(c) 2018, The Washington Post.

Florence made landfall Friday morning as a hurricane, but the storm has only just begun for the Carolinas. Beyond Friday's torrential rain, multi-foot storm surge and widespread power outages, Florence will continue to batter the region through early next week. The storm's winds will slowly weaken, but some of its most devastating effects may be yet to come.

Through the weekend, the massive storm - containing a zone of tropical-storm-force winds nearly 400 miles wide - will drift inland, engulfing much of South Carolina and southern North Carolina.

The National Weather Service says nearly 5 million people could witness at least 10 inches of rain.

"Although coastal storm surge flooding will gradually subside. . .it cannot be emphasized enough that another serious hazard associated with slow-moving Florence is and will be extremely heavy rainfall," the National Hurricane Center said.

Already, some areas in Southeast North Carolina have seen up to 20 inches of rain, and are fast closing in on North Carolina's rainfall record for a tropical storm or hurricane. Most river gauges in Southeast North Carolina were reporting moderate to major flooding.

"Catastrophic flash flooding to continue/worsen," said a bulletin from the National Weather Service, which had issued flash flood watches and warnings over most of eastern North Carolina and northeast South Carolina.

The Associated Press reports the storm may unload 18 trillion gallons of rain on the Southeast and Mid-Atlantic. "Florence's 18 trillion gallons is as much water as there is in the entire Chesapeake Bay," its article says. "It's also enough to cover the entire state of Texas with nearly four inches (10 centimeters) of water."

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On Friday night, the storm will drift southwest into northeast South Carolina, around which the most severe effects will occur.

The storm surge, the rise in seawater above normally dry land at the coast, will continue to inundate some areas - especially around high tide.

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Peak storm surge forecast from the National Hurricane Center

- The Neuse, Pamlico, Pungo and Bay rivers - 8 to 12 feet

- Cape Fear t o Salvo, N.C. - 3 to 5 feet

- Myrtle Beach to Cape Fear - 2 to 4 feet

A disastrous amount of rain will exacerbate the storm surge. While winds blow onshore through Monday, rainfall runoff will pile on top of the storm surge in some locations, particularly around inlets where rivers drain into the Atlantic Ocean, like Cape Fear and Jacksonville, North Carolina.

These same areas that are inundated with storm surge will also be raked by strong winds Friday night sustained at 30 to 50 mph with gusts as high as 50 to 70 mph - strong enough to bring down trees and knock out power.

On Saturday, winds back off to 25 to 35 mph with stronger gusts. By Sunday evening, winds should fall below 20 mph, according to the National Weather Service in Wilmington.

Into the weekend, as the winds start to slacken at the coast and the surge subsides, the rains are likely to keeping coming.

"We are forecasting flooding at multiple rivers in our area, some approaching major and record levels, starting tomorrow [Saturday] into next week," the Weather Service office in Wilmington tweeted.

Forecast models suggest the heaviest rain will focus near the coast around the South Carolina and North Carolina border. Through Sunday evening, 20 to 25 inches of rain could fall in southeast North Carolina and far northeast South Carolina on top of what has already fallen. These are numbers that far exceed what fell on the same region during Hurricane Matthew in 2016.

Enough rain will likely fall to break North Carolina's record for a tropical storm - 24 inches - set near Wilmington during Hurricane Floyd in 1999, said Greg Carbin, chief of forecast operations at the Weather Service's national prediction center.

Florence is forecast to "drop almost double" the volume of rain over the state compared to Floyd, tweeted Ryan Maue, meteorologist for weathermodels.com.

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Flood threat spreads across the Carolinas this weekend into Monday

Even toward the interior of the Carolinas, away from the coast, rainfall totals of 5 to 10 inches will be widespread from Raleigh to Charlotte to Columbia, with isolated higher amounts.

Flash flooding and river flooding is likely if not inevitable over the next several days as torrential rain falls on saturated ground.

Nearly 14 million people are under a flash flood watch, meaning water levels could rise swiftly at any moment. As the water drains off mountains and hills through the weekend, rivers will swell beyond their banks and flood surrounding communities and farms.

The National Weather Service office in Greenville-Spartanburg, S.C. warned of an "increased threat of landslides and debris flows across the mountains and foothills this weekend" due to the forecast for up 8 to 12 inches of rain.

Flooding from heavy rains is the second-leading cause of fatalities in tropical storms and hurricanes that make landfall.

While the storm's winds will abate as it moves inland, they could continue to gust to tropical-storm-force - especially near its core. With all of the rain saturating the ground, trees will become vulnerable to falling.

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Flood threat shifts north early next week

The rain threat may not stop in the Carolinas. The storm, far weaker by early next week, is expected to track north along the Appalachian Mountains from Sunday to Wednesday. The still-soggy storm could drop several inches of rain on northern Georgia and eastern Tennessee as it turns north.

Southwest Virginia, West Virginia, the Ohio River Valley, and western and central Pennsylvania could receive several inches of rain from Florence's remnants on Monday and Tuesday that could lead to flash flooding, depending on where the heaviest rainfall rates develop.

If the remnants track far enough east, Northern Virginia, Maryland, and Washington, may see a period of heavy rain. In these areas, the ground has already been saturated by weeks of higher-than-normal rainfall. Flash flooding, downed trees and power outages will be possible in these regions early next week before Florence finally departs the East Coast on Wednesday or Thursday.

florence-flooding

(c) 2018, The Washington Post.

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