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.

[6] The JTWC followed suit, and upgraded the low's chances of development into medium at 14:30 UTC,[7] 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.

[8] However, by 18:30 UTC the next day, the JTWC cancelled the TCFA as 08U moved over the western Top End region of Australia.

[11] The system later moved inland over the Cape York Peninsula, before entering the Coral Sea on 30 December.

[12] 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.

[14] Around the same time, the BoM reported that Seth reached peak intensity, with maximum 10-minute sustained winds of 55 knots (102 km/h; 63 mph).

The storm would later weaken into a Category 1 tropical cyclone,[1] as it became elongated due to a dry air intrusion and strong northwesterly wind shear.

[16] By the next day, the BoM reclassified Seth as a subtropical low, as convection was still confined to the south of its broad center.

[1] As Seth's precursor low was forming in the Timor Sea, cyclone warnings were issued in parts of the Northern Territory, including the city of Darwin.

[21] Eight children were rescued after a massive swell turned a calm estuary into deep, fast-moving waters.

[23] Dangerous swell and high tides washed away hundreds of turtle nests before they were due to hatch.