"Florence is going to drop tremendous amount of rain, not as much as [Hurricane] Harvey brought to Texas but there will nearly definitely be flooding", Salna says.
It has so far reached a maximum wind speed of 90 miles per hour.
Forecasters say the hurricane's strength is expected to fluctuate but it still will be a risky storm by the time it reaches the coast of SC or North Carolina on Thursday.
Forecasts are coming into agreement on the storm hitting near the border of North and SC.
The storm's 175-mile (282 km) wide "cone of probability" suggests the storm will likely hit North Carolina's coast near Cape Fear, but storms fall outside the cone one-third of the time, according to The Weather Channel.
NOAAAn NOAA map shows the probability of tropical-storm-force winds hitting the US East Coast between Sunday and Friday.
To develop a forecast cone, scientists look at a variety of models that determine the path the storm will most likely take.
Meteorologist Eric Holthaus had a three-letter reaction to the news that Hurricane Florence is now a category 4 storm with 115mph winds and higher gusts. Although North Carolina is still expected to see the worst of the storm, a more southern track would mean that states further north - like Virginia - could be spared of some nasty weather.
More than 1.5 million people have been ordered to evacuate coastal areas ahead of the storm because of both destructive winds and a storm surge that could place normally dry land under at least 10 feet of water.
To put things in perspective, any storm surge taller than 12 feet is "life-threatening", National Hurricane Center Director Ken Graham said.
Hurricane Isaac is holding steady in strength over the Atlantic.
While a statement from Ralph Northam, Virginia Governor, added that it could be the "most significant hurricane event in decades" before warning of "catastrophic inland flood, high winds and power outages".
Shifts in the hurricane's path are still possible, The Washington Post noted.
North Carolina Governor Roy Cooper declared a state of emergency in North Carolina, as did his counterparts in neighbouring sc and Virginia.