Hurricane Katia
A NASA image showed Tropical Storm Katia moving northwest over the Atlantic Ocean. The storm was upgraded to a Category 1 hurricane on Thursday. Reuters

Hurricane Katia grew Monday night into a major Category 4 storm, the second highest level, as it moved over the Atlantic Ocean, the U.S. National Hurricane Center said.

At 11 a.m. Monday, Katia was a Category 2 hurricane with maximum sustained winds of 110 miles per hour. On Monday night, the sustained winds increased to 135 miles (215 kilometers) per hour. Katia was about 450 miles (725 kilometers) south of Bermuda, moving northwest at 10 miles per hour.

Some fluctuations in strength are expected during the next 24 hours, followed by a slow weakening, the National Hurricane Center said.

Satellite images indicate that Katia has strengthened considerably during the past several hours, Channel 6 News reported, quoting John Cangialosi, a hurricane specialist at the center. Fluctuations in the intensity of Katia are likely during the next 12 to 24 hours, perhaps due to eyewall replacement cycles, as the major hurricane remains in a favorable environment.

As Hurricane Katia strengthened, forecasters warned of dangerous rip currents along the U.S. East Coast and Bermuda.

Florida has no direct threat from Katia but people along the East Coast should beware of a potential from rough ocean conditions and a high rip current risk due to the swells created by Katia, Florida emergency management officials said.

The storm was pushing hurricane-force winds outward up to 60 miles and tropical storm-force winds outward up to 205 mph.

For the next 36 hours, Katia is expected to continue travelling northwestward before turning to the north within 48 to 72 hours.

Hurricane is expected to move between the East Coast and Bermuda before turning east and tracking out to sea by next weekend, forecasters said.

Forecasters expect Katia to slow its progress on late Tuesday or early Wednesday, as it moves into an environment of moderate shear and slightly cooler waters.

Katia's path is similar to Earl, which sent high waves to the East Coast in 2010 but didn't make a direct strike on land.

Katia became the 11th named storm of the Atlantic hurricane season which runs from June 1 to Nov. 30. Irene became the first to reach hurricane strength, before ravaging a path up the East Coast last week, and power remains out in some areas and the cleanup may take weeks, if not months for some.

But even though the U.S. might be spared by Katia, figures released last month from NOAA's Climate Prediction Center suggest an above-normal hurricane season in the Atlantic this year. The outlook calls for 14 to 19 named storms, with seven to 10 becoming hurricanes and three to five expected to become a major hurricane, Category 3 or higher.