Hurricane Florence intensified into a fierce Category 4 hurricane Monday and doubled in size during the day as it churned toward the Carolina coast.
In a 5 p.m. update, National Hurricane Center forecasters said sustained winds had increased to 140 mph, with the storm likely to make landfall along the North or South Carolina coast by Thursday. Hurricane-force winds doubled over 12 hours, extending 40 miles from the storm’s center by Monday evening. As it nears the U.S. coast, winds speeds could rise and fall as the intense storm undergoes eyewall replacements, broadening the storm and increasing the areas potentially impacted even more.
The storm is also expected to continue intensifying, and could become a Cat 5 before it hits the coast, forecasters said.
“Unfortunately, the models were right. Florence has rapidly intensified into an extremely dangerous hurricane,” they said.
Florence was located about 525 miles south, southeast of Bermuda, moving to the west, northwest at 13 mph.
During the day, the storm began turning to the north, northwest and is expected to speed up over the next two days. By late Wednesday, it should begin to make a turn toward the northwest.
A system developing in the Caribbean may also influence the storm, increasing uncertainty in the track forecast. The latest track takes the storm slightly east, forecasters said, but warned that at four to five days in advance, the margin in error for track forecasts can still be greater than 140 nautical miles.
Florence could trigger heavy flooding as it lingers over areas already drenched by heavy rain, forecasters said. Storm surge watches could be issued for North and South Carolina and Virginia by Tuesday morning. Heavy flooding could extend hundreds of miles inland over the Carolinas and mid-Atlantic, forecasters said.
As it crosses the Atlantic tonight, wave heights up to 35 feet may be near the storm’s center, the NHC said. Swells up to eight feet high could also begin hitting the Atlantic coasts in the Virgin Islands, Puerto Rico and the Turks and Caicos this evening.
Farther west, Isaac also continues to roll west toward the Caribbean, where it’s expected to cross the Lesser Antilles on Thursday. The compact storm, with hurricane winds extending just 10 miles from the center, was located more than a thousand miles east of the Windward Islands Monday evening, with sustained winds of 75 mph.
Because the storm is so small, forecasters are less certain about how intense Isaac may become. It’s expected to strengthen over the next couple of days as it faces little wind shear. But after that, outflows from Florence could weaken it. But small storms tend to grow and shrink more rapidly than large storms. It’s still likely to be near or at hurricane strength when it crosses the islands, forecasters said, with winds around 90 mph.
Behind Isaac, Helene has strengthened with maximum sustained winds estimated to be at 105 mph. Helene is expected to continue strengthening over the next two days as it rolls west and become a major, Cat 3 hurricane over the next 12 to 24 hours, with sustained winds of 115 mph. It’s likely to continue west for the next two days, but then turn to the northwest, parallel and far from the U.S. coast, when Isaac knocks an opening into a steering ridge.
Forecasters are also watching a system in the Northwest Caribbean that could become a tropical depression later this week as it moves across the western Gulf of Mexico. Monday afternoon, they gave the system a 50 percent chance of forming over the next five days.