Hurricane researcher William Gray lowered his 2007 forecast slightly Friday, citing cooler water in the Pacific and more atmospheric dust from Africa in predicting 15 named storms and eight hurricanes off the East and Gulf coasts of the U.S.
Gray's forecast also calls for four of the hurricanes to be intense.
Gray has been forecasting hurricanes for more than two decades, and his predictions are watched closely by emergency responders and others in coastal areas.
On May 31, at the outset of hurricane season, Gray had called for 17 named storms and nine hurricanes, five of them intense.
Despite the reduction, Gray said, the indicators tracked by his team at Colorado State University call for 60 percent more storm activity than the long-term average.
The June 1-Nov. 30 Atlantic hurricane season averages 9.6 named storms, 5.9 hurricanes and 2.3 intense hurricanes per year. "Nobody knows for sure, but if the future is like the past, there are these precursor signals out there in the atmosphere on a global scale" that indicate an active season, he said.
Gray's new forecast calls for three named storms and two hurricanes, one intense, in August; five named storms and four hurricanes, two intense, in September; and five named storms and two hurricanes, one intense, in October and November combined. Gray's forecast of 15 named storms includes two that have already occurred this season.
The latest forecast put the chances of an intense hurricane hitting the U.S. coastline at 68 percent for the rest of this season.
Surface temperatures in the Pacific, and Saharan Desert dust blocking solar energy from warming the Atlantic, are part of a complex web of factors in hurricane formation, Gray said. Others include atmospheric pressure and Atlantic water temperatures that are affected by circulation patterns, he said.
A study issued this week said the number of tropical storms developing in the Atlantic each year has more than doubled over the past century and coincided with rising sea-surface temperatures.
The study, published online by Philosophical Transactions of the Royal Society of London, said the increase in sea temperature was largely the byproduct of human-induced climate warming.
Gray, whose views on global warming are controversial among some climate scientists, disputed that conclusion. He said changes in the number and intensity of hurricanes are cyclical and not caused by human influence.
"We don't attribute this to human-induced global warming," he said. "These are just natural changes."
There were 10 named Atlantic storms last year and five hurricanes, two of them major. None of the hurricanes hit the U.S. coast.
Chances of an intense hurricane hitting the East Coast, including the Florida Peninsula, this season are 43 percent, Gray said. For the Gulf Coast from the Florida Panhandle to Brownsville, Texas, the probability is 44 percent.
The forecast also called for above-average risk of a major hurricane making landfall in the Caribbean.
The devastating 2005 season set a record with 28 named storms, 15 of them hurricanes. Four hurricanes hit the U.S., including Katrina, which devastated parts of the Gulf Coast.