Hurricane Debbie hits Florida—and that's just the beginning » Yale Climate Connection

More than a foot of rain has fallen in western Florida

Debbie has flooded western Florida: Radar estimated 14-18 inches of rain in two counties south of Tampa Bay (see tweet below). Observations from the CoCoRaHS rain gauge network showed many readings over 11 inches in the 24 hours ending at 7 a.m. ET Monday, with the highest reading in Manatee County being 16.04 inches. Flooding closed Interstate 75 south of Tampa, killing a truck driver who was swept away Monday morning.

Catastrophic extreme rainfall expected in coastal Georgia and South Carolina

The main damage caused by Debbie will be flooding from heavy rains. 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. As of Monday morning, the National Hurricane Center is forecasting 16 to 20 inches of rain across much of the Georgia and South Carolina Coastal Plain, including parts from the Georgia-South Carolina border to Charleston, South Carolina. 20 -30 inches of rainfall. Such rainfall could break South Carolina's all-time tropical cyclone rainfall record of 23.63 inches from Hurricane Florence in 2018.

At 4:39 a.m. ET on Monday, the National Oceanic and Atmospheric Administration's (NOAA) Excess Rainfall Discussion listed northeastern Florida to southeastern South Carolina as a “high risk” area for heavy rainfall on Monday, with heavy rainfall expected in Georgia and Tuesday. Much of South Carolina's coastal plain will enter the high-risk zone. 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. The three-day outlook warns that “widespread, multiple flash flooding is possible, with the potential for severe and potentially catastrophic flooding.”

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 location of Debbie's greatest flood threat is difficult to estimate, as it depends heavily on how long the center remains offshore. With water temperatures of 27-28 degrees Celsius (81-82°F) offshore South Carolina, Debbie will have plenty of thermal energy to re-intensify once the center submerges Tuesday afternoon into Thursday morning. 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. On Monday morning, GFS and European models agreed that Debbie's flooding of the southeastern U.S. would eventually end by Friday, when the storm is expected to be affected by a trough of low pressure and accelerate northeastward, rising up the U.S. East Coast.

Disadvantages of the Saffir-Simpson Scale: Comparison to Hurricane Idalia (2023) and Hurricane Florence (2018)

As regular Eye of the Storm readers know, the Saffir-Simpson intensity scale is based solely on maximum sustained winds; by design, it does not take into account any other very real dangers of landfalling hurricanes. Rainfall is a classic example. As ocean waters continue to warm and the atmosphere becomes wetter, hurricanes bring more rain, and inland flooding has accounted for more than half of all U.S. landfall hurricane deaths since 2013. The ratings don't actually tell us how much flooding occurred.

Debbie made landfall in the Big Bend, Florida area, almost exactly where Category 3 Hurricane Idalia made landfall on August 30, 2023, with sustained winds of 115 mph and a storm surge of 8 to 12 feet. “Idalia” killed 12 people and caused $3.6 billion in damage, which is a very low loss figure for a major hurricane that made landfall in the United States. This happens because Idalia is located on a sparsely populated coast and does not wander on land. The storm's maximum rainfall amounts were 5 to 10 inches, falling in a relatively narrow area from northern Florida to southern North Carolina (Figure 1). So while Idalia was rated a Category 3 storm when it made landfall, its effects were more typical of what one would expect from a Category 1 storm, and the name Idalia did not derive from the hurricane's name. Exit from the name rotation list.

Hurricane Florence was a large and very slow-moving hurricane that made landfall as a Category 1 storm in Wrightsville Beach, North Carolina on September 18, 2018, bringing damaging storm surges of up to 10 feet and more than Wind gusts of 100 mph. However, Florence caused $30 billion in damage and 53 storm-related deaths, much of it from heavy rains inland, causing flood levels in many rivers to exceed previous records. Much of North Carolina and nearly half of South Carolina received more than 5 inches of rain, with most areas receiving 20-35 inches (Figure 2). Florence broke all-time tropical cyclone precipitation records in North Carolina (35.93 inches) and South Carolina (23.81 inches). Florence's Category 1 hurricane cost $30 billion, ranking 14th.^th-The costliest hurricane in U.S. history—behind only 11 major hurricanes, one Category 2 hurricane (Ike in 2008) and one Category 1 hurricane (Sandy in 2012). So while Florence was rated a Category 1 storm when it made landfall, its effects were more typical, similar to a Category 3 or 4 storm.

Likewise, if Debbie's rainfall is as intense and widespread as the strong agreement between forecast models and official outlooks suggests, Debbie could cause more than $10 billion in damage, making the impact of this Category 1 hurricane more like a Category 3 or 4 storm. Debbie's name is expected to be retired at the end of the 2024 hurricane season.

Read: How sea level rise causes billions of dollars in additional damage during hurricanes

Debbie's storm surge

Debbie brought three to four feet of storm surge to much of Florida's west coast, from Naples to Tampa Bay, causing minor to moderate coastal flooding. Tide gauges measured the highest storm surge in Cedar Key, recording surges of more than six feet. Fortunately, the peak occurred at low tide and the final water level at the station was only the sixth-highest on record. Here are the six highest water levels in Cedar Key since 1914 (the above are higher water levels, 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, Tropical Storm Josephine, October 8, 1996
5) 5.13 feet, March 13, 1993, storm of the century
6) 4.66 feet, August 5, 2024, Hurricane Debbie

Debbie brought the third-highest water ever recorded to Naples, Florida, where accurate records date back to 1965. This is behavior one would expect to see in the era of climate change, as rising sea levels make it easier to set new high-water records. Here are the six highest water levels in Naples since 1965 (the above are higher water levels, or MHHW):

1) 6.18 feet, September 28, 2022, Hurricane Ian (gauge failed before peak water level was recorded)
2) 4.02 feet, September 10, 2017, Hurricane Irma
3) 3.11 feet, August 4, 2024, Hurricane Debbie (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

Debbie brought the fifth-highest water levels ever recorded to Fort Myers, Florida, where accurate records date back to 1965. :

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

Note that there were multiple reports of 8 to 12 feet of flooding in the Naples area and 7 to 11 feet of flooding in the Fort Myers area as a result of Hurricane Donna in 1960.