Forecasters continued to sound warnings Saturday about a very large and dangerous storm that is expected to combine with another weather system and lash a large coastal area from the Carolinas to Southern New England with high winds, heavy rains and extreme flooding and high tides. To prepare, Islanders spent the day stocking up on flashlights, batteries and gasoline, while shipyards removed remaining boats from the water and tree companies made quick work of a flurry of requests to remove troublesome branches. In Oak Bluffs, an unusual object was in place: a Cape Wind site testing rig had been moved into the harbor for safekeeping during the storm; it was one of the visible presences in the harbor. There, and across other Island towns, few but emergency and municipal boats remained.