Tropical Storm Sean (2011)
Encyclopedia
Tropical Storm Sean was a tropical cyclone
that affected the island of Bermuda
in November of the 2011 Atlantic hurricane season
. The nineteenth tropical system and the eighteenth named storm of the 2011 season, Sean developed from a non-tropical low pressure area on November 8. Initially subtropical
, the system displayed gale
-force winds over a portion of the Atlantic Ocean
. However, several hours after classification, the storm began to display tropical characteristics, and it was noted by the National Hurricane Center that Sean had transitioned into a fully tropical storm. Moving in an erratic motion to the southwest of Bermuda, Sean steadily strengthened to reach a peak intensity of 65 mph (100 km/h) before curving towards the north and eventually the northeast. On November 11, Sean passed by the island of Bermuda, bringing tropical storm force winds and heavy rainfall before becoming extratropical
early on November 12, and was absorbed by a neighboring extratropical storm, soon afterwards.
. While crossing Bermuda, an extratropical low pressure detached from the frontal system. By November 7, deep convection
began to consolidate around the surface center of the low pressure area. The system continued to organized, and by early on November 8, satellite imagery indicated that the low-level circulation was becoming well-defined and gale
-force winds were being reported near the center. By 0900 UTC
, the National Hurricane Center
initiated advisories on Subtropical Storm Sean, with initial wind speeds of 45 mph (75 km/h). After becoming a subtropical cyclone, Sean quickly began to developed curved bands of deep convection. However, Sean had not transitioned into a tropical cyclone since it remained in the vicinity of an upper-level low. Later on November 8, Sean moved away from the upper-level low, and there was evidence of a warm core. As a result, Sean was declared fully tropical at 2100 UTC on that day.
Under light wind shear and marginally warm sea surface temperature
s, Sean saw little change to its structure and intensity through the early hours of November 9. Over the next few hours, the structure of the storm improved as the convection in the storm became more organized. It was then noted of the possibility of an eye feature
developing, and the National Hurricane Center noted that its official intensity forecast might have been conservative. Shortly thereafter, Sean strengthened slightly and reached a wind speed of 65 mph (100 km/h). Though none of the computer models showed intensification into a hurricane, the National Hurricane Center predicted otherwise, due to light wind shear. Early on November 10, cloud top temperatures near the center of Sean had warmed, and the storm intensity had remained steady. Satellite imagery in the early hours of November 10 indicated that Sean continued to become better organized, noting increased convective banding and several attempts at developing an eye feature. However, subsequent satellite images also showed that the eye remained ragged and that Sean became more compact than on the previous day. By then, the National Hurricane Center no longer predicted Sean would strengthen into a hurricane, though it was mentioned that intensification into a hurricane was still possible. Later on November 10, it was noted that Sean appeared less organized on satellite imagery, though an reconnaissance aircraft
indicated that the storm was not weakening. It was at this time that the storm reached its lowest central pressure of 983 mb, which it maintained through the early hours of November 11. As the day continued, wind shear began to increase, and Sean began to weaken. Hours later, the storm was described as a "strongly sheared cyclone," and the storm continued to decrease in intensity and condense in size. At the same time, a cold front
was approaching from the west, which was associated with a powerful extratropical cyclone
, centered over Newfoundland
.
By the afternoon hours, it was noted that Sean was losing tropical characteristics, with stable
air entering the southern half of the storm. The storm became post-tropical early on November 12 as it merged with the cold front, and was absorbed into the extratropical storm, soon afterwards.
issued a tropical storm watch for Bermuda on November 8 at 1200 UTC. Almost 48 hours later, the tropical storm watch was upgraded to a tropical storm warning.
Tropical Storm Sean brought rough seas to the east coast of Florida
, which drowned one swimmer in Jensen Beach
.
