It takes two to tango, Southwest Monsoon & Western Ghats are a pair that dances to a matching tune. A tune that provides for heavy rains to one side of the mountains while the other side of the mountains pretty much end up dry and a perfect example of Rain Shadow region as we have read in Geography classes when young. In this post we have attempted to provide a few examples on how the Western Ghats at various locations play both the roles, Good Cop & Bad Copy, to parts of Tamil Nadu during the Southwest Monsoon season.
No where else this is clearer than in Kanyakumari and Tirunelveli districts. The Western Ghats divides Kanyakumari & Tirunelveli district from North to South all the way from the Kodaiyar dam till Kavalkinaru in the plains. To the East of the Western Ghats lies Tirunelveli and to the West lies Kanyakumari district. While places in Kanyakumari district like Kuzhithurai Thuckalay etc has recorded in excess of about 40 cms since the start of Southwest Monsoon Season on June 1st, during the same period places like Tenkasi, Ambasamudram & Manimuthar etc in Tirunelveli district have recorded less than 10 cms. 
If Western Ghats creates a divide between districts during Southwest Monsoon in one part of TN it creates a different impact in places little further north. Let us look at how one particular range plays a crucial role in places around Valparai being one of the wettest in South India and why Chinna Kallar is called the Cherrapunjee of Tamil Nadu. Though for many Valparai is a hill station it pretty much sits like a plateau in the Western Ghats facing the low lands of Kerala on one side and ringed by the Anaimudi range on the other side.
Not only does the orographic lift helps the Valparai plateau get good rains during Southwest Monsoon season the neighbouring Anaimudi range which towers in excess of 2000 mts ensures the moisture does not cross easily resulting in the clouds dumping most of the rains in the Valparai plateau resulting in places like Chinna Kallar getting more than 500 cms on most years regularly.
Similarly if we move further north to the Nilgiris range of Western Ghats we can see huge variations within the Nilgiris district as places that face the monsoon end up getting almost twice the amount of rains places slightly further to the East get during the same period. Places like Devala, which on a good day will allow you to see the distance Arabian Sea, is one of the wettest places of Tamil Nadu as it sits on the edge of the Nilgiris range.
The Nilgiris range pretty much ensures North Kerala becomes one of the most active regions during Southwest Monsoon ensuring the monsoon clouds keep climbing to find a way past the mountains, a perfect case of the clouds trying to soak as much moisture as possible finally dumping all the rains unable to hold any more moisture as the clouds rise up in an effort to cross over Nilgiris range.