Unusually long duration and heavy rainfall on 5-6 February 2021 causes devastating floods in Semarang. The heavy rainfall is produced by two mesoscale convective systems (MCSs). The first MCS develops at 13Z on 5 February 2021 over the southern coast of Sumatra and propagates towards Semarang. The second MCS develops over the north coast of Semarang at 18Z on 5 February 2021, which later led to the first peak of precipitation at 21Z on 5 February 2021. These two MCSs eventually merge into single MCS, producing the second peak of precipitation at 00Z on 6 February 2021. Analysis of moisture transport indicates that the strong and persistent northwesterly wind near the surface induced by CENS prior to and during the event, creates an intensive meridional (southward) tropospheric moisture transport from the South China Sea towards Semarang. In addition, the westerly flow induced by low-frequency variability associated with La-Nina and the tropical depression associated with tropical cyclone formation over the North of Australia, produces an intensive zonal (eastward) tropospheric moisture transport from the Indian Ocean towards Semarang. The combined effects of the zonal and meridional moisture transports provide favorable conditions for the development of MCSs, and hence, extreme rainfall over Semarang.