Hurricane Sally
Hurricane Sally was a destructive and slow-moving tropical cyclone that was the first hurricane to make landfall in the U.S. state of Alabama since Ivan in 2004, coincidentally on the same date in the same place. The eighteenth named storm and seventh hurricane of the extremely active 2020 Atlantic hurricane season, Sally developed from an area of disturbed weather which was first monitored over the Bahamas on September 10. The system grew a broad area of low-pressure on September 11, and was designated as a tropical depression late that day. Early the next day, the depression made landfall at Key Biscayne and subsequently strengthened into Tropical Storm Sally that afternoon. Moderate northwesterly shear prevented significant intensification for the first two days, but convection continued to grow towards the center and Sally slowly intensified. On September 14, a center reformation into the center of the convection occurred, and data from a hurricane hunter reconnaissance aircraft showed that Sally had rapidly intensified into a strong Category 1 hurricane. However, an increase in wind shear and upwelling of colder waters halted the intensification and Sally weakened slightly on September 15 before turning slowly northeastward. Despite this increase in wind shear, it unexpectedly re-intensified, reaching Category 2 status early on September 16 before making landfall at peak intensity at 09:45 UTC on September 16, near Gulf Shores, Alabama, with maximum sustained winds of and a minimum central pressure of. The storm rapidly weakened after landfall before transitioning into an extratropical low at 12:00 UTC the next day. Sally's remnants lasted for another day as they moved off the coast of the Southeastern United States before being absorbed into another extratropical storm on September 18.
Numerous watches and warnings were issued in anticipation of the imminent approach of Sally, and several coastline counties and parishes on the Gulf Coast were evacuated. In South Florida, heavy rain led to localized flash flooding while the rest of peninsula saw continuous shower and thunderstorm activity due to the asymmetric structure of Sally. The area between Mobile, Alabama, and Pensacola - Gulf Breeze, Florida took the brunt of the storm with widespread wind damage, storm surge flooding, and over of rainfall in the first 24 hours and over 30 inches in 48 hours. Numerous tornadoes also occurred as well. Damage is estimated at $7.3 billion. Sally was the costliest of several destructive 2020 hurricanes whose names were not retired by the World Meteorological Organization following the season, along with Hanna, Isaias, Delta, and Zeta.
Meteorological history
Toward the end of August 2020, 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. The National Hurricane Center 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 low steadily coalesced with accompanying convection becoming more organized as the system approached the archipelago on September 11. Contrary to the NHC's forecasts, 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 east-southeast of Miami, Florida. 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. 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. 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. A prominent central dense overcast, displaced about 50–60 mi 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. Following a burst of deep convection with tops colder than, Sally underwent a brief period of rapid intensification. Its winds increased to 85 mph by 18:00 UTC at the end of this phase, constituting its initial peak intensity. 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.
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. 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. Its eye became increasingly well-defined and aircraft observations revealed a major increase in both flight-level and surface winds; elevated winds up to 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 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 within 24 hours with rainfall rates up to 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. The storm reached Category 2 intensity by 06:00 UTC as it approached Mobile Bay and continued to intensify until its center reached land. The hurricane ultimately made landfall at peak strength in Gulf Shores at 09:45 UTC with maximum sustained winds of 110 mph and a minimum pressure of 965 mbar. 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 and a peak gust of. These were observed atop an mast and were not representative of surface values.
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. 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. 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. 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. 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.