Monday, October 26, 2009

Hurricane Home Stretch

The World Series is upon us, and the match is now set (Yankees-Phillies, folks). With World Series time, I think about Halloween, cool-to-cold nights, and the end of the Atlantic Hurricane Season. We have about a month left of hurricane season (it officially ends November 30), and here's what typically happens during the last month of the season:
Not much activity, right? This map above shows the prevailing track of any storms that do form in November, and also indicates that the probability (compared to the height of the season) is less likely. Here's what September's climatology looks like, so you get the perspective:
So far this 2009 season, we've had eight named storms in the Atlantic Basin, two of which became hurricanes (Bill and Fred, both major hurricanes). An "average" season has about 10 named storms, with six becoming hurricanes, and 2-3 of those becoming major (Category 3 or stronger) hurricanes. So you can see by the math, that this season has been a little below average for total storms, but still produced two major hurricanes. There's still one month left before we close the book on this year's hurricane season, but this is typically when tropical activity starts winding down.
So why the below-average season? There is some research linking El Nino-La Nina with tropical activity. During a La Nina year, Atlantic cyclone development may be more likely, while during an El Nino year (like this year), less likely. The reverse happens for the Pacific Basin. So this year, often when it appeared that a system might organize in the Atlantic enough to strengthen to hurricane status, wind shear would disrupt the organization process. Could El Nino be to blame? It's certainly possible.

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