South-west monsoon was activated over Sri Lanka from 24 May 2017 and a very low upper air wind convergence was formulated over the eastern sea of Sri Lanka which was absorbing westerly winds. As a result of this, heavy rains were received on 25 of may to the South-western watersheds in the country. Large amount of rains were received within 12 hours in SW regions including Namunuthanna (619mm) of, Bulathsinghela (419mm), Morawaka (406mm) and Walasmulla (437mm) leading riverine oods of the Kalu, Nilwala and Gin rivers.

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