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...
that affected the island 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...
in November of the 2011 Atlantic hurricane season
2011 Atlantic hurricane season
The 2011 Atlantic hurricane season is tied for the third most active season on record with 1887, 1995 and 2010. It began on June 1, 2011, and ended on November 30, 2011, however these dates conventionally delimit the period of each year when most tropical cyclones develop in the Atlantic basin...
. The nineteenth tropical system and the eighteenth named storm of the 2011 season, Sean developed from a non-tropical low pressure area on November 8. Initially subtropical
Subtropical cyclone
A subtropical cyclone is a weather system that has some characteristics of a tropical and an extratropical cyclone. As early as the 1950s, meteorologists were unclear whether they should be characterized as tropical or extratropical cyclones. They were officially recognized by the National...
, the system displayed gale
Gale
A gale is a very strong wind. There are conflicting definitions of how strong a wind must be to be considered a gale. The U.S. government's National Weather Service defines a gale as 34–47 knots of sustained surface winds. Forecasters typically issue gale warnings when winds of this strength are...
-force winds over a portion of the 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...
. However, several hours after classification, the storm began to display tropical characteristics, and it was noted by the National Hurricane Center that Sean had transitioned into a fully tropical storm. Moving in an erratic motion to the southwest of Bermuda, Sean steadily strengthened to reach a peak intensity of 65 mph (100 km/h) before curving towards the north and eventually the northeast. On November 11, Sean passed by the island of Bermuda, bringing tropical storm force winds and heavy rainfall before becoming extratropical
Extratropical cyclone
Extratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are a group of cyclones defined as synoptic scale low pressure weather systems that occur in the middle latitudes of the Earth having neither tropical nor polar characteristics, and are connected with fronts and...
early on November 12, and was absorbed by a neighboring extratropical storm, soon afterwards.
Meteorological history
On November 4, a frontal system moved off the East Coast of the United StatesEast Coast of the United States
The East Coast of the United States, also known as the Eastern Seaboard, refers to the easternmost coastal states in the United States, which touch the Atlantic Ocean and stretch up to Canada. The term includes the U.S...
. While crossing Bermuda, an extratropical low pressure detached from the frontal system. By November 7, deep convection
Atmospheric convection
Atmospheric convection is the result of a parcel-environment instability, or temperature difference, layer in the atmosphere. Different lapse rates within dry and moist air lead to instability. Mixing of air during the day which expands the height of the planetary boundary layer leads to...
began to consolidate around the surface center of the low pressure area. The system continued to organized, and by early on November 8, satellite imagery indicated that the low-level circulation was becoming well-defined and gale
Gale
A gale is a very strong wind. There are conflicting definitions of how strong a wind must be to be considered a gale. The U.S. government's National Weather Service defines a gale as 34–47 knots of sustained surface winds. Forecasters typically issue gale warnings when winds of this strength are...
-force winds were being reported near the center. By 0900 UTC
Coordinated Universal Time
Coordinated Universal Time is the primary time standard by which the world regulates clocks and time. It is one of several closely related successors to Greenwich Mean Time. Computer servers, online services and other entities that rely on having a universally accepted time use UTC for that purpose...
, 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...
initiated advisories on Subtropical Storm Sean, with initial wind speeds of 45 mph (75 km/h). After becoming a subtropical cyclone, Sean quickly began to developed curved bands of deep convection. However, Sean had not transitioned into a tropical cyclone since it remained in the vicinity of an upper-level low. Later on November 8, Sean moved away from the upper-level low, and there was evidence of a warm core. As a result, Sean was declared fully tropical at 2100 UTC on that day.
