"100s of miles of coastline at risk". 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.
Beyond the damage from wind and high surf, Hurricane Florence is anticipated to slow over the Carolinas where it might dump up to 30 inches of rain over quite a bit of North and SC.
Once it makes landfall, Florence is predicted to stall and remain over North Carolina for at least 24 hours - increasing the threat of risky flooding, NHC director Ken Graham said on Monday.
National Weather Service officials in Honolulu urged residents to remain careful, noting that the difference between a Category 1 hurricane and a tropical storm is just 5 miles per hour wind speed.
Isaac has become the fifth hurricane of the 2018 Atlantic season. Residents in this part of the east coast should prepare now for the likelihood of a major hurricane at landfall.
Monster storm Mangkhut surpassed Florence's strength on Tuesday, with peak winds of about 260 km/h, and the storm is forecast to strengthen further - closing in on sustained winds of 280 km/h - as it surges toward Hong Kong later this week.
The massive storm is threatening the East Coast of the USA with an expected landfall late Thursday.
FEMA administrator Brock Long is warning that the hurricane will be a "devastating event" and urging Americans to evacuate if they've been asked to leave their homes.
The devastation from 17 named storms, 10 hurricanes and 6 six major hurricanes reached near apocalyptic proportions in some regions, causing an estimated $282billion worth of damage.
Lining up behind Florence, Isaac was about 1,150 miles east of the Windward Islands with top winds of 75 mph early Monday, moving west at 14 mph.
"As this weather system moves westward across the tropics, warm ocean air rises into the storm, forming an area of low pressure underneath", the NOAA adds.
There are three hurricanes and a tropical wave in the Atlantic Ocean, and forecasters predict Hurricane Florence will make landfall later this week. Isaac is expected to be near hurricane strength when it moves through the central Lesser Antilles, with some weakening forecast afterward on Friday.
Risky storms are also threatening parts of the U.S.in the Pacific Ocean. Isaac became a tropical storm again approaching the Caribbean, while Hurricane Helene was veering northward, no threat to land.
Unlike the Atlantic and eastern Pacific basins, tropical storm season never ends in the western Pacific. But forecasters say residents should still beware of the chance for risky flooding and wind.
He says the federal government is ready to respond to the Category 4 storm.