The study is aimed to commence the concept of soft computing technique in the form of rough set theory to view the onset of monsoon over Kolkata (22° 32′, 88° 20′), which is situated over Gangetic West Bengal (GWB), India. The concept of rough set theory is primarily based on the condition–decision rule. The method is thus suitable for the present study, where the consideration is to observe the number of cyclonic disturbances during pre-monsoon months as condition and the onset of monsoon whether it would be right time, advance or delay, that is considered as decision. The strength, certainty and coverage factors for the decision algorithms are computed with 19 years’ (1990–2008) data. The result of the study reveals that the number of pre-monsoon cyclonic disturbances including depression, deep depression, cyclonic storm, severe cyclonic storm, very severe cyclonic storm and super cyclonic storm has a immense impact on predicting the onset of southwest monsoon over Kolkata. The validation has been done with data from 2009, 2010 and 2011.