Meanwhile, Hurricane Florence was 580 miles south, south east of Bermuda with winds of 115 mph and strengthening into a major storm.
By Sunday, Isaac was still expected to be a tropical storm as it continues straight west across the Caribbean south of Jamaica. Moving west-northwest near 17 miles per hour with a turn to the northwest expected on Wednesday, Helene is not expected to pose any threat to land.
Florence is expected to be an extremely unsafe major hurricane through Thursday night.
That was the nation's most expensive hurricane, although that record fell three years later when Hurricane Andrew struck South Florida.
The center of Florence was located about 1 100km southeast of Bermuda, the NHC in its 03:00 GMT on Monday advisory.
Hurricane Florence's size is "staggering", National Hurricane Center Director Ken Graham warned.
The storm may sit over parts of the mid-Atlantic for days, like Harvey did in Texas past year, bringing major flooding to an area that has already seen above-average rainfall in recent months. The state's emergency management agency said it's "preparing for the possibility of a large-scale disaster".
Florence is one of three hurricanes in the Atlantic Ocean, along with Helene and Isaac.
Dominica, still recovering from the battering it took from Hurricane Maria last September, is preparing for the passage of Hurricane Isaac that meteorological officials say could pass near or over the island by Wednesday night. It took Matheson and her husband almost the whole day Monday to drive the 60 miles off the barrier island. And with little wind shear to pull the storm apart, Florence's hurricane wind field was expected to expand over the coming days, increasing its storm surge and inland wind threats, and raising the likelihood of life-threatening freshwater flooding.
Tropical-storm-force winds extend outward up to 45 miles (75 km) from the center.
"We are in the bull's-eye", North Carolina Governor Roy Cooper said at a news conference.
For more general information on the 2018 Atlantic hurricane season, check out this hurricane season guide by Live Science, a Space.com sister site.
Issac delivers swell to the Lesser Antilles as it approaches the islands and also enhances trade winds around the northeast Caribbean, helping to send swell to some of the northern coasts as well.
Hurricanes need warm ocean water to form, so hurricane season comes when the Atlantic Ocean is at its warmest.
It added that maximum sustained winds were near 75 miles per hour with higher gusts.
Hurricane conditions are possible within the hurricane watch area by Thursday morning, with tropical storm conditions possible early Thursday in both the hurricane and tropical storm watch areas.