The 2011 Atlantic hurricane season is tied with 1887, 1995, 2010, and the following 2012 season for the third highest number of named storms since record-keeping began in 1851. The season officially began on June 1 and ended on November 30, dates which conventionally delimit the period during each year in which most tropical cyclones develop in the Atlantic Ocean. However, the first tropical storm of the season, Arlene, did not develop until nearly a month later. The final system, Tropical Storm Sean, dissipated over the open Atlantic on November 11. Despite unfavorable conditions, seven storms reached Category 1 strength and four of those storms reached major hurricane status (Irene, Katia, Ophelia and Rina). Katia and Ophelia reached Category 4 status and tied with strongest winds, yet Ophelia reached overall peak. (Operationally, the year featured eighteen tropical storms; in post-season analysis, the National Hurricane Center (NHC) determined that an unnamed tropical storm developed in late August, north of Bermuda.)