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Posted at 04:01 PM ET, 11/28/2011

2011 Atlantic hurricane season by the numbers: how accurate were forecasts?


Hurricane Irene spins off the North Carolina Outer Banks on August 27, 2011. (NOAA)
Hurricane season in the tropical Atlantic officially ends November 30. After predictions for a busy season, how did it pan out?

NOAA recently released two recaps, noting there were:

* 19 tropical storms, third most on record since 1851, 8 above average (11)
* 7 hurricanes, one above average (6); and three major hurricanes, also one above average (2).
* 120 fatalities and $11 billion in property and infrastructure damage.

NOAA said the storm totals include a post-storm upgrade of tropical storm Nate to hurricane status, and the addition of a short-lived, unnamed tropical storm that formed in early September between Bermuda and Nova Scotia.

2011 hurricane season in 4.5 minutes, courtesy NOAA

While the number of hurricanes was near average, Wunderground.com’s Jeff Masters said an unusually small number of the 19 tropical storms reached hurricane strength - just 37% compared to an average of 55-60%. For this reason (despite the much above average number of named storms), Accumulated Cyclone Energy(ACE), a combined metric for tropical cyclone duration and intensity, was above average but not off the charts. Atmospheric scientist Ryan Maue - who tracks the indicator on his website - reports this year’s ACE in the Atlantic basin was 124 compared to a 1981-2010 average of 104, or about 20% above average.

Of the seven hurricanes, just Irene (on August 28) made landfall, the first hurricane to strike the U.S. coast since Ike in 2008. Irene alone accounted for 55 deaths and $10+ billion in damage according to NOAA. The only other named storm to make landfall in the U.S. was tropical storm Lee on September 4. Lee caused major flooding from northern Virginia into Pennsylvania and New York State. (One other tropical system to affect land, Don, was a tropical depression when it washed ashore in south Texas on July 29.)

No major hurricane (category 3 or higher) struck the U.S. coast extending a historic drought for such storms. Roger Pielke Jr. posted it’s been 2,232 days since a major hurricane made landfall in the U.S. (Wilma in 2005), the longest period on record.

How accurate were pre-season forecasts for Atlantic hurricane activity? In general, pretty accurate. The forecast average across 6 groups was for 15 named storms (compared to 19), 8 hurricanes (compared to 7) and 4 major hurricanes (compared to 3). In short, forecasters slightly underestimated the total number of named storms while slightly overestimating the number/intensity of hurricanes.

By  |  04:01 PM ET, 11/28/2011

Categories:  Latest, Tropical Weather

 
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