The 2011 Atlantic hurricane season: another strangely active one

Wednesday marks the final day of the 2011 Atlantic hurricane season, and it was another very odd year. The season featured a huge number of named storms--nineteen--tying 2011 with 2010, 1995, and 1887 as the 3rd busiest year for tropical storms. Only 2005 and 1933 had more named storms since record keeping began in 1851. However, 2011 had an unusually low percentage of its named storms reach hurricane strength. The year started out with eight consecutive tropical storms that failed to reach hurricane strength--the first time on record the Atlantic has seen that many storms in row not reach hurricane strength. We had a near-average average number of hurricanes in 2011--seven--meaning that only 37% of this year's named storms made it to hurricane strength. Normally, 55 - 60% of all named storms intensify to hurricane strength in the Atlantic. There were three major hurricanes in 2011, which is one above average, and the total Accumulated Cyclone Energy (ACE)--a measure of the destructive potential of this season's storms--was about 20% above average. The rare combination of near-record ocean temperatures but unusually dry, stable air over the Atlantic is no doubt at least partially responsible for 2011's unusually high count of named storms, but near-average number of hurricanes and ACE. Both 2010 and 2011 had nineteen named storms, making it the second busiest 2-year period in the Atlantic behind 2004 - 2005. Even when one considers that 2 - 4 tropical storms from both 2010 and 2011 would likely have been missed before the advent of satellites, the tropical storm activity of 2010 - 2011 is still very remarkable (in 2011, Tropical Storm Franklin, Tropical Storm Jose, and the unnamed 19th tropical storm of September 1 would probably have been missed before satellite technology came along, since they were all weak, short-lived storms that did not impact land or shipping.)


FIgure 1. Tracks for the Atlantic tropical cyclones of 2011.

Another below-average hurricane season for the U.S.
For the second consecutive year, despite a near-record number of named storms in the Atlantic, the U.S. had far fewer strikes by tropical storms and hurricanes than average. Favorable steering currents steered most of the storms in 2010 and 2011 past Bermuda and out to sea. During 2010, only one tropical storm hit the U.S., despite a season with the 3rd highest number of named storms--nineteen. Only two named storms hit the U.S. in 2011: Tropical Storm Lee, which hit Louisiana with 60 mph winds, and Hurricane Irene, which hit North Carolina on August 27 with 85 mph winds, and made two additional landfalls in New Jersey and New York the next day. Tropical Storm Don hit Texas on July 29 as a tropical depression and did not count as a landfalling named storm, according to post analysis by NHC. Wind shear and dry air from the Texas drought made Don rapidly weaken before landfall on Padre Island National Seashore north of Brownsville. During the 15-year active hurricane period from 1995 - 2009, 33% of all named storms in the Atlantic hit the U.S., and 30% of all Atlantic hurricanes hit the U.S. at hurricane strength. The U.S. averaged seeing six named storms per year, with four of them being hurricanes and two being intense hurricanes. Thus, the landfall of only three named storms in a two-year period is a major departure from what happened the previous fifteen years. The past six years is the first six-year period without a major hurricane strike on the U.S. since 1861 - 1868. The last major hurricane to hit the U.S. was Category 3 Hurricane Wilma of October 2005. One caveat to keep in mind, though: Hurricane Ike and Hurricane Gustav of 2008 both hit the U.S. as strong Category 2 hurricanes, and had central pressures characteristic of Category 3 hurricanes. Had these storms occurred more than 65 years ago, before the Hurricane Hunters, Ike and Gustav would likely have been classified as Category 3 hurricanes at landfall (assuming that few quality wind observations would have been available at landfall, which is usually the case.)


Figure 2. The scene in Nassau in the Bahamas at daybreak on August 25, 2011 during Hurricane Irene. Image credit: Wunderblogger Mike Theiss.


Figure 3. The eye of Hurricane Irene as seen by hurricane hunter and wunderblogger LRandyB on August 24, 2011, when the hurricane was approaching the Bahama Islands.

The strongest, deadliest and longest-lived storms of 2011
The strongest hurricane of 2011 was Hurricane Ophelia, which peaked as a Category 4 hurricane with 140 mph winds and a central pressure of 940 mb on October 2, when it was just northeast of Bermuda. Ophelia hit Southeast Newfoundland as a tropical storm with 70 mph winds on October 3, but caused little damage. The strongest hurricane at landfall was Hurricane Irene, whose 120-mph eyewall winds raked Crooked Island, Long Island, Rum Cay, Cat Island, Eleuthera, and Abaco Island in the Bahamas. Wind gusts as high as 140 mph were reported in the Bahamas.The longest-lived storm of 2011 was Hurricane Phillipe, which lasted 15 days, from September 24 to October 8. The most damaging storm was Hurricane Irene, which caused an estimated $7.2 billion in damage from North Carolina to New England, according to re-insurance broker AON Benfield. Irene was also the deadliest storm of 2011, with 55 deaths in the Caribbean and U.S.



Figure 3. Pre-season Atlantic hurricane season forecasts issued by seven major forecast groups. The average of these forecasts called for 15 named storms, 8 hurricanes, 4 intense hurricanes, and an ACE index 150% of normal. The actual numbers were 19 named storms, 7 hurricanes, 3 intense hurricanes, and an ACE index 120% of normal.

Pre-season hurricane forecasts did a decent job
The pre-season Atlantic hurricane season forecasts issued by seven major forecast groups were generally decent. The average of these forecasts called for 15 named storms, 8 hurricanes, 4 intense hurricanes, and an ACE index 150% of normal. The actual numbers were 19 named storms, 7 hurricanes, 3 intense hurricanes, and an ACE index 120% of normal. Phil Klotzbach and Bill Gray of Colorado State will be releasing their end-of-season verification and summary of the 2011 Atlantic hurricane season on November 30.


Figure 4. Portlight volunteers at work in Pink Hill, North Carolina, after Hurricane Irene.

Portlight disaster relief efforts for 2011
My favorite disaster relief charity, Portlight.org, has posted a summary of their efforts during the hurricane season of 2011. Portlight mobilized in the wake of Hurricane Irene to help out in North Carolina, Delaware, and Maryland on cleanup efforts, food, and supply distribution. Portlight also provided financial assistance to survivors, including a commercial fisherwoman and single mother of two who lost her boat and home in the storm, after having been diagnosed with breast cancer two days before Irene struck. See the portlight blog for the full story; donations are always welcome. Wunderground is proud to be a major sponsor of Portlight again this year.

Next post
On Wednesday, I plan to look at 2011's worst hurricane--Hurricane Irene--and the lesson it should have given us regarding the hurricane vulnerability of New York City.

Jeff Masters