Tropical Cyclone Seth was a strong tropical cyclone whose main impacts came after it degenerated into a remnant low. The eighth tropical low and the fourth tropical cyclone of the 2021–22 Australian region cyclone season, Seth originated from a tropical disturbance in the Timor Sea and caused severe flooding in southeast Queensland and hazardous surf along the southeastern coast of Australia.
Overall, Seth caused 4 fatalities, 2 each in Queensland and New South Wales. Damage from the storm totaled $75 million USD at the least. Due to the damage caused by Seth in Queensland and New South Wales, the name Seth was retired and replaced with Stafford.
Meteorological history
On 21 December, the Bureau of Meteorology (BoM) began to monitor the northern Arafura Sea and Timor Sea for possible development of a tropical low. Two days later, the tropical low formed within a monsoon trough of low pressure over the eastern Timor Sea, with the BoM designating it as Tropical Low 08U. The agency initially gave 08U a moderate chance of development into a tropical cyclone in the next 3 days. At 00:30 UTC on 24 December, the Joint Typhoon Warning Center (JTWC) started to monitor the low, giving the unofficial designation Invest 97S and a low chance of development within the next 24 hours. At that time, broad and flaring convection was surrounding the low's partially exposed low-level circulation center, and was within a favorable environment of sea surface temperatures, low to moderate vertical wind shear, and strong poleward and equatorial outflow aloft. The BoM then increased 08U's chances of development into high at 04:30 UTC. The JTWC followed suit, and upgraded the low's chances of development into medium at 14:30 UTC, before subsequently increasing it into high with the issuance of a Tropical Cyclone Formation Alert (TCFA) at 21:00 UTC that same day, noting the low's consolidating center and deep convection to its southern quadrant. However, by 18:30 UTC the next day, the JTWC cancelled the TCFA as 08U moved over the western Top End region of Australia. The BoM then decreased the low's chances of development into medium by 26 December. As it moved east-southeast, it emerged into the Gulf of Carpentaria by the next day, while becoming elongated. The JTWC then upgraded 08U's chances of development into medium as it moved over the Gulf. The system later moved inland over the Cape York Peninsula, before entering the Coral Sea on 30 December.
Upon entering the Coral Sea, the JTWC re-issued its TCFA at 02:30 UTC, as 08U was consolidating under an environment of warm sea surface temperatures, lowering wind shear, and a developing anticyclone providing improved equatorial and poleward outflow. The system was also positioned near a monsoonal flow to its north that was producing near gale-force winds, and strong southeasterlies between its broad circulation and land, which aided its development. Gale-force winds were briefly reported at Holmes Reef later that same day, however, deep convection was not organized enough for the BoM to upgrade 08U into a tropical cyclone. By 00:00 UTC the next day, the tropical low intensified into a category 1 tropical cyclone in the Australian scale, with the BoM naming it as Seth. The JTWC later canceled its TCFA again, and classified Seth as a subtropical cyclone, due to the storm possessing both tropical and mid-latitude cyclone characteristics. Seth then intensified into a Category 2 tropical cyclone six hours later. The JTWC started issuing advisories on the storm at 15:00 UTC that same day, noting that the storm transitioned into a well-defined tropical cyclone, with deep convection surrounding its center. Around the same time, the BoM reported that Seth reached peak intensity, with maximum 10-minute sustained winds of . The storm would later weaken into a Category 1 tropical cyclone, as it became elongated due to a dry air intrusion and strong northwesterly wind shear.
As Seth moved south under a mid-latitude upper low, it became strongly influenced by the low and started to exhibit subtropical characteristics, prompting the BoM to reclsssify the storm as ex-tropical cyclone at 06:00 UTC on 1 January. The JTWC followed suit, reclassifying the storm as a subtropical cyclone. By the next day, the BoM reclassified Seth as a subtropical low, as convection was still confined to the south of its broad center. As the steering influence became weak, the low moved slowly northwest for the next few days, before turning west on 6 January and crossing over the Queensland coast by the next day and dissipating overland.