The team now anticipates 10 named storms forming in the Atlantic basin between June 1 and Nov. 30. Four of the storms are predicted to become hurricanes, and of those four, 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 June's prediction of 11 named storms, five hurricanes, and two major hurricanes. Long-term averages are 9.6 named storms, 5.9 hurricanes, and 2.3 major hurricanes per year.
“We have witnessed the development of an El Nino event over the past couple of months,” said William Gray, who is in his 26th year of forecasting hurricanes. “These conditions are expected to intensify to a moderate El Nino over the next few months. El Nino events tend to be associated with increased levels of vertical wind shear and decreased levels of Atlantic hurricane activity.”
“Tropical Atlantic sea surface temperatures anomalies have warmed somewhat since our early June prediction and surface pressures have fallen somewhat,” said Phil Klotzbach, the lead author of the forecasts. “But the negative influences of El Nino-induced strong Caribbean Basin and Main Development Region vertical wind shear typically dominate over surface pressure and sea surface temperature in the tropical Atlantic.”
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 46 percent compared with the last-century average of 52 percent.
The hurricane forecast team's probabilities for a major hurricane making landfall on various portions of the U.S. coast:
* A 27 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 26 percent chance that a major hurricane will make landfall on the Gulf Coast from the Florida Panhandle west to Brownsville, Texas (the long-term average is 30 percent).
New with this year’s forecasts are probabilities that a major hurricane will make landfall in the Caribbean and Central America. The team lowered that probability to 37 percent (the long-term average is 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 website. 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. Probabilities for each coastal state are now also available.
The website is the first publicly accessible Internet tool that adjusts landfall probabilities for regions, states, 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 website.
Currently observed climate factors are similar to conditions that occurred during 1957, 1963, 1965, and 2002 seasons. The average of these four 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 85 percent of the average season. By comparison, 2008 witnessed tropical cyclone activity that was about 160 percent of the average season.
Still, despite the reduced forecast, Gray and Klotzbach advise coastal residents not to change their hurricane preparedness measures 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.