By Dinah Voyles Pulver

Hurricane Florence exploded in size and strength Monday, putting the coast of the Southeastern United States on high alert for potentially deadly storm surge, devastating winds and intense, flooding rainfall by Thursday.

Florence doubled the size of its hurricane-force wind field in 12 hours on Monday and increased its maximum sustained winds from 105 mph to 140 mph, making it an exceptionally dangerous Category 4 hurricane. And, the National Hurricane Center warned the storm could continue to strengthen, possibly building to 155 mph by Tuesday.

But relief washed over Volusia and Flagler counties as the local forecast improved, without even tropical storm-force winds expected along the coast. Still, forecasters warned waves of up to 9 feet, life-threatening rip currents, and possible erosion could occur on Wednesday and Thursday as Florence passes offshore on its way north.

Hurricane Center specialists were watching three hurricanes and two other systems with a potential to develop into storms, but it was Florence -- forecast to make a landfall along the North Carolina coast on Thursday -- that riveted the attention of residents and officials in five states from Georgia to Maryland. With the storm's exact track by no means certain, evacuations were ordered in North and South Carolina and Virginia as officials prepared for the rapidly intensifying storm.

At 11 p.m., Hurricane Florence was 1085 miles east-southeast of Cape Fear, North Carolina, with maximum sustained winds of 140 mph, moving west-northwest at 13 mph.

The storm began its forecast turn to the west-northwest earlier in the day. Its central barometric pressure plummeted to 939 millibars as a well-defined eye formed and a wall of convective storms built up around the center.

Florence is forecast to be in a low wind shear environment and even warmer ocean waters on Tuesday, which would allow it to continue strengthening, wrote Eric Blake, one of the Hurricane Center's hurricane specialists in a discussion Monday afternoon. Even though fluctuations in intensity are possible in massive storms, the center had "high confidence that Florence will be a large and extremely dangerous hurricane, regardless of its exact intensity."

In North Carolina, where Florence is forecast to make landfall sometime Thursday as a Category 4 storm with sustained winds of 140 mph, Gov. Roy Cooper said the state is "bracing for a hard hit."

Even with a storm packing the most intense winds the state has seen in years, the bigger threat from Florence is expected to be water, from storm surge and rainfall. After it strikes the coast, Florence is forecast to slow down and linger, becoming a "prolonged and exceptionally heavy rainfall event," dumping intense rainfall which could extend inland and northward across the mid-Atlantic for "hundreds of miles."

"It's not just the coast," said Hurricane Center Director Ken Graham.

Locally, after a rough day on Volusia County beaches Sunday, with a drowning and numerous rescues, crowds were considerably lighter Monday, said Capt. Andrew Ethridge with Beach Safety and Ocean Rescue.

The Weather Service has forecast a high risk of "life-threatening rip currents" along Volusia and Flagler county beaches this week.

"We're looking at a forecast for 6- to 9-foot surf on Wednesday, which is considerable for our area," Ethridge said. With the expected high tides, the "beach may be closed for driving" much of the time.

Area surfers, meanwhile, were making the most of Florence's early swells.

"The last couple of days the ocean have looked like a surfer magazine," Ethridge said. "The waves have been beautiful and the weather has been perfect. Surfers are loving it, but its' dangerous for our swimmers."

In Flagler County, emergency management chief Jonathan Lord said they're "cautiously optimistic."

"We're always concerned because storms do things and don't always follow rules," Lord said. However, tidal flooding in low-lying areas from the seasonal high tides unrelated to Florence are expected to be a problem this week.

Elsewhere, the Hurricane Center watched Hurricane Isaac and Hurricane Helene. Isaac had maximum sustained winds of 75 mph and was moving west about 1,000 miles east of the Lesser Antilles. It's forecast to cross the Antilles on Thursday and then weaken in the Caribbean.

An unexpected area of storms and clouds in the northwestern Caribbean could strengthen as it moves into the Gulf of Mexico later this week, the Hurricane Center said, putting coastal residents of northeastern Mexico, southern Texas and Louisiana on alert.

Hurricane Helene, meanwhile, spun harmlessly northwest in the Atlantic, far to the east with maximum sustained winds of 105 mph.

Ahead of Florence, governors in South Carolina, North Carolina, Virginia and Maryland prepared for "catastrophic" flooding. Experts have warned for years of the danger hurricanes pose to a region stretching from Virginia Beach to Charleston, South Carolina, where the land is sinking and the ocean is rising at some of the highest rates on the East Coast.

In North Carolina, emergency officials warned the entire state to be "alert and ready," not just for a coastal event, but also for flooding and landslides. On the Outer Banks, Dare County issued a mandatory evacuation for Hatteras Island starting Monday at noon, and a mandatory evacuation for the remainder of the county starting Tuesday at noon.

In South Carolina, Gov. Henry McMaster ordered an evacuation beginning at noon Tuesday for an estimated 1 million residents in the eight counties along the coast. McMaster said storm surge could reach as high as 10 feet. To clear an evacuation route, the governor ordered a rare reversal of traffic lanes on eastbound Interstate 26 and U.S. 501 to allow residents to flee the coast.

In Virginia, Gov. Ralph Northam ordered a mandatory evacuation in low-lying areas of Hampton Roads and the Eastern Shore, affecting an estimated 245,000 residents. Northam urged all Virginia residents to prepare for the storm, which he says will affect the entire state. The U.S. Navy was sending nearly 30 ships out to sea to ride out the storm.

Two airlines -- American and Southwest -- began allowing customers to change their flight plans for the week if their travel was in Florence's potential path.

Meanwhile, in Hawaii, Hurricane Olivia is forecast to impact much of the state with strong winds, heavy rainfall and high surf. Rainfall is forecast at between 10 to 15 inches, but as high as 20 inches in isolated areas, the National Weather Service said.

Swells and surf generated by Hurricane Olivia will continue to build slowly and reach damaging levels on some east-facing shores Tuesday and Wednesday. Surf heights along east facing shores are expected to reach up to 20 feet on Maui and the Big Island, and 10 to 15 feet on other islands. That's forecast to result in significant beach erosion and overwash onto vulnerable coastal roadways.

(c)2018 The News-Journal, Daytona Beach, Fla.