Slow-moving Debby threatens catastrophic flooding to Carolinas, Virginia

Debby will dump a tremendous amount of rain that may exceed 2 feet in some locations, triggering life-threatening flooding and immense property damage.

Slow-moving Debby is positioning itself to cause catastrophic flooding in the southeastern United States this week. Inches of rain will pour down in hours, and feet of rain will drown some communities over several days. AccuWeather meteorologists warn that life-threatening flooding will expand from coastal to inland locations and from urban areas to small streams and major rivers.

Flash flooding and coastal flooding have already inundated parts of Georgia and South Carolina into Tuesday morning, but more problems are coming to many more areas. Flash flooding and storm surge are typically the most dangerous forms of flooding, while river flooding tends to be a slow-moving disaster. All are expected to occur as Debby slowly moves up the Atlantic coast.

AccuWeather meteorologists continue to say the slow-moving storm will have dangerous, damaging and potentially devasting consequences.

"Debby has certainly slammed on the brakes on Tuesday as we were concerned about with its forward speed down to a few miles per hour, compared to the 10-15 mph speed it had over the Gulf of Mexico this past weekend," AccuWeather Lead Hurricane Expert Alex DaSilva said.

Light steering winds will cause Debby to drift and even meander rather than move swiftly along into Thursday. This means some areas may have pouring rain much of the time over a two- to three-day period. Rainfall rates of 1-3 inches per hour at times in some places will add up over the hours and days. The AccuWeather Local StormMax™ rainfall for Debby in the Southeastern states is 32 inches.

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This will put Debby on a short list of hurricanes that brought devastating flooding to the Carolinas. Hurricane Matthew unloaded up to 20 inches of rain over several days in October 2016. Two years later, Hurricane Florence topped Matthew with up to 36 inches of rain which set rainfall records for a tropical system in the region. Both systems brought catastrophic flooding.

People living along unprotected rivers from near the Georgia border through the Carolinas and into southern Virginia should be prepared for moderate to major flooding, regardless of prior abnormally dry to drought conditions this summer. Some of the major rivers at risk of flooding include but are not limited to the Pee Dee, Lumber, Tar and Congaree.

"People should be prepared to seek higher ground and to know of an evacuation route and have a plan in place," AccuWeather Meteorologist Brandon Buckingham said.

"Water will enter rivers and other waterways that flow toward the coast. Water flowing toward the coast, coupled with the persistent onshore flow, can back up the rivers because they won't be able to drain as they normally do, further amplifying what can already be major flooding concerns along and near the coastal areas,” explained AccuWeather Chief Meteorologist Jon Porter.

Because Debby will spend a bit more time near or over the Gulf Stream along the Georgia and South Carolina coast into Thursday, it will be in position to pump tropical moisture from the Atlantic to the Carolinas, DaSilva explained.

"This proximity may also allow Debby to maintain some strength and possibly [strengthen] for a time before steering breezes finally pick the storm up later Thursday and begin to accelerate the storm to the north and northeast later this week and this weekend," DaSilva said.

This faster movement will limit direct rainfall from Debby to a day or so rather than two to three days from northern Virginia to New England. Still, prior rain from non-tropical systems may have saturated the landscape. That, coupled with Debby's additional effects, will lead to urban, small stream, and river flooding from the mid-Atlantic to New England.

The slow movement and proximity of Debby to the Atlantic in the Southeastern states into Thursday will create plenty of problems in coastal areas, ranging from constant bashing due to waves and storm surge to torrential rain, gusty winds, and even a tornado and waterspout threat.

Storm surge, or water level rise above typical tide levels in coastal areas, will range from 1 to 6 feet along the Carolina coast, with a rise topping 6 feet in some areas of the upper South Carolina coast, including the Myrtle Beach area.

Wind gusts associated with Debby will be the strongest in the vicinity of spiral bands and severe thunderstorms. Gusts of hurricane force (74 mph or greater) can still occur along portions of the Carolina coast into Thursday.

Tropical storm-force gusts ranging from 40-60 mph will be more common and capable of causing property damage, knocking over trees and triggering power outages. Some of the strong wind gusts will also be transported well inland over the Carolinas and eastern Virginia.

Because Debby will linger near the coast and much of the storm's eastern flank will be exposed to the warm, moist waters of the Atlantic and large bays, some tornadoes can occur in the spiral bands from the storm.

The tornado risk will not be limited to the coastal Carolinas but will extend northward into coastal areas of the mid-Atlantic and even the south shore of New England as the storm picks up forward speed.

Coastal population centers along tidal rivers will face a double whammy from storm surge flooding followed by flooding triggered by heavy rain. In some cases, as Porter warned earlier, both may occur at the same time, such as in Savannah, Georgia, and Charleston, South Carolina.

"Charleston is sinking at a rate of about 0.15 of an inch per year, which may not seem like much, AccuWeather Senior Meteorologist and Climate Change Expert Brett Anderson said. But, combine that with rising sea levels along the East Coast, and this has greatly increased the risk of coastal flooding in low-lying places."

Since Charleston is only about 10 feet above sea level, flooding has been occurring even during high tides without a storm. Sea level rise during both tropical and non-tropical storms puts coastal population centers at greater risk.

"Since 1960, sea level along large portions of the southeastern U.S. coast has risen 5-10 inches, "Anderson explained, "The frequency of coastal flooding along the Gulf and East Coast has significantly increased over the past 60 years, with the greatest increase over the past 15 years."

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