Hurricane Debby hits Florida – and that’s just the beginning » Yale Climate Connections

Already over a foot of rain in western Florida

Debby has already deluged western Florida: Radar-estimated rainfall amounts of 14-18 inches occurred in two counties just south of Tampa Bay (see Tweet below). Observations from the CoCoRaHS rain gage network had many readings in excess of 11 inches for the 24 hours ending at 7 a.m. EDT Monday, with a maximum of 16.04 inches in Manatee County. Flooding from the deluge closed I-75 south of Tampa, where a trucker died on Monday morning after being swept from the road.

Catastrophic extreme rainfall expected for coastal Georgia and South Carolina

The main damage from Debby is going to be flooding from its torrential rains. The trough of low pressure pulling Debby to the northeast is not expected to be strong enough to finish the job, leaving the storm stranded in a region of weak steering currents near the Southeast U.S. coast for multiple days. As of Monday morning, the National Hurricane Center was predicting 16-20 inches of rain for much of the coastal plain of Georgia and South Carolina, with a pocket of 20-30 inches from the Georgia-South Carolina border to Charleston, South Carolina. Such amounts could topple the South Carolina all-time state precipitation record for a tropical cyclone – 23.63 inches from Hurricane Florence in 2018.

NOAA’s Excessive Rainfall Discussion at 4:39 a.m. EDT Monday placed northeastern Florida through southeastern South Carolina in a “High Risk” of heavy rainfall for Monday, with a High Risk area projected for Tuesday for most of the coastal plains of Georgia and South Carolina, and for Wednesday over most of coastal South Carolina and a portion of coastal North Carolina. 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. The outlook all three days warned of “widespread and numerous instances of flash flooding with significant and potentially catastrophic flooding likely.”

Although most of South Carolina is in moderate to severe drought, widespread rains in excess of 10 inches are sure to cause damaging flooding, especially near the coast where onshore winds and a storm surge will prevent rainwater from draining efficiently.

The exact location of Debby’s maximum flood threat is difficult to estimate since it will heavily depend on how much time the center remains offshore. With water temperatures offshore of South Carolina being 27-28 degrees Celsius (81-82°F), Debby will have plenty of heat energy available to re-intensify once the center is over water on Tuesday afternoon through Thursday morning. With weak steering currents and a storm likely to be very near the coast, slight and unpredictable wobbles in the track will make big differences in how much rain Debby ultimately dumps. The Monday morning runs of the GFS and European models were in agreement that Debby’s deluge over the Southeast U.S. would finally end on Friday, when the storm is expected to get picked up by a trough of low pressure and accelerated to the northeast, up the U.S. East Coast.

Where the Saffir-Simpson scale falls short: A comparison with Hurricane Idalia (2023) and Hurricane Florence (2018)

As regular readers of Eye on the Storm know, the Saffir-Simpson intensity scale is based solely on maximum sustained winds; by design, it does not consider any of the other very real hazards of a landfalling hurricane. Rainfall is a prime example. As hurricanes fed by ever-warmer waters and a moister atmosphere dump ever more rainfall, inland flooding has accounted for more than half of all deaths from landfalling U.S. hurricanes since 2013. Yet a Saffir-Simpson rating tells us practically nothing about how much flooding a given hurricane might produce.

Debby came ashore in Florida’s Big Bend region in virtually the same location where Category 3 Hurricane Idalia made landfall on August 30, 2023, with sustained winds of 115 mph and an eight- to 12-foot storm surge. Idalia killed 12 people and caused $3.6 billion in damage – a very low damage figure for U.S. landfalling major hurricane. This occurred because Idalia hit a sparsely populated coast and did not dawdle over land; the storm’s heaviest rains of five to 10 inches fell along a relatively narrow swath from northern Florida to southern North Carolina (Fig. 1). Thus, even though Idalia was rated a Category 3 storm at landfall, its impacts were more typical of what one might expect from a Category 1 storm, and the name Idalia was not retired from the rotating list of hurricane names.

