By early November 2024, a moderately active Madden-Julian Oscillation (MJO) moved eastwards into the Indian Ocean, aligning with an equatorial Rossby wave.[1][2] On 14 November, the Australian Bureau of Meteorology (BoM) noted that a tropical low could may form to the west of Sumatra, Indonesia, and pre-designated it as 01U.[3] This came into fruition 9 days later, when both the BoM and the United States Joint Typhoon Warning Center (JTWC) reported that the system had formed approximately 350 nmi (650 km; 400 mi) to the northwest of Cocos (Keeling) Islands, aided by a westerly wind burst that also contributed to the formation of Cyclone Fengal.[2][4] Moving generally to the southeast, the low struggled to organize under an environment of moderate to high vertical wind shear, offset by 29–30 °C (84–86 °F) sea surface temperatures and improving vorticity signature.[5] At 20:30 UTC on 26 November, the JTWC issued a Tropical Cyclone Formation Alert on 01U, as an ASCAT pass revealed gale-force winds wrapping the southwest of the low's strengthening but still disorganized low-level circulation centre.[6] By 15:00 UTC the next day, the JTWC upgraded the low into a tropical storm in the Saffir-Simpson scale, noting a well-defined but partially exposed centre.[7] The BoM would not follow suit until early 28 November, when the cyclone gained gale-force winds throughout its entire quadrant, becoming a category 1 tropical cyclone in the Australian scale and earning the name Robyn.[2]
By 12:00 that same day, both the BoM and JTWC reported that Robyn reached its peak intensity, becoming a category 2 tropical cyclone with 10-minute sustained winds of 55 knots (102 km/h; 63 mph).[2][8] However, strong wind shear and significant mid-level dry air entrainment made the system to rapidly weaken, prompting the BoM to downgrade Robyn to a category 1 tropical cyclone by 29 November.[9][8] Six hours later, the BoM downgraded Robyn below tropical cyclone intensity,[2] before issuing their final advisory at 12:00 UTC that same day.[10] The JTWC followed suit three hours later, noting that the system had rapidly weakened, with a fully exposed centre.[11] The remnants of Robyn then drifted westward, moving out of the Australian area of responsibility in early December.[2]
Heavy rains killed ten people in Karo Regency, North Sumatra,[13] including nine from one landslide alone.[14] In Padang Lawas Regency flooding killed five people and destroyed two houses.[15] In Sibolangit, flooding caused four deaths, injured nine and left two missing by 24 November.[16] On 26 November, a landslide occurred in the town, killing nine and injuring 24.[17][18] Severe flooding in South Tapanuli Regency killed two people, injured 76, destroyed 16 houses and damaged 345 others, with three villages recording severe damage.[19][20][21] In Medan, floodwaters damaged 7,699 homes, affecting 24,874 residents.[22] Damage of the flooding in North Aceh Regency was Rp3.4 billion (US$213,000).[23]
^ abcdefPaterson, Linda (23 January 2025). Tropical Cyclone Robyn (01U) (Report). Tropical Cyclone Report. Perth, Western Australia: Australian Bureau of Meteorology. Retrieved 18 April 2025.
Extratropical cyclone, remnant low, tropical disturbance, or monsoon depression
