Hurricane Helene (2006)
Encyclopedia
Hurricane Helene was the ninth tropical storm
, fourth hurricane, and strongest hurricane of the 2006 Atlantic hurricane season
.
Helene was a long-lived Cape Verde-type hurricane
that formed in the extreme southeastern part of the North Atlantic Ocean
, peaking as a Category 3 hurricane on the Saffir-Simpson hurricane scale
as it traversed the central Atlantic. It never affected land until the very end of its lifespan as a weak extratropical system, which had minor impact in the northern British Isles
.
. It was well-organized from the start, and on September 11, even before it emerged off the coast, the National Hurricane Center
believed that it could quickly develop into a tropical depression. That is indeed what happened, and it became Tropical Depression Eight on the morning of September 12. The enormous size of the depression made it fairly slow to develop, combined with some easterly wind shear
in the eastern Atlantic and influence of the Saharan Air Layer
to the north as it tracked south of the Cape Verde
islands. Convection was also slow to build in, with little banding at first. The initial strengthening was delayed as a result. However, on September 13, the organization improved as banding became better defined, and that evening, the depression strengthened into Tropical Storm Helene.
The storm slowly strengthened over the warm waters of the eastern Atlantic thereafter as it tracked west-northwest. Some dry air from the Saharan Air Layer continued to slow development on September 14 as Helene remained a low-end tropical storm. However, a bout of intensification took place on September 15 as shear diminished. The storm became much better organized with well-defined banding, and leveled off that evening at just under hurricane intensity. On the morning of September 16, the storm began to develop a ragged eye
, and the storm strengthened to hurricane status. The intensity leveled off for a while as a marginal Category 1 hurricane
, as the intensification was slowed by the presence of moderate wind shear despite the absence of significant dry air. Late that evening, the storm began to slowly intensify once again.
On September 17, the intensification became more rapid and Helene quickly became a Category 2 hurricane that morning as the eye became clearer and surrounded by deep convection. Helene also turned more northward and slowed down in the central Atlantic, which was in response to a weakness in the subtropical ridge
farther north created by Gordon to the north. The northward turn shifted the course away from any land areas. The intensification continued through the afternoon, and that evening, Helene strengthened into a major hurricane with 115 mph (185 km/h) and a minimum central pressure of 962 mbar. As shear remained low and the oceans remained warm, Helene intensified slightly more, peaking as a solid Category 3 hurricane with 120 mph (195 km/h) winds early on September 18. At that time, Helene and Gordon were at roughly the same longitude
in the open ocean. Operationally, Helene was estimated to have had 125 mph (205 km/h) winds, but the post-storm analysis downgraded it slightly. As Gordon moved eastward on the afternoon of 18th, a narrow ridge built in, forcing Helene westward. The eyewall collapsed somewhat, and Helene weakened slightly to a Category 2 hurricane, where it leveled off for about 48 hours until September 20 due to a lengthy eyewall replacement cycle
and an elongated cloud pattern. On the 20th, Helene turned back to the northwest and weakened slightly due to a slight increase in wind shear. That afternoon, it was downgraded to a Category 1 hurricane, which Helene remained until becoming extratropical.
Early on September 21, the motion shifted to the north as it moved along the northwest periphery of the subtropical ridge. That general motion continued throughout the day, however, Helene began to turn more northeastward that evening along the edge of the subtropical ridge, east of Bermuda
. A decrease in wind shear and relatively warm water around 81°F (27°C) kept the intensity mostly stationary, keeping Helene as a Category 1 hurricane. The intensity held around 80 mph (130 km/h) before strengthening slightly late on September 22, despite losing some tropical characteristics (it was operationally downgraded to a tropical storm for a brief period) as it accelerated northeast in the north Atlantic. A QuikSCAT
analysis early on September 23 confirmed a high-end Category 1 hurricane with 90 mph (145 km/h) winds.
