Thursday, April 05, 2012

More on the Tropics for 2012

Following up on Tom's post the other day about tropical season and the potential for an El Niño to develop, yesterday we were treated to the annual pre-release of the Colorado State University Atlantic Hurricane Seasonal Outlook.

Dr.'s Klotzbach and Gray usually release an outlook in December with just some pre-thoughts. Then they release their second forecast in April, followed by their final forecast on June 1st and an update to that August 3rd. The CSU team has basically set the standard for seasonal outlooks for hurricanes. While any outlook 2 months in advance of the start of the season and 5 months ahead of the statistical peak of the season carries a lot of risk, there may be some use or skill with this year's forecast.

As Tom alluded to, with the potential for an El Niño to develop in summer, this may have a mitigating impact on the tropics. The CSU team makes this clear as well. Their forecast? 10 named storms, 4 hurricanes, and 2 major hurricanes. This 10/4/2 slashline is close to the 10/6/2 long-term slashline but falls well short of the 15/8/4 of the "active period" we've been in since 1995. They also indicate that landfall probabilities are down across the board, with no real bend from climatology, which favors places like Florida and the Caribbean.

Colorado State University's April Atlantic
Basin Hurricane Forecast
Credit: Colorado State University

Specifically looking at the US, I did some research awhile back to see how hurricane season behaved in years that saw spring or summer bring developing El Niños. I identified a bunch of years: 1951, 1963, 1968, 1976, 1986, 2004, 2006, and 2009 as possible "analogs" for the tropics. Those years averaged a slashline of 10/6/3...not far off the long term average. I narrowed those down a bit, focusing on years that show the Atlantic Ocean in a "warm phase" or +AMO.

Those years narrowed down to 1951, 1963, 2004, 2006, and 2009. That produced a slashline of 11/6/3...similar to the long term average again. In all of those years, the US was struck a total of 15 times...9 by tropical storms and 6 times by hurricanes (including 2004's Ivan which struck Alabama as a hurricane and then later Louisiana as a tropical storm after looping around). Eight of the 15 storms struck Florida, with two in the Carolinas, two in Alabama, two in Louisiana, and one in Texas. The dispersal is similar to what you'd expect, with Florida at highest risk, and the Carolinas and Gulf at enhanced risk.

While again, I don't put much stock into predicting the tropics in April, I think the CSU team's forecast is as good as any right now, and using climatology as a guide for landfall risk seems to be a fairly good method as well. Obviously, we'll see what happens going forward, and remember, all it takes is one storm to make the year "active" or memorable.