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The Colorado State University hurricane forecast team today predicted a slightly below-average 2009 Atlantic basin season based on a cooler-than-normal tropical Atlantic and the greater potential for a weak El Nino during the bulk of the hurricane season.
The team now anticipates 11 named storms forming in the Atlantic basin between June 1 and Nov. 30. Five of the storms are predicted to become hurricanes, and of those five, two are expected to develop into major hurricanes (Saffir/Simpson category 3-4-5) with sustained winds of 111 mph or greater.
The scientists reduced their forecast from April’s prediction of 12 named storms, six hurricanes and two major hurricanes. Long-term averages are 9.6 named storms, 5.9 hurricanes and 2.3 major hurricanes per year.
"We believe that there is a slightly greater chance of a weak El Nino developing this summer/fall than there was in early April," said William Gray, who is beginning his 26th year forecasting hurricanes at Colorado State University. "El Nino conditions would likely increase levels of vertical wind shear and decrease Atlantic hurricane activity."
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 hurricane formation.
The team also updated its U.S. landfall probabilities. These probabilities are calculated based upon 20th century landfall statistics and then adjusted by the latest seasonal forecast.
"The probability of a major hurricane making landfall along the U.S. coastline is 48 percent compared with the last-century average of 52 percent," said lead forecaster Phil Klotzbach. The hurricane forecast team’s probabilities for a major hurricane making landfall on various portions of the U.S. coast:
– A 28 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 28 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).
New with this forecast are landfall probabilities for the Caribbean and Central America. This season, the forecast team expects a 39 percent chance of a major hurricane tracking into the Caribbean, which is slightly lower than the long-term average of 42 percent.
Probabilities of tropical storm-force, hurricane-force and major 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 and landmasses in Central America are now available on the Landfall Probability Web site
Currently observed climate factors are similar to conditions that occurred during 1959, 1960, 1965, 2001 and 2002 seasons. The average of these five seasons had slightly below-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 90 percent of the average season. By comparison, 2008 witnessed tropical cyclone activity that was about 160 percent of the average season.
Gray and Klotzbach advise coastal residents not to change their hurricane preparedness measures because of a less active seasonal forecast since major hurricanes can devastate coastal communities in less active seasons.
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 will issue a final seasonal forecast update on Tuesday, August 4.
CSU RESEARCH TEAM
EXTENDED RANGE ATLANTIC BASIN HURRICANE FORECAST FOR 2009
-Released June 2, 2009-
Tropical Cyclone Parameters Extended Range
(1950-2000 Climatology Averages Forecast for 2009
Named Storms (9.6)* 11
Named Storm Days (49.1) 50
Hurricanes (5.9) 5
Hurricane Days (24.5) 20
Intense Hurricanes (2.3) 2
Intense Hurricane Days (5.0) 4
Accumulated Cyclone Energy (96) 85
Net Tropical Cyclone
Activity (100%) 90
* Numbers in ( ) represent average year totals based on 1950-2000 data.