Hurricane Florence will hammer the Carolinas and other parts of the Southeast for several days. While impacts from wind will be destructive, Florence’s rainfall is likely to trigger catastrophic flooding in some areas, and will flirt with all-time state rainfall totals from tropical cyclones.
Hurricanes are categorized based on winds. However, arguably the most dire concern with Florence is with water. Put simply, Florence is a “Category 5 flood threat.”
The National Hurricane Center didn’t mince words Tuesday.
— Jonathan Erdman (@wxjerdman) September 11, 2018
Tropical cyclones and their remnants are notoriously heavy rainfall producers. Harvey in 2017 is the most prolific recent example in the Lower 48 states.
Florence was already expected to produce heavy rain. Now the forecast has become even more ominous.
Florence is expected to slow down and move erratically for a period of time near the coast of the Carolinas or offshore.
A tropical cyclone’s rain potential is largely dependent on its forward speed, not its wind intensity. The slower it moves, the heavier the rain totals.
According to the National Hurricane Center, rainfall could total 20 to 40 inches in coastal North Carolina, and rainfall of 10 inches is possible in several other states from the Southeast to parts of the Middle Atlantic.
The forecast rain totals from Florence may approach or even exceed state rain records for tropical cyclones in North Carolina, South Carolina, Virginia and possibly West Virginia, according to research compiled by David Roth, meteorologist with NOAA’s Weather Prediction Center.
You don’t need rain nearly as heavy as what is being forecast near the coast of the Carolinas to trigger major flash flooding and river flooding in the Appalachians, due to the runoff enhancement of terrain.
Florence may produce heavy rain along the slopes of the Appalachians from north Georgia into West Virginia into at least early next week. This could lead to numerous rock and mudslides.
If that wasn’t enough, trees may be more easily toppled by winds than otherwise when soil is saturated from either heavy rain or storm surge flooding.
Now Add Storm Surge Flooding
Compounding this rainfall flood threat is ocean water the hurricane will push ashore.
Storm surge flooding will accompany the arrival of Florence’s center, with the highest storm surge generally to the east of the center.
Persistent onshore winds to the east or north of the center of Florence will keep water levels high for several tidal cycles even after the most intense, closest pass of the hurricane, again, due to its slow movement.
As always, this surge won’t just happen at the Atlantic coast, but will surge into inlets, bays, rivers and sounds.
With heavy rain falling, these elevated water levels won’t allow swollen rivers to drain efficiently, worsening flooding on those rivers.
Recent Major Inland Flood Events
The most notable recent inland flood from a tropical storm or hurricane was Hurricane Matthew, which produced a swath of 10 to 19 inches of rain from northeastern Florida through the eastern Carolinas and southeastern Virginia in October 2016, leading to flooding that topped levels from Floyd along the Neuse River at Goldsboro and Kinston, North Carolina.
The other most recent example in this region was Hurricane Floyd in 1999, when torrential rain on ground saturated from Tropical Storm Dennis a few weeks prior led to massive flooding in North Carolina.
Incredibly, the rainfall totals from Florence may top those during Floyd in parts of North Carolina.
Hurricane Irene in 2011 produced a swath of heavy rain up the East Coast, but it is probably most memorable for the massive inland flooding it triggered in Vermont and other parts of New England and New York state.
Arguably, the most notorious inland Eastern flood event in recent memory was also from a remnant tropical cyclone, Agnes, in June 1972, which resulted in $2.1 billion of damage in the U.S., most of which came from inland flooding.
Catastrophic inland flooding is likely in some areas from Florence, featuring both short-term flash flooding and lingering mainstem river flooding that could last for weeks.
According to a National Hurricane Center study, roughly 3 of every 4 deaths in tropical cyclones in the U.S. is from water, either from storm surge (49 percent) or rainfall flooding (27 percent). Only 8 percent of U.S. deaths are from wind.
All interests in the eastern U.S., including those inland from the coast, should be closely monitoring the forecast for Florence and have their preparedness plan in place.