By Kevin Gray
MIAMI (Reuters) - The 2013 Atlantic hurricane season will be "above average" with 18 tropical storms, nine of which will intensify into hurricanes, forecasters at Colorado State University predicted on Wednesday.
Four of the hurricanes will be major with sustained winds of at least 111 miles per hour, the leading U.S. storm research team said.
An average season brings about 12 tropical storms, six hurricanes and two major hurricanes in the Atlantic, Caribbean and Gulf of Mexico, according to CSU. The hurricane season runs from June 1 to November 30.
The prediction for a busier 2013 season was based on two factors, the researchers said. Hurricanes thrive on warm water and the Atlantic Ocean has warmed in recent months.
There is also little expectation of an El Nino effect this summer and fall.
El Nino is a warming of surface waters in the tropical Pacific that occurs every four to 12 years and has far-ranging effects around the globe. The weather phenomenon creates wind shear that makes it harder for storms to develop into hurricanes in the Atlantic-Caribbean basin.
The researchers said there was a 72 percent chance that a major hurricane will hit the U.S. coast this year, compared with a historical average of 52 percent.
There is a 48 percent chance a major hurricane will hit the U.S. East Coast, compared with a historical average of 31 percent, and a 47 percent chance one would hit the Gulf of Mexico coast, compared with an average of 30 percent.
The 2012 hurricane season spawned 19 tropical storms and 10 hurricanes, including Hurricane Sandy, which hit the northeastern United States in October after rampaging through the Caribbean.
Sandy killed more than 200 people and caused more than $71 billion of damage in New York and New Jersey.
(Reporting by Kevin Gray; Editing by Jane Sutton and Eric Beech)