Posted 23rd October Night:
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   

Reproduced Image from Vagaries blog of 15th October 2015.

With the 2010 rainy seasons hesitating to change between the SWM and NEM, an amateur's attempt at a brief Summary of the NEM.

-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 widhrawing 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.

-The main has its origin, its "Power House", is at the Mascrene Highs off Madagascar. The NEM has its origin round a large anti-cyclone over Siberia.

-The Upper winds during the NEM across the Bay are in a clockwise direction.That is an anti-cyclone is formed. At a level of 500 mb, the centre of the anticyclone shifts to central Burma region.

-The Upper Air temperatures show a Northward gradient in the NEM.Near 10N it is normally warmest, and dropping by 8c at 30N.

-Jet streams at 200 hpa are common to both the monsoons.

In the SWM, the Tropical Easterly Jet stream dominates the Peninsula, while the Sub-tropical Westerly Jet Stream is the feature of the NEM.

-The normal Sea Level Pressure during the NEM is a large system of low pressure over the Central Bay. It can extend into the Indian Peninsula as a trough.Towards the end of November,there is a shift southwards of this extended low.

-The general variations and fluctuations in intensity of this low pressure, governs the rainfall. When the trough is well defined, and the low is well marked, rainfall over the southern peninsula is good.

-During the NEM, an occasional burst of cold air from the Siberian High develops a low pressure sysytems 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.

Most of the NEM rains depend on depressions and cyclones from the Bay towards the East Coast of India.

Some points taken from Bose's book.