“In this transitional period [from September to October], we need to be alert to potential extreme weather such as short heavy rainfall with lightning or thunderstorms, whirlwinds and hailstorms,” BMKG meteorology deputy Guswanto said in a written statement on Monday.

The agency also predicted that heavy intensity rainfall along with lightning or thunderstorms would happen in several regions including North and West Sumatra, West Java, all provinces of Kalimantan, South Sulawesi, Maluku and Papua within the next week.

The public is asked to be alert to possible flooding, flash floods, landslides and whirlwinds that follow heavy rainfall.

Heavy rainfall followed by floods occurred in several parts of West Java and Jakarta on Monday. The water level of many rivers also reportedly rose due to the high volume of torrential rain.

“The heavy rainfall was caused by unstable atmospheric conditions which are strengthened with atmospheric Rossby waves [naturally occurring waves caused by the Earth's rotation and which affect the planet's weather and climate] and wind horizontal convergence,” Guswanto explained.

“These atmospheric phenomena increase the potential for rain clouds [nimbus] to emerge around the West Java region,” he added.

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