Hurricane Sally leaves a trail of devastating flooding in its wake across the US gulf coastline

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September 2020

Hurricane Sally made landfall on Wednesday 16 September as a Category 2 hurricane near Gulf Shores, Alabama. As it slowly progressed across the Florida Panhandle, the tropical depression brought extreme levels of rainfall, resulting in significant and widespread pluvial, fluvial and coastal flooding.

The most significant damage has been witnessed along the Alabama and Florida gulf coasts where the storm made landfall (NYTimes, 2020a). Storm surge occurred in coastal towns such as Orange Beach and Gulf Shores, AL, as well as Pensacola, FL, where a surge of over 5ft (1.5m) was observed, causing damage to cars, boats and properties. Cumulative rainfall levels in cities across the Alabama/Florida border exceeded 24 inches in a 48-hour period, leaving 500,000 homes and businesses without power (Reuters 2020).

Through late Wednesday and Thursday morning (17 September), the hurricane’s wind speed dropped significantly as it was downgraded to a tropical storm. Its forward progression (translational speed) increased and it rapidly veered inland across Georgia and North Carolina. While still projected to bring continued rainfall and further flooding remains possible in the Northern states, it is Hurricane Sally’s impact along the gulf coast that will be its most damaging legacy.

Extreme rainfall

Figure 1: Extract of the JBA Flood Footprint for Hurricane Sally (Source: JBA Risk Management). Purple represents pluvial (surface water) flood and light blue represents fluvial (river) flood.

The maximum recorded rainfall accumulated over a 24-hour period between 16 and 17 September 2020 was 13.64 inches (346mm), recorded near Brewton in Alabama state. Using daily historical rainfall data, JBA has generated return period estimates for the five sites with highest recorded rainfall in Alabama and Florida (Table 1). At Brewton, Alabama, the 24-hour accumulated rainfall of 13.64 inches (346mm) gives an estimated return period of 250 years (Table 1). Folley Creek, near Dixie in Alabama, saw rainfall totals of 11.81 inches (300mm), which could indicate a return period of up to 200 years for the rainfall received in a single day from Hurricane Sally.

Table 1: 24-hour rainfall recorded at different locations in Alabama, Florida and Georgia states and JBA return period estimate in years. (Data source: USGS 2020 and PSL Data; Data processing: JBA Risk Management)

As the storm moved inland towards eastern Alabama and south-west Georgia, it brought more heavy rainfall. Between 17 and 18 September, some areas in Georgia saw rainfall ranging from 3.15-8.26 inches (80 – 209mm) (USGS Water Resources, 2020). Based on data from USGS Water Resources as of 18 September, rainfall around North Carolina state ranged from 0.9-4.8 inches (23-103mm) (USGS Water Resources, 2020).

JBA conducted extreme value analysis for the locations in Table 1 using gridded historical rainfall record obtained for the United States. Figure 2 shows analysis for the station near Brewton, Alabama. With recorded 24-hour rainfall totalling 13.4 inches (340mm), Brewton has an estimated return period of 250 years.

Figure 2: Analysis for Murder Creek, near Brewton, Alabama. The blue line represents the 24-hour rainfall recorded at this site.

Figure 3: Total rainfall (mm) from 15 September to 17 September 2020. (Data source: NASA Global Precipitation Measurement, 2020)

One of the significant drivers of the inland flooding from this event has been the slow translational speed of the hurricane. This allowed time for the accumulation of extreme levels of precipitation to occur in the same locations across the US coastline. This can be seen in Figure 3 and Table 2 showing the 3-day rainfall totals recorded across the region and as well at specific gauges in Bellview town, Florida (30 inches) and Pensacola, Florida (24.8 inches).

Table 2: 3-day Accumulated Rainfall Totals (Source: CWOP/WeatherFlow/NWS LSR courtesy of AccuWeather)

Changing trends in hurricane behaviour

Compared to hurricanes that hit the US in the early 20th century, Hurricane Sally and Hurricane Harvey (2017) made landfall with a low translational speed. Prior to making landfall, Hurricane Sally’s forward speed was just 2mph (3.2kph) while its maximum sustained windspeed was only 105mph (169kph) (National Hurricane Centre, 2020).

A 2018 study highlighted a growing phenomenon whereby a warmer climate with more moisture in the atmosphere appears to slow down a hurricane’s forward momentum, while at the same time allowing it to carry more water and increase rainfall levels (Kosin, 2018). In addition, a climate study in Penn State suggested that warming in the Arctic resulted in a small temperature difference between the Arctic and the tropics, resulting in the slowing of the jet stream. This could affect other natural circulation phenomena in the mid-latitudes in North America.

This research suggests a changing trend in hurricane behaviour, and that climate change may have a role to play in slower moving hurricanes after they have made landfall.

To capture the impact of the inland flooding from this event, JBA has produced a river and surface water flood footprint. Please get in touch using the form above for more information, or email the event response team.

JBA Risk Management has nationwide return period flood maps for the USA at 5m resolution and a Global Flood Model to help you understand and manage flood risk more effectively.

References

BBC, 2020 https://www.bbc.com/news/world-us-canada-54183485

Kosin, 2018, Nature, accessed from https://www.nature.com/articles/s41586-018-0158-3

NASA Global Precipitation Measurement, 2020 https://gpm.nasa.gov/

National Hurricane Centre, 2020 https://www.star.nesdis.noaa.gov/goes/floater.php?stormid=AL192020

NYTimes, 2020a https://www.nytimes.com/2020/09/16/us/hurricane-sally-live.html

NYTimes, 2020b https://www.nytimes.com/2020/09/15/climate/hurricane-sally-climate-change.html

PSL data CPC US Unified Precipitation data provided by the NOAA/OAR/ESRL PSL, Boulder, Colorado, USA, from their Web site at https://psl.noaa.gov

USGS Water Resources, 2020 https://waterdata.usgs.gov/nwis/current/?type=precip&gropu_key=state_cd