MIAMI - Tropical Storm Katia formed over the open Atlantic Ocean on Tuesday, becoming the 11th named storm of the 2011 hurricane season, US forecasters said.The US National Hurricane Center said Katia was about 535 miles (855 km) southwest of the southernmost Cape Verde Islands and was moving quickly west northwestward across the Atlantic.It was too soon to gauge the storm's potential threat to the US East Coast or energy interests in the Gulf of Mexico with any confidence.But Katia was packing top sustained winds of about 40 miles (65 km) per hour and was forecast to become a powerful Category 2 hurricane by the weekend, the Miami-based hurricane center said.