Helene transitioned into a "hybrid" storm with both tropical and extratropical characteristics that afternoon, with both a deep warm core and an asymmetric, frontal-like appearance. After that, increased shear weakened Helene once again, although it remained a hurricane-strength storm until it was fully extratropical on the morning of September 24. After becoming extratropical, the storm weakened as it tracked eastward, becoming a gale center west of Ireland
early on September 27. It eventually merged with a larger extratropical low near the northern end of the British Isles
late on the 27th.
. As a weakened extratropical system, strong wind gusts were reported in Ireland
and northern Scotland
. The strongest reported wind gust in Ireland was at the Valentia Observatory, where 56 mph (91 km/h) gusts were reported. In Scotland, the strongest gust was on South Uist
Island in the Outer Hebrides
, where 74 mph (118 km/h) gusts were reported. No damage or fatalities were reported as a result of Helene.
Tropical cyclone
A tropical cyclone is a storm system characterized by a large low-pressure center and numerous thunderstorms that produce strong winds and heavy rain. Tropical cyclones strengthen when water evaporated from the ocean is released as the saturated air rises, resulting in condensation of water vapor...
, fourth hurricane, and strongest hurricane of the 2006 Atlantic hurricane season
2006 Atlantic hurricane season
The 2006 Atlantic hurricane season was significantly less active than the record previous season. It marked the first since 2001 in which no hurricanes made landfall in the United States, and was the first since 1994 that no tropical cyclones formed during October. Following the intense activity of...
.
Helene was a long-lived Cape Verde-type hurricane
Cape Verde-type hurricane
A Cape Verde-type hurricane is an Atlantic hurricane that develops near the Cape Verde islands, off the west coast of Africa. The average hurricane season has about two Cape Verde-type hurricanes, which are usually the largest and most intense storms of the season because they often have plenty of...
that formed in the extreme southeastern part of the North Atlantic Ocean
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...
, peaking as a Category 3 hurricane on the Saffir-Simpson hurricane scale
Saffir-Simpson Hurricane Scale
The Saffir–Simpson Hurricane Scale , or the Saffir–Simpson Hurricane Wind Scale , classifies hurricanes — Western Hemisphere tropical cyclones that exceed the intensities of tropical depressions and tropical storms — into five categories distinguished by the intensities of their sustained winds...
as it traversed the central Atlantic. It never affected land until the very end of its lifespan as a weak extratropical system, which had minor impact in the northern British Isles
British Isles
The British Isles are a group of islands off the northwest coast of continental Europe that include the islands of Great Britain and Ireland and over six thousand smaller isles. There are two sovereign states located on the islands: the United Kingdom of Great Britain and Northern Ireland and...
.
Meteorological history
In the second week of September, a strong tropical wave began to emerge off the coast of AfricaAfrica
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
. It was well-organized from the start, and on September 11, even before it emerged off the coast, the National Hurricane Center
National Hurricane Center
The National Hurricane Center , located at Florida International University in Miami, Florida, is the division of the National Weather Service responsible for tracking and predicting weather systems within the tropics between the Prime Meridian and the 140th meridian west poleward to the 30th...
believed that it could quickly develop into a tropical depression. That is indeed what happened, and it became Tropical Depression Eight on the morning of September 12. The enormous size of the depression made it fairly slow to develop, combined with some easterly wind shear
Wind shear
Wind shear, sometimes referred to as windshear or wind gradient, is a difference in wind speed and direction over a relatively short distance in the atmosphere...
in the eastern Atlantic and influence of the Saharan Air Layer
Saharan Air Layer
The Saharan Air Layer is an intensely dry, warm and sometimes dust-laden layer of the atmosphere which often overlies the cooler, more-humid surface air of the Atlantic Ocean. In the Sahara Desert region of North Africa, where it originates, it is the prevalent atmosphere, extending from the...
to the north as it tracked south of the Cape Verde
Cape Verde
The Republic of Cape Verde is an island country, spanning an archipelago of 10 islands located in the central Atlantic Ocean, 570 kilometres off the coast of Western Africa...
islands. Convection was also slow to build in, with little banding at first. The initial strengthening was delayed as a result. However, on September 13, the organization improved as banding became better defined, and that evening, the depression strengthened into Tropical Storm Helene.
