Yesterday, on November 11, 2011, Tropical Storm Sean dissipated in the Atlantic Ocean somewhere northwest of Bermuda. Although the season doesn't technically end until the end of November, and it is possible for a tropical storm or even a hurricane to form well into December, this is probably the end of the 2011 Hurricane Season in the Atlantic. In any case, any storms that form from now on will probably do little to alter the overall picture of the season.
Every hurricane season is alike in some ways, and every hurricane season is different in some ways. 2011 had much that was familiar, but had a few surprises.
The National Oceanic and Atmospheric Administration had a final forecast, a week before the start of hurricane season, predicting 12-18 named storms, 6 to 10 hurricanes, and 3 to 6 major hurricanes. Colorado State University (a major home of hurricane science, somewhat oddly considering Colorado's decidedly non-tropical weather) put the numbers at 16, 9 and 5. These numbers are what would be called a moderately busy season, although in past times they would have been considered very busy. However, since 1995, tropical storm activity in the Atlantic has been active, and the bar has been raised for what is considered a busy season. The predictions of a busy season were born out, although not exactly in the way predicted.
The Atlantic storm season is usually considered to begin on June first, although in this year, like in many years, it was only in late June that the first storm formed. And like in most years, and as could be expected, the early storms were usually weak. But 2011 was an unusual year in just how many weak storms formed before the season's first hurricane, Hurricane Irene, formed. Hurricane Irene formed on August 20th, and between then and late October, the season's six hurricanes would form, interspersed with some other short lived tropical storms. Like most storms that form over the open Atlantic, the season's hurricanes for the most part recurved to the northeast, dissipating in the north Atlantic and mostly causing lots of rain to fall on Canada's Maritime Provinces. The exception to this was Irene, which made landfall in the United States three times, bringing drenching rains (but a much lesser than expected storm surge) to the Mid-Atlantic region.
In many ways, the year had the expected pattern of an Atlantic tropical storm season: a beginning of smaller storms originating in the Caribbean, followed by a peak season full of Cape Verde Storms that originated in waves off of Africa, followed by a return to storms originating in the Caribbean. The season had two major unusual features.
First, there was a great number of tropical storms compared to the number of hurricanes that formed. The predicted number of tropical storms was underestimated, while the predicted number of hurricanes and major hurricanes was overestimated. There is a good possibility that this is an artifact of detection methods, in recent years, satellite monitoring and other technologies have allowed more tropical storms to be detected than was the case in previous years.
The other unusual aspect of the year's hurricane season was the extent to which rainfall, rather than wind or storm surge, was dangerous. One of the year's most destructive storms was Lee, which was a short lived, weak tropical storm that came ashore in Louisiana and quickly dissipated, but the remnants of which moved up the spine of the Appalachians, dropping rain as far away as upstate New York. This was a week after Irene had brought heavy rains to the same region, leading to even more damaging flooding. And this is perhaps the most unexpected thing that happened in 2011: that Vermont should be catastrophically affected by tropical storms, while Florida is untouched.
There is much debate, some of it scientific, and some of it not-so-scientific, about what has been happening in the Atlantic since 1995. The short term movers seem to be El Nino and La Nina, but there could be other things going on, including (obviously) anthropomorphic climate change, of the expected carbon dioxide variety, but also perhaps involving particulate pollution or any number of things. It could be that what we saw this year is a true sign of what is to come: a large number of small, wet and insidiously dangerous storms; and a hurricane season when areas up to the 40th parallel are now in play. Or it could be that this year's storms were just odd points on the bell curve of possibly hurricane seasons.
While there is much scientific debate about what is going on, and there is always the danger of overlearning lessons. Science relies on having as many data points as possible, but when dealing with hurricanes, those data points can cost 10s of billions of dollars. So while we don't yet know exactly what this season portends, it is best to learn sooner rather than later.