* Sandy interacting with polar trough over eastern U.S.
* Similar to ``Perfect Storm'' that formed in 1991
* Large swath of U.S. East Coast potentially in harm's way
(Recasts throughout with new info, details)
NASSAU, Oct 26 (Reuters) - Slow-moving Hurricane Sandy, a late season Atlantic storm unlike anything seen in more than two decades, slogged toward the U.S. East Coast on Friday after killing at least 31 people on a trail of destruction across the Caribbean.
Forecasters said the storm, with an expanding wind field already 550 miles (890 km) wide, had begun merging with a polar air mass over the eastern United States, potentially spawning a ``hybrid'' super storm that could wreak havoc along the U.S. East Coast.
``Its structure is evolving as we speak because it's interacting with this weather feature at higher levels of the atmosphere,'' said Todd Kimberlain, a forecaster at the U.S. National Hurricane Center in Miami.
``The models are suggesting that the storm could actually become better organized or intensify a little bit, not due to the normal processes than we would expect for a tropical cyclone but more related this weather feature,'' Kimberlain said.
Most of Florida was under a tropical storm warning, and watches extended up the U.S. coast through North Carolina. Winds and rains generated by Sandy were being felt in south Florida.
Late Thursday, Sandy weakened to a Category 1 storm as it tore though sparsely populated low-lying southeastern islands in the Bahamas, knocking out power and blowing rooftops off some homes.
One storm-related death was reported in the Bahamas. Police said it was under investigation, but it occurred in Lyford Cay, a wealthy enclave of New Providence island that is home to the likes of actor Sean Connery, hedge fund billionaire Louis Bacon and Canadian fashion mogul Peter Nygard.
Sandy's driving rains and heavy winds were blamed for a number of deaths elsewhere in the Caribbean.
The Cuban government said Sandy killed 11 people when it barreled across the island on Thursday. At least 16 other lives were lost in deeply impoverished Haiti and three people were killed in the neighboring Dominican Republic and Jamaica.
The Haitian dead included a family of five in Grand-Goave, west of the capital Port-au-Prince, killed in a landslide that destroyed their home, authorities said.
The Cuban fatalities were unusual for the communist ruled country that has long prided itself on protecting its people from storms by ordering mass evacuations.
The National Hurricane Center said Sandy was about 460 miles (770 km) south-southeast of Charleston, South Carolina, late Friday morning and packing maximum sustained winds of 80 miles per hour (130 km per hour) on Friday morning.
'A CATASTROPHIC STORM'
Sandy is forecast to remain a Category 1 hurricane as it moves over the Bahama, sending swirling rains and winds across area including Florida.
The Bahamas Electricity Corporation, which supplies power to most of the Bahamas, said Sandy had caused power outages on several islands.
Sandy was expected to move past the Bahamas by Friday evening and churn north toward the U.S. coast. Its forward speed had slowed to just 6 mph (9 kph), however, making its final trek across the central and northwest corner of the Bahamas island chain very slow going.
Forecasters said Sandy was expected to come ashore in the northeastern United States late Monday or early Tuesday, and it could still be packing winds that are at or near hurricane force.
Weather trackers said hard-hit areas, prone to storm surge and coastal or inland flooding, could span anywhere from the Carolinas up to Maine, with New York City and Boston both potentially in harm's way.
``There may be hurricane force winds but it would be more like a winter storm than anything else we're used to seeing,'' said Kimberlain.
He said Sandy, dubbed ``Frankenstorm'' by another government forecast on Thursday, was unique because of its integration with the polar trough over the United States.
``We went through this same sort of thing back about 20 years ago around Halloween in 1991 with the 'Perfect Storm,''' he said.
Amid final preparations for the crucial Nov. 6 presidential election, some forecasters are warning that Sandy could be more destructive than last year's Hurricane Irene, which caused billions of dollars in damage as it battered the U.S. Northeast and New England.
``This will then be a catastrophic storm for the Middle Atlantic and Northeast. It will not be a purely tropical system, with a core of powerful winds near the center, but rather more like a Nor'Easter, with strong winds over a larger area,'' said a forecast report on Friday from AccuWeather.com.
``Damaging winds will affect areas from Virginia up into New York and New England, leading to widespread power outages and property damage,'' it said.
At $4.3 billion in losses, last year's Irene ranks as one of the 10 costliest hurricanes, adjusted for inflation and excluding federally insured damage, according to the Insurance Information Institute, an industry group.
(Additional reporting by Jeff Franks and Nelson Acosta in Havana, Kevin Gray in Miami, Susana Ferreira in Port-au-Prince; Writing by Tom Brown; Editing by Lisa Shumaker and Vicki Allen)