Hurricane Sandy pounded the Bahamas with battering winds and rain on Friday, sweeping over the island chain after killing 21 people across the Caribbean and posing a menacing threat to the US East Coast.

Forecasters warned the late-season storm is expected to combine with a polar air mass near the United States next week, potentially producing a highly unusual and potent storm that could wreak havoc along the US East Coast. Late on 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 off rooftops of some homes.

Sandy’s torrential rains and heavy winds were blamed for the deaths of 21 people. The Cuban government said on Thursday night that 11 people died when the storm barreled across the island, most killed by falling trees or in building collapses in Santiago de Cuba province and neighboring Guantanamo province. Haiti’s civil protection office said nine people died even though the country did not suffer a direct hit from Sandy, and one person was killed by falling rocks. in Jamaica when the storm struck there on Wednesday.

The Cuban deaths were an unusually high number for the communist island which prides itself on protecting its people from storms by ordering mass evacuations. Winds and rains generated by Sandy were also being felt in south Florida. Early Friday, the US National Hurricane Center said Sandy was about 55 miles (90 km) southeast of Great Abaco Island in the Bahamas.