Experts Predict 'Average' Hurricane Season

By: Emily Narvaes Wilmsen, CSU Email
By: Emily Narvaes Wilmsen, CSU Email

FORT COLLINS - The Colorado State University forecast team issued a presss release today predicting an average 2009 Atlantic basin hurricane season based on the potential for a weak El Nino event and an observed cooling of tropical Atlantic sea surface temperatures.

The team lowered its forecast from December and now anticipates 12 named storms forming in the Atlantic basin between June 1 and Nov. 30. Six of the storms are predicted to become hurricanes, and of those six, two are expected to develop into intense or major hurricanes (Saffir/Simpson category 3-4-5) with sustained winds of 111 mph or greater.

This forecast has been reduced from the early December prediction of 14 named storms, seven hurricanes and three major hurricanes. Long-term averages are 9.6 named storms, 5.9 hurricanes and 2.3 intense hurricanes per year.

"We expect current weak La Nina conditions to transition to neutral and perhaps weak El Nino conditions by this year's hurricane season. If El Nino conditions develop for this year's hurricane season, it would tend to increase levels of vertical wind shear and decrease levels of Atlantic hurricane activity," said William Gray, who is beginning his 26th year forecasting hurricanes at Colorado State University.

The team has seen anomalous cooling of sea surface temperatures in the tropical Atlantic over the past few months. Cooler waters are associated with dynamic and thermodynamic factors that are less conducive for an active Atlantic hurricane season.

"Based on our latest forecast, the probability of a major hurricane making landfall along the U.S. coastline is 54 percent compared with the last-century average of 52 percent," said lead forecaster Phil Klotzbach of the Colorado State hurricane team. "We are calling for an average hurricane season this year -- about as active as the average of the 1950-2000 seasons."

These factors are similar to conditions that occurred during 1951, 1968, 1976, 1985 and 2001 seasons. The average of these five seasons had about average activity, and Klotzbach and Gray predict the 2009 season will have activity in line with the average of these five years.

The hurricane forecast team predicts tropical cyclone activity in 2009 will be 105 percent of the average season. By comparison, 2008 witnessed tropical cyclone activity that was about 160 percent of the average season.

The hurricane forecast team's probabilities for a major hurricane making landfall on U.S. soil are as follows:

- A 54 percent chance that at least one major hurricane will make landfall on the U.S. coastline in 2009 (the long-term average probability is 52 percent).

- A 32 percent chance that a major hurricane will make landfall on the U.S. East Coast, including the Florida Peninsula (the long-term average is 31 percent).

- A 31 percent chance that a major hurricane will make landfall on the Gulf Coast from the Florida Panhandle west to Brownsville (the long-term average is 30 percent).

The team also predicted average major hurricane landfall risk in the Caribbean.

The hurricane team's forecasts are based on the premise that global oceanic and atmospheric conditions - such as El Nino, sea surface temperatures and sea level pressures - that preceded active or inactive hurricane seasons in the past provide meaningful information about similar trends in future seasons.

The team began using a new early April statistical model last year.

"We have found that using two late-winter predictors and our early December hindcast, we can obtain early April hindcasts that show considerable hindcast skill over the period from 1950-2007," said Klotzbach. "This new forecast model also provided a very accurate prediction for the 2008 hurricane season."

Probabilities of tropical storm-force, hurricane-force and intense hurricane-force winds occurring at specific locations along the U.S. East and Gulf Coasts within a variety of time periods are listed on the forecast team's Landfall Probability Web site. The site provides U.S. landfall probabilities for 11 regions and 205 individual counties along the U.S. coastline from Brownsville, Texas, to Eastport, Maine. The Web site, available to the public at http://www.e-transit.org/hurricane, is the first publicly accessible Internet tool that adjusts landfall probabilities for regions and counties based on the current climate and its projected effects on the upcoming hurricane season. Klotzbach and Gray update the site regularly with assistance from the GeoGraphics Laboratory at Bridgewater State College in Massachusetts. In addition, probabilities for various islands in the Caribbean will be available in the early June update.

The team will issue forecast updates on June 2, August 4, September 2 and October 1. The August, September and October forecasts will include separate forecasts of August-only, September-only and October-only Atlantic basin tropical cyclone activity.


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