Tropical Storm Debby will continue to deluge the Southeast U.S. with torrential rains on Wednesday and Thursday as the slow-moving storm chugs leisurely along the coast of South Carolina. Though Debby’s center moved over water Tuesday night, the storm has not intensified much, and its rains have diminished considerably since Tuesday. The large storm totals piling up now are more a consequence of Debby’s lackadaisical movement than of extremely heavy rainfall rates. As a result, some of the worst localized flash-flood risk is on the decrease, while the larger-scale issue of river flooding is on the increase.
At 11 a.m. EDT Wednesday, Debby had intensified to 60 mph winds, and was located about 55 miles southeast of Charleston, South Carolina, moving slowly northeast at 5 mph. As drier air has infiltrated the inland side of Debby, showers and thunderstorms (convection) were almost all in the storm’s eastern half, giving the storm an asymmetric and somewhat disorganized appearance on radar – though with plenty of clouds evident through the circulation on daytime visible satellite imagery, as shown in the satellite loop below and the image at top.
Forecast for Debby
Debby’s center is expected to remain off the coast of South Carolina through Thursday morning, over waters of 28-29 degrees Celsius (82-84°F). However, the large and poorly organized storm is not well-structured to intensify rapidly, and NHC predicts that Debby’s top sustained winds will increase only another 5 mph, to 65 mph, by the time of its second landfall on Thursday morning. During this period, Debby will crawl along at a forward speed near 5 mph. This will allow the storm to dump additional heavy rains over a large portion of coastal South Carolina and southern North Carolina, generally just to the northeast of where the heaviest rains had fallen through early Wednesday.
Debby’s historic flooding over the Southeast U.S. will finally begin to wind down on Friday and Saturday, when a trough of low pressure will pull the storm to the northeast. However, Debby’s flood threat will then spread to the Mid-Atlantic and Northeast U.S., where widespread rains of 4-8 inches are expected (Fig. 1). NOAA’s Excessive Rainfall Discussion at 8:36 a.m. EDT Wednesday placed a large swath of Pennsylvania, New York, and Vermont in their “Moderate Risk” for excessive rains and flash flooding for Friday (see Tweet below).
A Debby-induced predecessor rain event (PRE) hits New York/New Jersey
Heavy rains can develop more than 500 miles poleward and downstream from a landfalling hurricane, in a phenomenon known as a predecessor rain event, or PRE. Torrential rains from just such a PRE deluged parts of the mid-Atlantic and Northeast on Tuesday, days before Debby itself was predicted to sweep through the region. Fortunately, Debby’s south-to-north movement across the interior Northeast on Friday will unfold perpendicular and just inland of the east-west corridor of Tuesday’s heavy rain, thus limiting the footprint area where the two intersect.
Significant flash flooding was reported Tuesday evening in the New York City area, particularly in the Bronx, closing some highways and leading to six rescues on the Cross Island Parkway. The heaviest rains fell in Bergen County, NJ, and Bronx County, NY, with over five inches of rain in less than eight hours. Some of the larger totals as of late Tuesday night included:
Bogota, NJ: 5.47″
New York (Fordham, Bronx), NY: 5.38″
New York (City Island, Bronx), NY: 4.76″
Port Washington (Nassau County), NY: 4.62″
Dix Hills (Suffolk County), NY: 3.91″
Caldwell, NJ: 3.96″
The highest one-day totals in the region from the CoCoRaHS volunteer rain network, as measured on Wednesday morning, included:
Pennington, NJ: 7.76″
Indiana, PA: 4.50″
Hebron, MD: 3.95″
Amityville, NY: 3.37″
Stamford, CT: 3.14″
Major river flooding in Florida, Georgia, and South Carolina
On Wednesday morning, major flooding was occurring at three rivers in Florida, two in Georgia, and one in South Carolina. Major flooding was predicted to occur later in the week at five rivers in North Carolina, two in South Carolina, and one in Florida.
Already 17-plus inches of rain in South Carolina
Rainfall totals just north of Debby’s center were generally less on Wednesday morning than on Tuesday morning, as measured by daily reports from CoCoRaHS. Widespread 4-8 inch amounts were reported across central and southeast South Carolina and adjacent parts of coastal Georgia and North Carolina. The highest totals by state for the 24 hours ending on Wednesday morning included:
- Murrells Inlet, SC: 8.00″
- Calabash, NC: 5.82″
- Lovettsville, VA: 2.38″
The highest four-day storm totals by state from Debby as of 10 a.m. EDT Wednesday, August 7 – spanning the period from Sunday to Wednesday morning – were all from the CoCoRaHS network. They included the following, as compared to the all-time all-time state precipitation records for a tropical cyclone:
- Florida: 19.67″, 2.8 miles ESE of Lake City (state record 45.20″ from Easy 1950)
- South Carolina: 17.27″, 5.3 miles SE of Summerville (state record 23.63″ from Florence 2018)
- Georgia: 13.44″, 5.3 miles NNE of Rincon (state record 27.85″ from Alberto 1994)
- North Carolina: 7.22″, 1.9 miles NE of Calabash (state record 35.93″ from Florence 2018)
NOAA’s Excessive Rainfall Discussion at 8:36 a.m. EDT Wednesday placed the coastal South Carolina/North Carolina border region at a “High Risk” for heavy rainfall of 4-8 inches on Wednesday, warning, “This area has the opportunity for a 48 hour totals exceeding 15 inches, leading to the threat for a prolonged life-threatening flash flood scenario.”
The High Risk area projected for Thursday covered a similar region, but expanded into central North Carolina, with 3-6 inches of rain expected. Only about 4% of days in the U.S. feature High Risk areas, but they account for about one-third of all flood fatalities and some 80% of flood-related damage.
Lower odds for development of the Caribbean tropical wave
A tropical wave moving across the central Caribbean has a declining chance of organizing into a tropical cyclone. The wave is moving through a moist environment and traveling atop sea surface temperatures around 1 degree Celsius (1.8 degrees Fahrenheit) warmer than average, but persistent northwesterly wind shear has kept its field of showers and thunderstorms disorganized. In its 8 a.m. EDT Wednesday Tropical Weather Outlook, the National Hurricane Center gave this system a near-zero chance of becoming at least a tropical depression by Thursday and only a 10 percent chance by next Wednesday. No other systems are expected to develop in the Atlantic over at least the next week.
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