In a 2 p.m. advisory, forecasters said sustained winds remained at 60 miles per hour, down from 75 miles per hour Monday.
A tropical storm warning means that winds of 36 miles per hour or higher are possible. "It's just not anywhere near a level of Maria", forecaster Eric Blake said.
"Hurricane Joyce is also in the Atlantic so we need to see how those two systems are going to move around each other because that could have an impact".
Its expected sluggishness is becoming more common, possibly a result of climate change, said National Oceanic and Atmospheric Administration climate scientist and hurricane expert Jim Kossin.
"It's just extremely busy and it's even busier than previous year and all at once", Blake said.
Isha Renta, a meteorologist with the weather service's Baltimore/Washington forecast office, said there could be threats of isolated flooding where persistent storms and rain set up.
Tropical Storm Isaac, which passed between the Caribbean island nations of Dominica and Martinique late Wednesday and early Thursday, is now moving nearly due west across the eastern Caribbean Sea. "It's a nasty interplay that makes it very hard".
"It's possible that the west coast of Wales and Cornwall could get some winds as well, but we will get a clearer picture of what to expect very soon". But forecasters also said the opposite could happen when it brushed up against strong wind shear.
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However, Isaac is still predicted to trigger tropical-storm-force winds and life-threatening flash flooding. Some fluctuation in wind speeds are expected, and weakening forecast Thursday, but it's still expected to be a fierce storm triggering unsafe flooding.
Officials in several states have declared states of emergency, including in the Carolinas, Georgia, Virginia and Maryland, where coastal areas are still recovering from summer storms. Some flooding was forecast early in the day for parts of North Florida, but those forecasts were adjusted later to include high swells and minor coastal flooding at high tide.
Panovich: Right now it looks like landfall is going to occur sometime early Friday morning between, I would say, 4 a.m. and probably about 8 a.m., but it might actually never make it all the way in. Given its expected slow speed once it rolls ashore, they warned the heavy rain and storm could spread across a wide area.
At 11am (1500 GMT), Florence was over the Atlantic Ocean about 145 miles (230km) east-southeast of Wilmington, North Carolina, and moving northwest at 10 mph (17 kph), the NHC said. Those pipelines carry fuel from the Gulf Coast to much of the eastern United States.
As the trough moves east, it will disrupt the winds that steer the hurricane, Stewart said.
When staffers realized a nearby Bojangles would stay open, NBC's Matt Bradley says, they opened back up. "Prepare for long power outages". Hurricane Helene is veering toward Europe.
The impact of Florence will be widespread, with destructive winds, life-threatening storm surge, risky surf, torrential rainfall, flooding and the potential for tornadoes.