By Rene Rodriguez, David J. Neal And Monique O. Madan

The 2018 hurricane season blew into high gear on Sunday.

In its 11 p.m. Sunday advisory, the National Hurricane Center upgraded Tropical Storm Isaac to a Category 1 hurricane, with winds of nearly 75 mph.

Isaac became the fifth hurricane of the 2018 Atlantic Season and the third storm being actively tracked in a busy weather weekend.

But the gravest threat to the U.S. remains Hurricane Florence, which is expected to strengthen considerably by Monday night and remain "an extremely dangerous major hurricane" through Thursday, according to the Hurricane Center.

Florence continued its west-northwest crawl toward the southeastern U.S. coast, packing maximum sustained winds of 90 mph.

The center of the hurricane is predicted to move over the southwestern Atlantic Ocean between Bermuda and the Bahamas by Wednesday and could intensify into a Category 4 storm, with winds reaching 130-156 mph, before it makes U.S. landfall sometime Thursday or early Friday.

Forecasters said it was too soon to determine the exact timing, location and magnitude of storm surges and flooding Florence will cause.

None of the three hurricanes presents an immediate threat to South Florida.

But Hurricane Center officials warned that anyone in coastal and inland locations from South Carolina to the mid-Atlantic region should "closely monitor the progress of Florence, ensure that they have their hurricane plan in place, and follow any advice given by local officials."

At a news conference on Sunday, South Carolina Governor Henry McMaster warned of a "likely strong hit" on the state and urged residents to begin making preparations now.

"Pretend, assume, presume that a major hurricane is going to hit right smack dab in the middle of South Carolina and is going to go way inshore," McMaster said. "So be prepared. Be ready."

North and South Carolina and Virginia have all declared states of emergency in anticipation of Hurricane Florence.

As of Sunday night, hurricane-force winds extended outward up to 25 miles from Florence's center, while tropical storm winds extended up to 125 miles.

Hurricane Isaac, with winds of 75 mph, is located roughly 1,300 miles east of the Windward Islands and is expected to continue moving westward, across the lesser Antilles and into the eastern Caribbean Sea, by Wednesday night or Thursday.

Even farther away is Hurricane Helene and her 85 mph winds, which are 195 miles south southwest of the Cabo Verde Islands off the western coast of Africa. Helene is expected to turn north well ahead of any proximity to the U.S. East Coast and is forecast to start weakening by Tuesday.

The 2018 hurricane season lasts until Nov. 30. Forecasters had predicted between nine and 13 named storms this season, four to seven of them hurricanes. Thus far, nine storms have been named.

The next National Hurricane Center advisory is scheduled for 5 a.m. Monday.

(c)2018 Miami Herald