The storm slowly strengthened over the warm waters of the eastern Atlantic thereafter as it tracked west-northwest. Some dry air from the Saharan Air Layer continued to slow development on September 14 as Helene remained a low-end tropical storm. However, a bout of intensification took place on September 15 as shear diminished. The storm became much better organized with well-defined banding, and leveled off that evening at just under hurricane intensity. On the morning of September 16, the storm began to develop a ragged eye
Eye (cyclone)
The eye is a region of mostly calm weather found at the center of strong tropical cyclones. The eye of a storm is a roughly circular area and typically 30–65 km in diameter. It is surrounded by the eyewall, a ring of towering thunderstorms where the second most severe weather of a cyclone...
, and the storm strengthened to hurricane status. The intensity leveled off for a while as a marginal Category 1 hurricane
Saffir-Simpson Hurricane Scale
The Saffir–Simpson Hurricane Scale , or the Saffir–Simpson Hurricane Wind Scale , classifies hurricanes — Western Hemisphere tropical cyclones that exceed the intensities of tropical depressions and tropical storms — into five categories distinguished by the intensities of their sustained winds...
, as the intensification was slowed by the presence of moderate wind shear despite the absence of significant dry air. Late that evening, the storm began to slowly intensify once again.
On September 17, the intensification became more rapid and Helene quickly became a Category 2 hurricane that morning as the eye became clearer and surrounded by deep convection. Helene also turned more northward and slowed down in the central Atlantic, which was in response to a weakness in the subtropical ridge
Subtropical ridge
The subtropical ridge is a significant belt of high pressure situated around the latitudes of 30°N in the Northern Hemisphere and 30°S in the Southern Hemisphere. It is characterized by mostly calm winds, which acts to reduce air quality under its axis by causing fog overnight, and haze during...
farther north created by Gordon to the north. The northward turn shifted the course away from any land areas. The intensification continued through the afternoon, and that evening, Helene strengthened into a major hurricane with 115 mph (185 km/h) and a minimum central pressure of 962 mbar. As shear remained low and the oceans remained warm, Helene intensified slightly more, peaking as a solid Category 3 hurricane with 120 mph (195 km/h) winds early on September 18. At that time, Helene and Gordon were at roughly the same longitude
Longitude
Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. It is an angular measurement, usually expressed in degrees, minutes and seconds, and denoted by the Greek letter lambda ....
in the open ocean. Operationally, Helene was estimated to have had 125 mph (205 km/h) winds, but the post-storm analysis downgraded it slightly. As Gordon moved eastward on the afternoon of 18th, a narrow ridge built in, forcing Helene westward. The eyewall collapsed somewhat, and Helene weakened slightly to a Category 2 hurricane, where it leveled off for about 48 hours until September 20 due to a lengthy eyewall replacement cycle
Eyewall replacement cycle
Eyewall replacement cycles, also called concentric eyewall cycles, naturally occur in intense tropical cyclones, generally with winds greater than 185 km/h , or major hurricanes...
and an elongated cloud pattern. On the 20th, Helene turned back to the northwest and weakened slightly due to a slight increase in wind shear. That afternoon, it was downgraded to a Category 1 hurricane, which Helene remained until becoming extratropical.
Early on September 21, the motion shifted to the north as it moved along the northwest periphery of the subtropical ridge. That general motion continued throughout the day, however, Helene began to turn more northeastward that evening along the edge of the subtropical ridge, east of Bermuda
Bermuda
Bermuda is a British overseas territory in the North Atlantic Ocean. Located off the east coast of the United States, its nearest landmass is Cape Hatteras, North Carolina, about to the west-northwest. It is about south of Halifax, Nova Scotia, Canada, and northeast of Miami, Florida...
