# Meteorological history of Hurricane Sally

The meteorological history of hurricane Sally is the life cycle of this strong tropical cyclone which brought prolific rainfall to the Southeastern United States in September 2020. Sally originated in early September from a trough over the western Atlantic Ocean associated with Tropical Storm Omar. This system gradually organized as it moved southwest toward the Bahamas, eventually becoming a tropical depression over the archipelago between Andros Island and Bimini on September 11. Steering along a general west course by a ridge, the depression made landfall near Miami, Florida, early the next day. While traversing the Everglades it strengthened into a tropical storm, the 18th of the season. Turning west-northwest, Sally steadily organized with moderate wind shear being an inhibiting factor. A brief reprieve in shear allowed the storm to rapidly intensify to a hurricane on September 14, with winds reaching 85 mph (140 km/h). Steering currents soon collapsed and Sally became nearly stationary south of Mobile Bay.
Wind shear increased again on September 15, disrupting the hurricane's structure and causing it to weaken slightly. The hurricane's slow movement led to a prolonged period of heavy rain across southern Alabama and the Florida Panhandle with multiple convective bands sweeping across the area. Later in the day, Sally turned north and north-northeast toward the coastline. A significant increase in upper-level divergence fostered re-intensification and Sally rapidly intensified for a second time early on September 16. In the hours leading up to its second landfall, Sally became a Category 2 hurricane and attained maximum sustained winds of 110 mph (175 km/h) as it moved ashore near Gulf Shores, Alabama. Once over land, the combined influences of friction and shear took a toll and the hurricane quickly weakened to a tropical depression by September 17. Accelerating northeast across Georgia and The Carolinas, Sally transitioned into an extratropical cyclone before dissipating on September 18.

## Background

The 2020 Atlantic hurricane season was the most active on record, with 30 tropical or subtropical storms developing across the basin. The activity was fueled by above-average sea surface temperatures, a strong west African monsoon, generally weak wind shear, and a La Niña. September proved to be especially active, with a record ten named storms developing. Sally was one of five simultaneous tropical cyclones in the basin on September 14. Storms formed at an exceptionally fast pace, with early formation records being set by 27 of them. The United States saw an unprecedented number of landfalling systems, with 11 striking the country and a record-tying 6 were hurricanes. Every coastal county from Texas to Maine had tropical cyclone-related watches and warnings issued at some point during the year. The season featured nine storms, including Sally, that underwent rapid intensification, tying the record set in 1995 and 2010. Between September 13 and 15, National Oceanic and Atmospheric Administration researchers deployed newly developed hurricane ocean gliders within Sally to measure ocean data.
Hurricane Sally marked the fourth consecutive year the United States was struck by a slow-moving tropical cyclone which produced prolific rainfall after Harvey in 2017, Florence in 2018, and Imelda in 2019. The increase in these prolific rain events is at least partially attributed to climate change, namely an increase in available atmospheric moisture. A 2018 study in Nature by James Kossin found that the average speed of tropical cyclones worldwide slowed by about 10 percent since the mid-20th century and another study in 2019 by Kossin and Timothy Hall revealed a 15 percent slowing along the North American coastline. This in turn leads to enhanced coastal rainfall from tropical cyclones; however, Hall stated this could be from natural climate variability. A study by Michael E. Mann suggests temperature rises in the Arctic causes a slowing of the jet stream, leading to prolonged blocking patterns that slow the movement of cyclones.

## Origins and intensification

Toward the end of August 2021, a shortwave trough interacted with a decaying frontal boundary and led to the development of Tropical Storm Omar. This weak tropical cyclone later became embedded within a broad mid-level trough well to the north of Bermuda as it dissipated on September 5; an extension of this trough propagated southwest as Omar moved over the North Atlantic. Over the next five days, the system remained disorganized as it approached the Bahamas.$^{:3}$ The National Hurricane Center (NHC) first noted the potential for tropical cyclogenesis on September 9, anticipating the system to not become a tropical cyclone until reaching the Gulf of Mexico several days later. A broad surface slow steadily coalesced with accompanying convection becoming more organized as the system approached the archipelago on September 11. Contrary to the NHC's forecasts,$^{:13}$ the system developed into a tropical depression, the nineteenth of the season, by 18:00 UTC that day while situated between Andros Island and Bimini in the Bahamas, or about 115 mi (185 km/h) east-southeast of Miami, Florida.$^{:3}$ A subtropical ridge over the Southeastern United States steered the system generally west-northwest toward the Florida Peninsula. Around 06:00 UTC on September 12, the depression made landfall near Cutler Bay, Florida, with maximum sustained winds of 35 mph (55 km/h).$^{:18}$ Within hours, multiple weather stations began recording sustained tropical storm-force winds and the system became Tropical Storm Sally by 12:00 UTC while situated over the Everglades.$^{:3}$ This marked the earliest formation of a season's 18th named storm, surpassing the previous record of October 2 set in 2005 with Hurricane Stan.
As Sally emerged over the eastern Gulf of Mexico later on September 12, banding features became increasingly defined over the southern portion of its circulation. One of these bands became somewhat stationary, pivoting over the Florida Keys and Florida Bay while producing heavy rainfall in the region. As Sally rounded the southern edge of the ridge and turned west-northwest, modest wind shear inhibited significant intensification over the next day.$^{:3}$ A prominent central dense overcast, displaced about 50–60 mi (85–95 km) from the circulation center, blossomed early on September 13. A convergence zone along the backside of the storm became established over the Florida Peninsula within an environment highly favoring extreme rainfall rates. A temporary reprieve in wind shear enabled Sally's core to become more vertically aligned and convective bursts occurred throughout the day into September 14.$^{:3}$ Following a burst of deep convection with tops colder than −112 °F (−80 °C), Sally underwent a brief period of rapid intensification. Its winds increased to 85 mph (140 km/h) by 18:00 UTC at the end of this phase, constituting its initial peak intensity.$^{:3}$ Nearby land-based Doppler weather radar depicted a developing mid- to upper-level eye within the hurricane's core at this time. The northern outer bands of the hurricane began impacting the Florida Panhandle early on September 14, primarily along the coast of Apalachee Bay. Rainfall continued to spread west into Alabama and slightly inland and its persistence led to soil saturation of 85 percent well-before the storm's core arrived. Operationally, the NHC upgraded the storm to low-end Category 2 status at 21:00 UTC, but this was determined to be due to transient eyewall features and not a true estimate of the storm's intensity.

