After hesitation, finally South West Monsoon withdraws from the country and allows North East Monsoon winds to set in ...
The Satellite image below shows and indicates the South West Monsoon withdrawal winds over India. And also indications of NE upper winds along TN coast and Southern Peninsula indicating advance of North East Monsoon.
-The SWM onset is well defined, and follows a well coursed out Northward progress. And can be discerned with reasonable accuracy on charts.
For the NEM, it is not so.
In fact, on many occasions, there is no clear indication between the withdrawing SWM and setting NEM. Often one tends to merge into another.
So, setting a date for the commencement of NEM is difficult, and sometimes not possible.
-During the NEM, an occasional burst of cold air from the Siberian High develops a low pressure systems over the Equatorial regions of the South China Seas. These systems move slowly westwards thru Sarawak/Eastern Malaysia coast, and remnants of these cross over into the Bay and form depressions.
Also browse thru the article analysing the NEM over the years in Vagaries blog of 23rd September 2016 here