Hurricane Florence was a large and very slow-moving hurricane that made landfall as a Category 1 storm at Wrightsville Beach, North Carolina, on September 18, 2018, with a destructive storm surge of up to 10 feet and wind gusts over 100 mph. However, the majority of the $30 billion damage caused by Florence – and its 53 storm-related deaths – came from torrential rainfall inland, which caused many rivers to surpass previous record flood heights. Rains in excess of five inches fell across most of North Carolina and almost half of South Carolina, with a substantial area receiving catastrophic rains of 20-35 inches (Fig. 2). Florence broke the all-time tropical cyclone precipitation records for both North Carolina (35.93 inches) and South Carolina (23.81 inches). Florence’s $30 billion price tag ranks this Category 1 hurricane as the 14^th-costliest hurricane in U.S. history – behind 11 major hurricanes, one Category 2 hurricane (Ike of 2008), and one Category 1 hurricane (Sandy of 2012). Thus, even though Florence was rated a Category 1 storm at landfall, its impacts were more typical of what one might expect from a Category 3 or 4 storm.

Similarly, if the rains from Debby are as intense and widespread as implied in the strong agreement among forecast models and official outlooks, Debby is likely to cause damage in excess of $10 billion, making this Category 1 hurricane more like a Category 3 or 4 storm in its impacts. Expect to see Debby’s name retired at the conclusion of the 2024 hurricane season.

Read: How sea level rise contributes to billions in extra damage during hurricanes

Storm surge from Debby

Debby brought a storm surge of three to four feet to much of the west coast of Florida, from Naples to Tampa Bay, bringing minor to moderate coastal flooding. The highest storm surge at a tide gage was measured at Cedar Key, where over six feet of surge was recorded. Fortunately, the peak surge occurred when the tide was going out, and the station ended up with only its sixth-highest water level on record. Here are the top six water levels at Cedar Key since 1914 (above mean higher high water, or MHHW):

1) 6.84 feet, August 30, 2023, Hurricane Idalia
2) 5.98 feet, September 2, 2016, Hurricane Hermine
3) 5.41 feet, August 31, 1985, Hurricane Elena
4) 5.15 feet, October 8, 1996, Tropical Storm Josephine
5) 5.13 feet, March 13, 1993, Storm of the Century
6) 4.66 feet, August 5, 2024, Hurricane Debby

Debby brought the third-highest water level on record to Naples, Florida, where accurate records go back to 1965. Five of the city’s top-six water levels since 1965 have occurred in the past eight years. This is the kind of behavior one would expect to see in an era of climate change since sea level rise makes it easier to set new high-water records. Here are the top six water levels at Naples since 1965 (above mean higher high water, or MHHW):

1) 6.18 feet, September 28, 2022, Hurricane Ian (gage failed before highest level was recorded)
2) 4.02 feet, September 10, 2017, Hurricane Irma
3) 3.11 feet, August 4, 2024, Hurricane Debby (data from the new North Naples Bay station)
3) 3.11 feet, December 22, 1972, Winter storm
5) 3.08 feet, January 17, 2016, Winter storm
6) 3.02 feet, September 28, 2023, Hurricane Idalia

Debby brought the fifth-highest water level on record to Fort Myers, Florida, where accurate records go back to 1965. Here are their top six water levels (above mean higher high water, or MHHW):

1) 7.26 feet, September 28, 2022, Hurricane Ian
2) 3.41 feet, November 23, 1988, Tropical Storm Keith
3) 3.31 feet, September 14, 2001, Hurricane Gabrielle
4) 3.30 feet, June 18, 1982, Unnamed subtropical storm
5) 3.27 feet, August 4, 2024, Hurricane Debby
6) 3.2 feet, August 30, 2023, Hurricane Idalia

Note that there were multiple reports of inundations from 1960’s Hurricane Donna of eight to 12 feet in the Naples area and seven to 11 feet in the Fort Myers area.