Under light wind shear and marginally warm sea surface temperature
Sea surface temperature
Sea surface temperature is the water temperature close to the oceans surface. The exact meaning of surface varies according to the measurement method used, but it is between and below the sea surface. Air masses in the Earth's atmosphere are highly modified by sea surface temperatures within a...
s, Sean saw little change to its structure and intensity through the early hours of November 9. Over the next few hours, the structure of the storm improved as the convection in the storm became more organized. It was then noted of the possibility of an eye feature
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...
developing, and the National Hurricane Center noted that its official intensity forecast might have been conservative. Shortly thereafter, Sean strengthened slightly and reached a wind speed of 65 mph (100 km/h). Though none of the computer models showed intensification into a hurricane, the National Hurricane Center predicted otherwise, due to light wind shear. Early on November 10, cloud top temperatures near the center of Sean had warmed, and the storm intensity had remained steady. Satellite imagery in the early hours of November 10 indicated that Sean continued to become better organized, noting increased convective banding and several attempts at developing an eye feature. However, subsequent satellite images also showed that the eye remained ragged and that Sean became more compact than on the previous day. By then, the National Hurricane Center no longer predicted Sean would strengthen into a hurricane, though it was mentioned that intensification into a hurricane was still possible. Later on November 10, it was noted that Sean appeared less organized on satellite imagery, though an reconnaissance aircraft
Hurricane Hunters
The Hurricane Hunters are aircraft that fly into tropical cyclones in the North Atlantic Ocean and Northeastern Pacific Ocean for the specific purpose of directly measuring weather data in and around those storms. In the United States, the Air Force, Navy, and NOAA units have all participated in...
indicated that the storm was not weakening. It was at this time that the storm reached its lowest central pressure of 983 mb, which it maintained through the early hours of November 11. As the day continued, wind shear began to increase, and Sean began to weaken. Hours later, the storm was described as a "strongly sheared cyclone," and the storm continued to decrease in intensity and condense in size. At the same time, a cold front
Cold front
A cold front is defined as the leading edge of a cooler mass of air, replacing a warmer mass of air.-Development of cold front:The cooler and denser air wedges under the less-dense warmer air, lifting it...
was approaching from the west, which was associated with a powerful extratropical cyclone
Extratropical cyclone
Extratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are a group of cyclones defined as synoptic scale low pressure weather systems that occur in the middle latitudes of the Earth having neither tropical nor polar characteristics, and are connected with fronts and...
, centered over Newfoundland
Newfoundland
Newfoundland usually refers to either:* Newfoundland, the former name of Newfoundland and Labrador, a Canadian province in the eastern part of Canada* Newfoundland , an island that forms part of the province of Newfoundland and Labrador...
.
By the afternoon hours, it was noted that Sean was losing tropical characteristics, with stable
Convective instability
In meteorology, convective instability or stability of an airmass refers to its ability to resist vertical motion. A stable atmosphere makes vertical movement difficult, and small vertical disturbances dampen out and disappear...
air entering the southern half of the storm. The storm became post-tropical early on November 12 as it merged with the cold front, and was absorbed into the extratropical storm, soon afterwards.
Preparations and impact
Prior to becoming a named system, the non-tropical precursor low brought near-tropical storm force winds and light rainfall to Bermuda for three days. Shortly after becoming subtropical, the Bermuda Weather ServiceBermuda Weather Service
The Bermuda Weather Service is Bermuda's national meteorological service. It provides public, marine, tropical and aviation weather forecasts as well as warnings and climatolological services. The service began operations under contract from the Department of Airport Operations, Ministry of...
issued a tropical storm watch for Bermuda on November 8 at 1200 UTC. Almost 48 hours later, the tropical storm watch was upgraded to a tropical storm warning.
Tropical Storm Sean brought rough seas to the east coast of Florida
Florida
Florida is a state in the southeastern United States, located on the nation's Atlantic and Gulf coasts. It is bordered to the west by the Gulf of Mexico, to the north by Alabama and Georgia and to the east by the Atlantic Ocean. With a population of 18,801,310 as measured by the 2010 census, it...
, which drowned one swimmer in Jensen Beach
Jensen Beach, Florida
Jensen Beach is a census-designated place in Martin County, Florida, United States. The population was 11,100 at the 2000 census. It is part of the Port St...
.
External links
- The National Hurricane CenterNational Hurricane CenterThe 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...
's advisory archive on Tropical Storm Sean