. A decrease in wind shear and relatively warm water around 81°F (27°C) kept the intensity mostly stationary, keeping Helene as a Category 1 hurricane. The intensity held around 80 mph (130 km/h) before strengthening slightly late on September 22, despite losing some tropical characteristics (it was operationally downgraded to a tropical storm for a brief period) as it accelerated northeast in the north Atlantic. A QuikSCAT
QuikSCAT
The QuikSCAT is an earth-observing satellite that provided estimates of wind speed and direction over the oceans to National Oceanic and Atmospheric Administration and others. This "quick recovery" mission intended to replace the NASA Scatterometer , which failed in June 1997...
analysis early on September 23 confirmed a high-end Category 1 hurricane with 90 mph (145 km/h) winds.
Helene transitioned into a "hybrid" storm with both tropical and extratropical characteristics that afternoon, with both a deep warm core and an asymmetric, frontal-like appearance. After that, increased shear weakened Helene once again, although it remained a hurricane-strength storm until it was fully extratropical on the morning of September 24. After becoming extratropical, the storm weakened as it tracked eastward, becoming a gale center west of Ireland
Ireland
Ireland is an island to the northwest of continental Europe. It is the third-largest island in Europe and the twentieth-largest island on Earth...
early on September 27. It eventually merged with a larger extratropical low near the northern end of the British Isles
British Isles
The British Isles are a group of islands off the northwest coast of continental Europe that include the islands of Great Britain and Ireland and over six thousand smaller isles. There are two sovereign states located on the islands: the United Kingdom of Great Britain and Northern Ireland and...
late on the 27th.
Impact
While a tropical system, Helene never approached land. Three ships were caught in the outer bands of Helene; the strongest of which reported 56 mph (91 km/h) sustained winds early on September 23 in the north Atlantic. Though it remained far away from the island, the hurricane produced rough waves in BermudaBermuda
Bermuda is a British overseas territory in the North Atlantic Ocean. Located off the east coast of the United States, its nearest landmass is Cape Hatteras, North Carolina, about to the west-northwest. It is about south of Halifax, Nova Scotia, Canada, and northeast of Miami, Florida...
. As a weakened extratropical system, strong wind gusts were reported in Ireland
Ireland
Ireland is an island to the northwest of continental Europe. It is the third-largest island in Europe and the twentieth-largest island on Earth...
and northern Scotland
Scotland
Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...
. The strongest reported wind gust in Ireland was at the Valentia Observatory, where 56 mph (91 km/h) gusts were reported. In Scotland, the strongest gust was on South Uist
South Uist
South Uist is an island of the Outer Hebrides in Scotland. In the 2001 census it had a usually resident population of 1,818. There is a nature reserve and a number of sites of archaeological interest, including the only location in Great Britain where prehistoric mummies have been found. The...
Island in the Outer Hebrides
Outer Hebrides
The Outer Hebrides also known as the Western Isles and the Long Island, is an island chain off the west coast of Scotland. The islands are geographically contiguous with Comhairle nan Eilean Siar, one of the 32 unitary council areas of Scotland...
, where 74 mph (118 km/h) gusts were reported. No damage or fatalities were reported as a result of Helene.
See also
- 2006 Atlantic hurricane season2006 Atlantic hurricane seasonThe 2006 Atlantic hurricane season was significantly less active than the record previous season. It marked the first since 2001 in which no hurricanes made landfall in the United States, and was the first since 1994 that no tropical cyclones formed during October. Following the intense activity of...
- Timeline of the 2006 Atlantic hurricane seasonTimeline of the 2006 Atlantic hurricane seasonThe 2006 Atlantic hurricane season was the first since 2001 in which no hurricanes made landfall in the United States, and the first since 1994 that no tropical cyclones formed during October. This timeline documents all the storm formations, strengthening, weakening, landfalls, extratropical...
- List of storms in the 2006 Atlantic hurricane season
External links
- The NHC's archive on Hurricane Helene