Wednesday marked the end of the 2011 hurricane season, and as you can see from the time-lapse animation above, it was a fairly active one. While the season started slow, the storms began to swirl more frequently in July (around the one minute mark in the animation). The final tally: 19 named tropical storms, the third highest number on record. Of those storms, 7 developed into hurricanes, including 3 major hurricanes.
That's a pretty close match to the season forecast issued by the National Oceanic and Atmospheric Administration (NOAA). As I reported when the hurricane season began on 1 June the forecast called for 12 to 18 named storms, 6 to 10 hurricanes, and 3 to 6 biggies. Researchers at NOAA's Climate Prediction Center make the forecasts by feeding satellite data into sophisticated climate models that run on a government supercomputer.
So why did the tropical storm number come in at the high end, while the hurricane and major hurricane numbers came in on the low end? Jae Schemm, who oversees the computer models, explains that there were several hard-to-predict factors. Tropical storms strengthen by drawing energy and moisture from the warm ocean waters they travel over, and in 2011 sea surface temperatures were warmer than average, but not by much. During the 2010 season ocean temperatures in the Caribbean reached a record high (pdf), and the 2010 season had the 12 hurricanes to prove it.
Also, says Schemm, there's the matter of where the storms formed in 2011. "Quite a number of tropical storms formed slightly north of the main development region," she says. That main region extends from the west coast of Africa to the Caribbean between 10 and 20 degrees north latitude--in the image below, it's the rectangular box--where conditions are most favorable for storms to develop and turn into hurricanes. But many of 2011's storms were outside the box.
The area north of the main development region also had stronger than average wind shear this year, says Schemm. "That suppressed the strengthening of tropical storms in that area, so they didn’t quite form into full hurricane strength," she says. The image above was produced in May, when the 2011 hurricane forecast was issued, and as you can see the forecasters expected reduced wind shear throughout much of the hurricane zone.
Hey, it's tough to predict the weather. But even though 2011 didn't produce many hurricanes, the ones that did come along packed a wallop. Hurricane Irene, which barreled through the Caribbean in late August before taking a right and plowing up the east coast of the United States, caused 43 deaths and more than $7.3 billion in damage, according to NOAA.