Hurricane Florence became better organized as it continued to grow in size Tuesday, on track to produce dangerous flooding when it slams the coast of the Carolinas later this week.
In a 5 p.m. advisory, forecasters at the National Hurricane Center said sustained winds had increased to 140 mph. The Category 4 storm is expected to continue strengthening Tuesday night and Wednesday, although it could again weaken slightly on Thursday. Forecasters also increased the projected height of powerful storm surges, with water now likely to be six to 13 feet above ground between Cape Fear and Cape Lookout. Between 15 to 25 inches of rain could accumulate over the Carolinas and parts of Virginia, they said.
Florence was located about 785 miles east, southeast of Cape Fear moving at 17 mph Tuesday evening. The storm’s hurricane-strength wind field, which grew another 20 miles Tuesday, extends 60 miles from the center. Tropical storm force winds span more than 300 miles.
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While Florida will be spared Florence’s high winds and heavy rain, deep swells and dangerous rip currents will likely hit the state’s east coast, beginning Wednesday in South Florida through the end of the week.
Throughout Tuesday, emergency managers repeatedly warned residents in the storm’s path to take Florence seriously and ordered mandatory evacuations for more than a million people. Storm surge and hurricane warnings were issued at 5 p.m. for most of the Carolina coast.
Only five major hurricanes have hit North Carolina since records began in 1851, raising fears that residents will be ill-prepared. That’s compared to 36 strikes by major hurricanes in Florida, where more than 6.5 million were ordered to evacuate before Irma struck last year.
“This is typically because storms hitting North Carolina encounter cooler waters as they approach the coast relatively speaking compared with those hitting Florida,” Colorado State University meteorologist Phil Klotzbach said in an email. “Climatologically, vertical shear gets stronger as you head north out of the deep tropics, so these storms often get sheared somewhat and weaken before they hit the coast.”
About 20 percent of all hurricanes making landfall in North Carolina have been at major hurricane strength, he said. About a third are in Florida.
“While North Carolina is hit fairly often by hurricanes, the issue with Florence is that not many hurricanes that make landfall are as strong as it is likely to be,” he added.
Dangerous flash floods and river flooding throughout the Carolinas and mid-Atlantic states will remain a risk through the end of the week and early next week, forecasters said.
As the storm moves toward the coast, it is being steered by atmospheric flows moving across the U.S. colliding with a high pressure ridge northwest of Bermuda. The blocking ridge is expected to keep Florence moving on a west-northwest track over the next two days. However, uncertainty over the storm’s track increased Tuesday evening when track models predicted those steering currents would collapse at 72 hours.
By Thursday, when it nears the coast, Florence is expected to slow considerably, worsening the threat of dangerous flooding. Once it crosses the coast, forecasters say Florence should quickly weaken to a tropical depression.
Farther west and on course to strike the Lesser Antilles, Isaac remained a tropical storm moving west toward the Caribbean Tuesday. At 5 p.m., forecasters said sustained winds remained at 70 mph as the storm chugged west at 17 mph. It was located about 670 miles east of the Lesser Antilles and is expected to be at or near hurricane strength when it crosses the islands Thursday. A three- to five-foot storm surge could sweep across parts of the islands along with three to five inches of rain.
On Tuesday afternoon, hurricane watches were issued for Guadeloupe, Martinique and Dominica near the center of the island chain. A tropical storm watch was issued for the northern islands.
Forecasting Isaac’s intensity has been complicated by its changing size. Overnight, the compact storm grew, with tropical storm force winds extending 105 miles from its center. It encountered warm waters and low wind shear, which should have allowed it to strengthen, which it did not. Those conditions should remain, so forecasters warned Isaac could still increase wind speed over the next day. However, they say it no longer looks like the storm will become a major hurricane as models suggested earlier and should weaken as it moves into the Caribbean.
Helene also continued churning westward near the Cape Verde Islands, but is not expected to threaten the U.S. coast. Sustained winds reached 105 mph on Tuesday evening as the storm moved northwest at 12 mph. It’s expected to continue moving west Wednesday, then speed up and make a sharp turn to the northeast. It should slow to a tropical storm on Thursday, forecasters said.
Forecasters are also watching a disturbance nearing the Gulf of Mexico that could bring heavy rainfall and gusty winds to Cuba Tuesday. On Tuesday afternoon, satellite imagery showed the system had become better organized during the day. It’s expected to continue strengthening as it crosses the Gulf and could become a tropical depression Thursday as it heads toward the Texas and Louisiana coasts. An Air Force plan may be sent to investigate it on Wednesday.
The mix of storms arrives just as the season, which ends Nov. 30, hits its historic peak. The first 10 days of September have generally produced the highest number of storms as seasonal Saharan dust dies down and conditions become ripe off the tropical Atlantic. It produced Irma and Maria, which dealt a lethal blow to Puerto Rico a year ago next week.