Cyclone Seth

Map plotting the storm's track and intensity, according to the Saffir–Simpson scale

Map key

Saffir–Simpson scale

  Tropical depression (≤38 mph, ≤62 km/h)
  Tropical storm (39–73 mph, 63–118 km/h)
  Category 1 (74–95 mph, 119–153 km/h)
  Category 2 (96–110 mph, 154–177 km/h)
  Category 3 (111–129 mph, 178–208 km/h)
  Category 4 (130–156 mph, 209–251 km/h)
  Category 5 (≥157 mph, ≥252 km/h)
  Unknown

Storm type

Tropical cyclone

Subtropical cyclone

Extratropical cyclone, remnant low, tropical disturbance, or monsoon depression

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.[2] 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.[1] The agency initially gave 08U a moderate chance of development into a tropical cyclone in the next 3 days.[3] 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 30–31 °C (86–88 °F) sea surface temperatures, low to moderate vertical wind shear, and strong poleward and equatorial outflow aloft.[4] The BoM then increased 08U's chances of development into high at 04:30 UTC.[5] The JTWC followed suit, and upgraded the low's chances of development into medium at 14:30 UTC,[6] 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.[7] However, by 18:30 UTC the next day, the JTWC cancelled the TCFA as 08U moved over the western Top End region of Australia.[8] The BoM then decreased the low's chances of development into medium by 26 December.[9] As it moved east-southeast, it emerged into the Gulf of Carpentaria by the next day, while becoming elongated.[1] The JTWC then upgraded 08U's chances of development into medium as it moved over the Gulf.[10] The system later moved inland over the Cape York Peninsula, before entering the Coral Sea on 30 December.[1]

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.[11] 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.[1] 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.[12] On January 1, the JTWC finally upgraded the low into a Category 1 tropical cyclone, giving it the number 04P. Later, Seth reached peak intensity and the BoM upgraded it into a Category 2 tropical cyclone, with winds of around 110 km/h (70 mph). As Seth moved south, the BoM downgraded Seth into a Category 1 tropical cyclone. The next day, the BoM classified Seth as a subtropical cyclone.[13]

The remnants of Seth caused major damage in Southern Australia, resulting in 5 fatalities.[14] Many beaches were closed in preparation for Seth.[15] Low-level flood warnings were issued due to Seth.[16] A 47-year-old woman drowned off the coast of New South Wales. Tens of hundreds of rescues were necessary according to lifeguards. Erosion was reported along the coast of New South Wales. Two emergency calls near Arrawarra. Hazardous conditions were reported as south as Sydney and Illawarra. One man died after being blown off of a rock by strong winds and drowning on Windang Island.[17] Another man died after being pulled unconscious due to strong surf. A large swell exposed many unexposed beaches, and further erosion was reported. Many roads and sidewalks suffered damage.[18] Waves in Kirra, Queensland were very high.[19] Damage from Seth totaled over $75 million.[20]

Due to its impacts on New South Wales and Queensland, the name Seth was retired after the 2021–22 season. It was replaced with Stafford.[21]

• Tropical cyclones in 2021
• Weather of 2021

Wikimedia Commons has media related to Cyclone Seth.

• Australian Bureau of Meteorology