Revealing the Key Drivers Conducive to the "Once-In-A-Century" 2021 Peninsular Malaysia Flood

In December 2021, Super Typhoon Rai caused significant devastation to the South Philippines and East Malaysia. In the meantime, an unprecedented flood event occurred in Peninsular Malaysia at 2,000 km west of the typhoon's path, causing comparable socioeconomic impacts as Rai. Record-breaking 3-day precipitation was received by Peninsular Malaysia during 16-18 December. Based on the storm tracking results, this study identified two mesoscale convective systems (MCSs) that were directly responsible for the flooding. The two MCSs were directly initiated by a tropical depression and sustained by an elongated easterly water vapor corridor originating from the Super Typhoon Rai. The return period and joint frequency analysis of key drivers indicate that the 3-day downpour was more severe than a "once-in-a-century" event. Historical records suggest such anomalous moisture channel has become more frequent in Southeast Asia, which alarms heightened attention in forecasting winter flood. On 16-18 December, Peninsular Malaysia received a record-shattering 3-day precipitation, resulting in catastrophic socioeconomic impacts. Due to the temporal coincidence with Super Typhoon Rai but far away in space, there were speculations that there might be a teleconnection between the two events. Our results reveal that their relationship could be more straightforward. Based on the analyses of storm tracking database and synoptic data records, we found that two consecutive mesoscale convective systems were responsible for the heavy precipitation, which were produced by a tropical depression that hovered over the peninsula. Meanwhile, Super Typhoon Rai provided a long-range water vapor transport, akin to adding fuel (i.e., moisture) to the engine (i.e., the tropical depression), and therefore, the precipitation over the peninsula was significantly enhanced. Such long-range moisture transport has become more frequent during the boreal winter season, posing an increasing risk of flooding in Southeast Asia. A stretched moisture channel from Typhoon Rai and a strong tropical depression are key synoptic drivers for the flooding eventReturn period and joint probability of key drivers indicate that the 2021 Peninsular Malaysia flood was more severe than "once-in-a-century"There is an increasing trend in such anomalous moisture channel, suggesting a rising risk of severe flooding in Southeast Asia