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* El Nino still expected but not until late in season
* Warm sea surface temperatures seen aiding storm formation
* Most active hurricane months are August through October (Adds quotes, detail, byline)
By Jane Sutton
MIAMI, Aug 9 (Reuters) - The U.S. weather agency NOAA predicted a slightly more active 2012 Atlantic hurricane season on Thursday, saying warming seas and the late arrival of El Nino would bring near-normal to above-normal storm activity.
Forecasters expect the June-through-November season will bring 12 to 17 tropical storms, with five to eight of those becoming hurricanes and two to three strengthening into major hurricanes.
That was a slight increase from the May forecast, when the agency predicted there would be nine to 15 tropical storms, with four to eight becoming hurricanes and one to three strengthening into major hurricanes. Major hurricanes have sustained winds of 111 miles per hour (178 km per hour) or higher and can cause devastating damage.
An average year brings about 12 tropical storms with six hurricanes and three major hurricanes.
The National Oceanic and Atmospheric Administration forecasters boosted their outlook in part because the season got off to a strong and early start.
Two tropical storms, Alberto and Beryl, formed in May before the season officially began on June 1. There have now been six tropical storms, two of which strengthened into hurricanes, and the season is just entering what is traditionally the most active period.
Sea-surface temperatures are higher than usual in the Atlantic region, which contributes to hurricane formation.
Although the hurricane-squelching El Nino pattern is still expected, it has not appeared yet. El Nino is a periodic warming of the tropical Pacific and brings shearing winds that hamper storm formation in the Atlantic.
The forecasters said it would likely form in August or September and that it would take a few weeks after that for its impact to reach the Atlantic.
"We don't expect El Nino's influence until later in the season," said Gerry Bell, NOAA's lead hurricane season forecaster.
El Nino is less welcome in other parts of the globe because it tends to bring heavy rain to Pacific Islands and the west coast of Central America, and crop-killing drought to Australia, Indonesia, the Philippines, Africa and India.
The U.S. Census Bureau estimates that 37.3 million people live along the Atlantic coast from Texas to North Carolina, the area most at risk from hurricanes.
The forecasters warned residents outside the hurricane belt to prepare as well. Inland flooding is the most dangerous aspect of a hurricane, as Hurricane Irene showed last year, said Laura Furgione, acting director of NOAA's National Weather Service.
Irene hit North Carolina and then chugged north along the U.S. Atlantic coast through New Jersey, New York, Vermont and New Hampshire, causing severe flooding in areas that rarely get hurricanes.
"In the last 30 years, inland flooding has caused more than half the deaths associated with tropical cyclones in the United States," Furgione said.
Editing by Vicki Allen