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Fewer Atlantic Hurricanes Forecast by CSU

Colorado State University (CSU) last week lowered its Atlantic hurricane forecast for 2009 to 12 named storms, with at least half of them likely to become hurricanes. Two of the storms are expected to develop into intense or major hurricanes (Saffir/Simpson Category 3-4-5) with sustained winds of 111 mph or more.

In a preliminary forecast in December, CSU's William Gray and Philip Klotzbach had estimated there would be 14 named storms between June 1 and Nov. 30 (see NGI, Dec. 15, 2008). A year ago the forecasters predicted that 15 Atlantic storms would form; they raised that outlook to 17 last August (see NGI, Aug. 11, 2008). The 2008 Atlantic hurricane season spawned 16 named storms and was one of the most destructive years on record from a damage perspective, according to CSU (see NGI, Nov. 24, 2008).

"We expect current weak La Nina conditions to transition to neutral and perhaps weak El Nino conditions by this year's hurricane season," said Gray, who is beginning his 26th year at CSU forecasting hurricanes. "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."

The team said it has seen anomalous cooling of sea surface temperatures in the tropical Atlantic over the past few months. Cooler waters are considered 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% compared with the last-century average of 52%," said Klotzbach. "We are calling for an average hurricane season this year -- about as active as the average of the 1950-2000 seasons."

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

Tropical cyclone activity in 2009 will be 105% of the average season, said the CSU team. By comparison, 2008 witnessed tropical cyclone activity that was about 160% of the average season.

CSU set these probabilities that a major hurricane would make landfall on U.S. soil:

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

CSU has scheduled tropical storm updates for June 2, Aug. 4, Sept. 2 and Oct. 1.

The latest CSU forecast is similar to two recent forecasts by AccuWeather.com and WSI Corp. AccuWeather.com's Chief Long Range Forecaster Joe Bastardi last month called for 13 named storms, including two intense hurricanes (see NGI, March 23). WSI's 2009 Atlantic forecast issued in December also predicted 13 named storms, three of them intense (see NGI, Jan. 5). WSI plans to update its tropical season forecast on April 22.

According to the National Oceanic and Atmospheric Administration (NOAA), an average Atlantic hurricane season has 11 named storms, which includes two major hurricanes. NOAA said the Atlantic's tropical storm development is increasing. Research indicates that the ocean has periods of 20-30 years when there are more storms than "normal." The current cycle began in 1995, the agency noted.

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