Tropical Storm Sean (2011)

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.

Meteorological history

On November 4, a frontal system moved off the East Coast of the United States.^[1] While crossing Bermuda, an extratropical low pressure detached from the frontal system.^[2] By November 7, deep convection began to consolidate around the surface center of the low pressure area.^[3] 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.^[4] By 0900 UTC, the National Hurricane Center initiated advisories on Subtropical Storm Sean, with initial wind speeds of 45 mph (75 km/h).^[5] 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.^[6] 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.^[7]

Under light wind shear and marginally warm sea surface temperatures,^[7] Sean saw little change to its structure and intensity through the early hours of November 9.^[8] 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.^[9] 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.^[10] Early on November 10, cloud top temperatures near the center of Sean had warmed, and the storm intensity had remained steady.^[11] 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.^[12] 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.^[13] 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,^[14] which it maintained through the early hours of November 11.^[15] As the day continued, wind shear began to increase, and Sean began to weaken.^[16] 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 just east of Newfoundland.^{[17] [18]} By the afternoon hours, it was noted that Sean was losing tropical characteristics, with stable air entering the southern half of the storm.^[19] The storm became post-tropical early on November 12 as it merged with the cold front, and was absorbed by the extratropical storm, soon afterwards.^[20]

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.^[21][22][23] Shortly after becoming subtropical, the Bermuda Weather Service issued a tropical storm watch for Bermuda on November 8 at 1200 UTC.^[24] Almost 48 hours later, the tropical storm watch was upgraded to a tropical storm warning.^[25] On the day before bypassing Bermuda, Sean produced 0.24 inches (6.1 mm) at the L.F. Wade International Airport.^[26] In addition, there were reported of sustained winds of 37 mph (60 km/h) and gusts to 56 mph (90 km/h) on November 11.^[27]

Tropical Storm Sean brought rough seas to the east coast of Florida, which drowned one swimmer in Jensen Beach.^[28]

See also

• 2011 Atlantic hurricane season
• Hurricane Olga (2001)

Reference

External links

• The National Hurricane Center's advisory archive on Tropical Storm Sean