A week after AccuWeather.com posted its spring prognostications for the 2011 Atlantic hurricane season, it's Colorado State University's turn. The mountain folks who never experience a hurricane are once again predicting an "above-average" storm season.
That's due in large measure to the fact that we are in a multi-decadal cycle that began in 1995 during which we should expect storm totals above the long-term (1950-2000) average in most years. The 16-year period between 1979 and 1994 saw 25 major (Cat. 3, 4 or 5) hurricanes. The 16 years since have witnessed 61.
The CSU scientists expect the cycle to continue for another 10 to 15 years before switching back to a long, less active phase.
The CSU forecast is also very close to the one issued by AccuWeather.com a week ago, except for a slightly more aggressive prediction on the number of "major" storms - Cat. 3 and above. Here's how the two predictions out so far compare:
AccuWeather.com, March 30: Named storms: 15. Hurricanes: 8. Major: 3
Colorado State, April 6: Named storms: 16. Hurricanes: 9. Major: 5
1950-2000 average: Named storms: 9.6. Hurricanes: 5.9. Major: 2.3
The new CSU forecast includes a reduction, by one named storm, in the numbers released in the school's December forecast.
The CSU team led by Phil Klotzbach, of the CSU Tropical Meteorology Project, points to several factors contributing to their calculations. "We expect that anomalously warm tropical Atlantic sea surface temperatures, combined with neutral tropical Pacific sea surface temperatures will contribute to an active season," he said.
The tropical Pacific is important because El Nino (above-average sea surface temperatures in the eastern and central tropical Pacific) tends to suppress hurricane development in the Atlantic, while La Nina or neutral temperature conditions tend to allow storm development. After a La Nina winter in 2010-2011, the sea surface temperatures in the Pacific have been warming up and are expected to be neutral this summer.
The Colorado team also looks at oceanic and atmospheric conditions in the Altantic and makes comparisons with those that existed during the summer in 29 prior years. They consider how those storm seasons turned out and then develop their "best estimate" of how the next season will look.
The closest analogs for the 2011 setup were found to be 1955, 1996, 2006 and 2008. All except 2006 had neutral or La Nina conditions, and all but 2006 were "very active" seasons.
Just where this year's storms can be expected to make landfall is impossible to predict with any precision. But the coastal regions of the United States have been extraordinarily lucky in recent years, said longtime hurricane forecaster William Gray, the other key member of the CSU team.
"Except for the very destructive hurricane seasons of 2004-2005, United States coastal residents have experienced no other major landfalling hurricanes since 1999. This recent 9 of 11-year period without any major landfall events should not be expected to continue," he said.
The team predicts a 72 percent chance that at least one major hurricane will make landfall in the U.S. The long-term average probability is 52 percent. The landfall chances for the East Coast, from Florida north, is put at 48 percent. The long-term average probability is 31 percent.
Last year in April, the CSU team predicted 15 named storms, 8 hurricanes and 4 major hurricanes. The actual count was 19 named storms (the third-most active season on record), 12 hurricanes, and 5 major hurricanes. No hurricanes made landfall in the U.S.