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A record‐breaking extreme rainfall event, the highest amount recorded since 1866, hit Indonesia's capital, Jakarta, in early January 2020. This torrential rainfall was mainly caused by the convergence of moisture‐rich air due to an unusual blocking of cross‐equatorial northerly surge over Northwest Java by the southerly winds induced by a cyclonic flow over the Indian Ocean. This condition caused a local increase in the amount of water vapor over Jakarta that eventually led to the formation of clouds and heavy precipitation. In addition, the concurrent occurrences of convectively active phases of equatorial waves (Kelvin, TD‐type, and eastward propagating inertia‐gravity waves) and Madden‐Julian Oscillation during the event also partly contributed to the enhanced local moisture over the region by increasing low‐level moisture flux convergence. Together, these large‐scale dynamical drivers provided a convective environment that fostered the development of a massive rain‐producing mesoscale convective system and, consequently, extreme rainfall over the region. Plain Language Summary: A record‐breaking heavy rainfall event, the highest in the 156‐year historical records, hit Indonesia's capital, Jakarta, in early January 2020. The flooding induced by extreme rainfall caused tremendous damage to infrastructure and had significant socioeconomic impacts. This study examines the atmospheric driving mechanisms underlying this extreme event. We found that heavy rains were triggered by an increase in humid air due to the blocking of the northerly surge over Northwest Java by the southerly winds. This condition created conditions for the development of deep convection and heavy rain over the region. In addition, the large‐scale moisture convergence induced by some types of equatorial waves and Madden‐Julian Oscillation during the event further enhanced local moisture and supported the development of convective cells and extreme rainfall. Deepening the understanding of the atmospheric mechanisms driving this event may provide valuable information for forecasting precipitation extremes over Jakarta in the future. Key Points: An exceptionally high daily rainfall accumulation of up to 377 mm caused severe flash floods in Jakarta in early January 2020The extreme rainfall was mainly caused by convergence of moisture‐rich air due to an unusual blocking of northerly surges by southerly windsThe occurrences of equatorial waves and Madden Julian oscillation partly contributed to the enhanced local moisture by increasing low‐level moisture convergence [ABSTRACT FROM AUTHOR]

On the basis of detailed analysis of a case study and long-term climatology, it is shown that equatorial waves and their interactions serve as precursors for extreme rain and flood events in the central Maritime Continent region of southwest Sulawesi, Indonesia. Meteorological conditions on 22 January 2019 leading to heavy rainfall and devastating flooding in this area are studied. It is shown that a convectively coupled Kelvin wave (CCKW) and a convectively coupled equatorial Rossby wave (CCERW) embedded within the larger-scale envelope of the Madden--Julian oscillation (MJO) enhanced convective phase, contributed to the onset of a mesoscale convective system that developed over the Java Sea. Low-level convergence from the CCKW forced mesoscale convective organization and orographic ascent of moist air over the slopes of southwest Sulawesi. Climatological analysis shows that 92% of December--February floods and 76% of extreme rain events in this region were immediately preceded by positive low-level westerly wind anomalies. It is estimated that both CCKWs and CCERW spropagating over Sulawesi double the chance of floods and extreme rain event development, while the probability of such hazardous events occurring during their combined activity is 8 times greater than on a random day. While the MJO is a key component shaping tropical atmospheric variability, it is shown that its usefulness as a single factor for extreme weather-driven hazard prediction is limited. [ABSTRACT FROM AUTHOR]