Tropical Storm Debbie is about to begin a week of hydrographic assault along the southeastern U.S. coast on Saturday after becoming the fourth named storm of the 2024 season in the Atlantic Ocean. Debbie is expected to reach hurricane strength before making landfall in Big Bend Bay on the northeastern Gulf Coast of Florida early Monday, bringing storm surge and high winds. But Debbie's biggest impact may come from days of heavy rain, as shifting currents could leave Debbie stranded off the southeastern coast of the United States and linger for days.
As of 5 p.m. ET Saturday, the newly upgraded Debbie was centered about 100 miles west-southwest of Key West, Florida, with maximum sustained winds of 40 mph. Debby's upper-level circulation was unusually large and strong for a minimal tropical storm, with multiple broad bands of showers and thunderstorms (convection) extending from southern Cuba to the west-central Florida coast. With ocean temperatures near 30 degrees Celsius (86°F), close to historical records, wind shear speeds of 5-10 knots, and moderate relative humidity of 65 to 70 percent, conditions are conducive to continued development.
As of Saturday, Debbie is being guided primarily by a strong upper-level ridge over the Atlantic Ocean, which is pushing the system west-northwest along the length of Cuba and into the southeastern Gulf of Mexico. An upper-level trough moving into the eastern United States this weekend will help Debbie move north and northeast. The models are in strong agreement for the expected retracement of Debbie, which will enter the northeastern Gulf Coast of Florida on Monday morning.
Debbie's size will inhibit the storm's rapid intensification until the storm's core becomes sharp. However, as the National Hurricane Center predicts, Debbie will have a chance of reaching hurricane strength from Sunday night until landfall due to extremely warm sea surface temperatures beneath its path. Over time, the storm's expanded convective shielding combined with decelerating motion will increase the substantial risk of large-scale flooding.
Extreme rainfall possible in Northeast Florida, Georgia and South Carolina
The trough of low pressure expected to pull Debbie northeastward was not strong enough for the task, causing the storm to linger for several days in an area of weaker steering current off the southeastern United States coast. Continued flooding will begin on Monday, with the storm expected to move at 7 to 9 miles per hour. This will be one of the slowest 20% of tropical cyclones in the region. As Debbie slows further, the storm is expected to take 36 to 48 hours to cross from the coast of Big Bend, Florida, to the coast of Georgia, a distance of only about 100 to 150 miles.
The National Hurricane Center predicts 8 to 12 inches of rainfall across the coastal plains of northeastern Florida, Georgia, and South Carolina (Figure 1 below), from near Jacksonville, Florida, to approximately 50 miles of the coast. There will be 12 to 16 inches of rainfall. The GFS model's 12Z run for Saturday predicts more than 20 inches of rain near the Georgia-South Carolina border. This could challenge the all-time precipitation record for a tropical cyclone in South Carolina: 23.63 inches of rain from Hurricane Florence in 2018.
At 4 p.m. EDT on Saturday, the National Oceanic and Atmospheric Administration's (NOAA) Excess Rainfall Discussion lists northeast Florida, Georgia and the South Carolina Coastal Plain as being at a “moderate risk” for heavy rainfall Monday and Monday night. ”, smaller areas of moderate risk are expected to develop along Florida’s Big Bend Coast and South Carolina on Sunday. Tuesday along the Georgia/South Carolina coast. The outlook warns that an upgrade to “high risk” is “likely” on Monday as confidence grows about the timing and location of the heaviest rainfall. In the United States, only about 4% of days are classified as high-risk areas, but these areas account for about one-third of all flood deaths and about 80% of flood-related losses.
Heavy rain of over six inches or more could extend into southern and eastern North Carolina, depending on the final speed and track of the system. Moisture transported along the coast north of the tropical cyclone itself could bring a total of 4-6 inches of moisture to southern New England by the end of the week.
Although much of South Carolina is in moderate to severe drought, widespread rainfall in excess of 10 inches is certain to cause damaging flooding, especially near the coast, where onshore winds and storm surge will prevent rainwater from draining effectively.
The exact severity of the flood threat in Georgia and the Carolinas is difficult to estimate because it depends heavily on how long the center remains at sea, which could exacerbate Debbie. Because the steering current is weak and the storm is likely to be very close to the coast, slight and unpredictable shaking in the track will have a large impact on the amount of rainfall Debbie ultimately dumps.
storm surge
Debbie was expected to bring a peak storm surge of 3-5 feet near the right side of the landfall site. The storm's large waves and shock waves will cause severe damage and erosion to beaches and dunes that were severely impacted by last year's landfall of Hurricane Idalia, which brought an 8- to 12-foot storm surge to Big Bend. and a storm surge of 3 -5 feet.
High tide in St. Marks, Florida's Big Bend area, is Monday at 12:44 am EDT (4:55Z) and again at 11:32 am EDT (15:32Z); low tide is at 7:15 am EDT (11:26Z) . The difference in water levels between high and low tide is only about 0.8 feet, so the timing of Debbie's landfall will not be a significant factor in determining the extent of coastal flooding. Five top hurricane tracking models run by 12Z on Saturday predict landfall will occur between 3 a.m. and 11 a.m. ET on Monday morning (8Z-15Z). At 11 a.m. ET on Saturday, the NHC forecast predicts landfall near 12 a.m. ET on Monday (8 a.m. ET), close to low tide time.
