move more toward the northeast on Wednesday.  On its anticipated track Subtropical Depression Four will move away from the northeast U.S.  The depression will remain south of Nova Scotia, but it could pass near southeastern Newfoundland on Thursday.

Tropical Storm Chantal Forms Southeast of Nova Scotia

Tropical Storm Chantal formed southeast of Nova Scotia on Tuesday night.  At 11:00 p.m. EDT on Tuesday the center of Tropical Storm Chantal was located at latitude 40.2°N and longitude 56.2°W which put it about 485 miles (780 km) southeast of Halifax, Nova Scotia.  Chantal was moving toward the east at 22 m.p.h. (35 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 1010 mb.

A small low pressure system that was near the coast of the Carolinas during the weekend began to exhibit tropical characteristics on Tuesday night and the National Hurricane Center designated the system as Tropical Storm Chantal.  Thunderstorms developed closer to the center of circulation when the low moved near the Gulf Stream and the strongest winds were occurring closer to the center.  Although the circulation around Chantal looked more tropical, the distribution of thunderstorms was still asymmetrical.  Many of the stronger storms were occurring in the eastern half of the circulation.  Bands in the western half of the circulation consisted primarily of showers and lower clouds.  Tropical Storm Chantal was far enough north that it was in the region of westerly winds in the upper levels.  Those winds were causing moderate vertical wind shear and they were the primary reason for the asymmetrical distribution of thunderstorms.

Tropical Storm Chantal will move through an environment only marginally favorable for intensification during the next several days.  Chantal will move over water where the Sea Surface Temperature is near 25°C.  It will continue to be in a region of moderate westerly winds in the upper levels, which will cause vertical wind shear.  Tropical Storm Chantal could intensify slowly if the upper level winds do not get any stronger.  However, Chantal could weaken if the upper level winds do get stronger.

Tropical Storm Chantal will move around the northern end of a ridge in the middle troposphere over the Atlantic Ocean.  The ridge will steer Chantal toward the east during the next 24 hours.  Chantal could move more toward the southeast on Thursday when it nears the western end of the ridge.  On its anticipated track Tropical Storm Chantal is forecast to make a slow loop in the middle of the Atlantic Ocean.

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.