## Peak intensity and landfall

By the start of September 15, steering currents collapsed and Sally largely stalled south of the Florida Panhandle and Mobile Bay with a slight drift west. A sudden increase in wind shear caused the hurricane's structure to degrade, with its core becoming asymmetric.$^{:3–4}$ An intrusion of dry air caused the eye to open up to the south. The hurricane's forward motion gradually shifted to the north and later north-northeast throughout the day as it approached the northwest side of a weak ridge. Although wind shear did not abate, upper-level divergence increased significantly and enabled the hurricane to unexpectedly intensify as it approached the Alabama coastline.$^{:4}$ Its eye became increasingly well-defined and aircraft observations revealed a major increase in both flight-level and surface winds; elevated winds up to 131 mph (211 km/h) were detected by Doppler weather radar. Convection around the core and in rainbands east of the center became more intense during this period. "Significant and life threatening flash flooding" began late on September 15 as rainfall intensified to rates up 3 in (76 mm) per hour along the coast, primarily between Gulf Shores, Alabama, and Destin, Florida. The Weather Prediction Center soon described it as "catastrophic flash flooding" as accumulations exceeded 18 in (460 mm) within 24 hours with rainfall rates up to 4 in (100 mm) per hour still occurring. Additional rainbands developed farther east near Panama City and Apalachicola, expanding the scope of the flood event.
Sally's northern eyewall began moving onshore in Baldwin County, Alabama, between Mobile Bay and Pensacola Bay around 05:00 UTC. Mesovortices within the eyewall battered the coastline with violent wind gusts during this time.$^{:4}$ The storm reached Category 2 intensity by 06:00 UTC as it approached Mobile Bay and continued to intensify until its center reached land.$^{:18}$ The hurricane ultimately made landfall at peak strength in Gulf Shores at 09:45 UTC with maximum sustained winds of 110 mph (175 km/h) and a minimum pressure of 965 mbar (hPa; 28.49 inHg).$^{:4}$ This coincidentally occurred on the 16th anniversary of Hurricane Ivan's landfall in the same location and just three hours later. Ingram Bayou saw the most intense winds, with sustained values reaching 113 mph (182 km/h) and a peak gust of 137 mph (220 km/h). These were observed atop an 59 ft (18 m) mast and were not representative of surface values.$^{:6}$

## Dissipation

Hurricane Sally's structure rapidly degraded as it progressed farther inland throughout September 16. Briefly crossing the border into the Florida Panhandle, the hurricane weakened to a tropical storm by 18:00 UTC as it moved back over southeastern Alabama.$^{:4}$ During this time, its forward motion steadily increased as it interacted with the mid-latitude westerlies. Convection surrounding the center eroded amid increasing wind shear and the influence of land-based friction. Excellent outflow aloft supported continued heavy rain across central Georgia in areas northeast of the center. Moist southerly flow from Sally interacted with a developing frontal boundary over the state and expanded the breadth of heavy rain. Sally degraded to a tropical depression by 06:00 UTC on September 17 as its surface circulation decoupled from its mid- to upper-level circulation, with the latter accelerating ahead.$^{:4}$ The system transitioned into post-tropical cyclone soon after as it merged with a frontal boundary over Georgia six hours later, and a defined warm front became established to the east-northeast.$^{:4}$ The trailing surface circulation of Sally continued to weaken before dissipating over South Carolina on September 18. A new low-pressure area developed over eastern North Carolina later that day in association with the aforementioned front and Sally's remnants.$^{:4}$ A convergence zone developed across northern North Carolina into southeastern Virginia as warm, moist air from the remnant system interacted with an unseasonably strong cold front, creating an axis of heavy rain across the region. The rain event associated with Sally finally ceased late on September 18 as the new system moved farther offshore.