Hurricanes and Tropical Storms - Annual 2011
2011 Season Summary:
The 2011 North Atlantic hurricane season had 19 named storms, seven hurricanes, and three major hurricanes. The 19 named storms tied with 2010, 1995, and 1887 as the third busiest year for tropical cyclones in the basin. The second most active tropical cyclone year for the Atlantic basin was 1933. An average season has 11 named storms, six hurricanes, and two major hurricanes. The seven storms which reached hurricane strength and the three that reached Category 3 status (major hurricane) are both near the average, despite the high number of tropical storms. One hurricane (Irene) and one tropical storm (Lee) made landfall in the U.S. during the 2011 season. Hurricane Irene was the first Hurricane since Ike in 2008 that made landfall in the nation.
The Accumulated Cyclone Energy (ACE) index of tropical cyclone activity also indicated an above-average season. The ACE index is used to calculate the intensity of the hurricane season and is a function of the wind speed of each tropical cyclone. The 2011 Atlantic hurricane season had an approximate ACE value of 125 (x104knots2), which is 20 percent above the 1981-2010 average ACE value of 104 (x104knots2). The highest ACE value on record of 250 (x104knots2) occurred in 2005. Although there were an extremely high number of tropical storms during the year, the near average number of hurricanes and major hurricanes kept the seasonal ACE value relatively low compared to more active years. The first eight storms of the season failed to reach hurricane strength, marking the first time in the Atlantic that this many consecutive storms in a single season failed to reach hurricane strength.
The above-average season was partially attributable to lower-than-average wind shear across the basin and warmer than average sea surface temperatures. The lower than average wind shear is consistent with the presence of La Niña conditions across the equatorial Pacific during most of the hurricane season. Drier-than-average conditions persisted as air from the African Sahara made its way into the middle layer of the atmosphere above the tropical Atlantic for most of the season. This likely inhibited many of the named storms from developing into stronger hurricanes.
Only two of 19 named storms, 11 percent of the storms, made landfall in the contiguous U.S. during the 2011 season, which is below average. Steering currents caused most of the storms that formed over the open waters of the Atlantic to curve out to sea, missing the U.S. as they moved northward and eventually northeastward. The storms that formed across the Caribbean were steered into Mexico and Central America. It should be noted that although only two named storms made landfall in the U.S., four storms — Arlene, Harvey, Nate, and Rina — made landfall across Central America and Mexico causing significant flooding, damage, and loss of life across those regions.
Hurricane Irene was the first hurricane to make landfall in the U.S. since Hurricane Ike hit the Gulf Coast in 2008. Irene formed from a tropical wave moving off the Cape Verde Islands on August 21st, and moved westward across the central North Atlantic. Irene made landfall on the 22nd over Puerto Rico as the storm strengthened into a hurricane. The storm then moved through the Bahamas as it strengthened to Category 3 strength and it curved more northerly in its track. Irene made landfall on the 27th near Cape Lookout, North Carolina as a Category 1 hurricane. The storm then moved back over the coastal Atlantic waters and made a second landfall near Little Egg Inlet, New Jersey, as a strong tropical storm. Irene moved over open water again, and made landfall as a tropical storm near Coney Island, New York on the 28th. Irene was a massive storm, with tropical storm force winds extending outward 300 miles (485 km). The storm was also slow moving as it traversed the Mid-Atlantic. Irene claimed at least 48 lives and caused over 7 billion U.S. dollars in damages in the U.S. and 3.1 billion U.S. dollars of damage in the Caribbean.
East Pacific Basin
2011 Season Summary:
The Eastern Pacific Basin experienced near average tropical cyclone activity during 2011. The basin had 15 names storms, 10 hurricanes, and six major hurricanes. On average the basin experiences 15 named storms, nine hurricanes and four major hurricanes. The ACE value of 121 (x104knots2) was slightly below the 1981-2010 average of 131 (x104knots2). The season got off to a strong start, with several storms developing early in June and July, when ENSO neutral conditions were observed across the equatorial Pacific. La Niña conditions re-developed in August, which is associated with increased wind shear across the eastern North Pacific, limiting tropical cyclone development.
The 2011 Hurricane season got off to an early start with Hurricane Adrian reaching Category 4 status in early June, with winds reaching 140 mph and a minimum central pressure 944 mb. The strongest storm of the season was Hurricane Dora, which reached Category 4 status with winds of 155 mph (just shy of Category 5 status) and a central minimum pressure of 929 mb during mid-July. Hurricanes Eugene, Hillary, and Kenneth all reached Category 4 strength during 2011, none of which made landfall.
The deadliest tropical cyclone of the 2011 East Pacific Hurricane season was Tropical Depression 12-E. The storm formed in mid-October and had tropical characteristics for only two days before making landfall and dropping copious amounts of rain over southwestern Mexico, Guatemala, Nicaragua, El Salvador, Honduras, and Costa Rica. Over a foot of rain was reported across the mountainous regions, causing extensive flooding and landslides. Nearly 40,000 people were directly impacted, and there were 80 confirmed fatalities.
Hurricane Kenneth, which formed in mid-November, was a unique storm, although it never directly impacted land. The hurricane reached Category 4 strength, with winds of 145 mph and a central minimum pressure of 943 mb. Kenneth was the latest-forming storm in the eastern North Pacific east of 140 degrees West since 1987 and the latest-forming storm since 1983. Kenneth was also the strongest storm to ever be observed this late in the season for the basin.