What the hurricane "has now lost in terms of maximum wind speeds near its centre, it has now gained in terms of sheer expansiveness", said Peter Mullinax, a meteorologist at Planalytics.
According to CNN, Wilmington is likely to receive eight months' worth of rain over the next three days.
The US National Hurricane Center described how winds of up to 90 miles (150 kilometers) per hour were pummeling the state of North Carolina.
The worst of the storm's fury had yet to reach coastal SC, where emergency managers said people could still leave flood-prone areas.
Florence was seen as a major test for the Federal Emergency Management Agency, which was heavily criticized as slow and unprepared a year ago for Hurricane Maria in Puerto Rico, where the storm was blamed for almost 3,000 deaths in the desperate aftermath.
Florence was downgraded to a Category 1 hurricane on Thursday evening with peak winds of 90 miles per hour (150 km/h).
About 1.7 million people in North Carolina, South Carolina and Virginia have been put under voluntary or mandatory evacuation orders to avoid what emergency officials called a "once in a lifetime" storm.
The National Hurricane Center said the storm's strength should continue through Thursday.
"Its slow movement will allow rain to continue to pile up in areas from Jacksonville to Wilmington seeing as much as 10" to 20" of rainfall.
Residents wade through deep floodwater to retrieve belongings from the Trent Court public housing apartments after the Neuse River went over its banks during Hurricane Florence September 13, 2018 in New Bern, United States.
Forecasters said conditions would only get more lethal when the storm smashed ashore early on Friday near the North Carolina-South Carolina line and crawled slowly inland.
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"It will be historic", Baker said of the rain from Florence.
Baker said the rain will bring damaging flash flooding to all areas of the Wilmington region, not just low-lying areas that are particularly vulnerable. "It will probably fall faster than it can leave". And we're going to have a lot of flooding.
Florence made landfall as a Category 1 hurricane at 7:15 a.m.at Wrightsville Beach, a few miles east of Wilmington, not far from the SC line, coming ashore along a mostly boarded-up, emptied-out stretch of coastline.
After it weakens inland, Florence is predicted to turn northward, raising concern about how additional rainfall - even from a significantly weakened storm - will impact the southwestern portion of the commonwealth, which is already water-logged.
NewsHour has also published maps of flooding predictions for the coast. This storm is extremely risky.
After landfall, the storm is expected to linger over the area for a few days, which would result in rising river levels and lakes.
National Hurricane Center: Florence about to make landfall in N. Carolina causing life-threatening storm surge.
Not everybody was heeding orders to evacuate, however.
Officials found a basketball-sized hole in the hotel wall and other life-threatening damage, with some cinder blocks crumbling and parts of the roof collapsing.
WXII-TV reports the city of New Bern said Friday that two out-of-state FEMA teams were working on swift-water rescues and more teams were on the way.
FlightAware doesn't anticipate closures at any East Coast hubs (Charlotte, Atlanta or any Washington-area airports), limiting "cascading disruption to the rest of the nation's airport system", Orsi said in a statement Thursday.