Hurricane Irene intensified into a Category 3 storm Wednesday, threatening much of the United States' East Coast in the coming days.

The storm had maximum sustained winds of 115 mph, and is expected to strengthen further "in the next day or so," the National Hurricane Center said Wednesday morning.

"This is a huge storm," said CNN Meteorologist Jacqui Jeras. "The cloud field is more than 800 miles across. The tropical storm force winds extend out 200 miles from the center."

The strengthening, which officially made Irene a major hurricane, came as the storm continued to pound the southeastern Bahamas and the Turks and Caicos Islands.

In Nassau, many people had evacuated by Wednesday morning. Shopkeepers were boarding up their storefronts, and residents prepared to hunker down for the storm's arrival.

At about 8 a.m. ET, the Hurricane Center said Irene was about 55 miles southeast of Acklins island in the Bahamas, and about 335 miles southeast of Nassau.

• Read More