October 20th marks the date under normal circumstances the onset of Northeast Monsoon is expected to start according to IMD with a margin of +/- 7 days. Often it coincides with the withdrawal of Southwest Monsoon from the country with a gap of couple of days. Southwest Monsoon officially has withdrawn from the country as of yesterday but this year Northeast Monsoon could see a delayed onset due to reasons explained in the post. Read on.
Last night saw scattered rains over many parts of South TN and some areas of Western Tamil Nadu. Neyyoor in Kanyakumari district saw 70 mm rains fall yesterday while Kangeyam in Tiruppur district saw 29 mm rains fall through the day.
As explained a few days back in our Northeast Monsoon – Subdued Onset post the wind pattern has not yet completely changed to Easterlies, if one sees the wind pattern during the onset of Northeast Monsoon 2014 we can see a clear Easterly wind pattern establish over Bay of Bengal 
On the contrary thanks to Typhoon Koppu we are expected to see disturbed wind pattern over the Bay with possibly the lower level winds being Westerlies for a few days in Bay of Bengal thereby preventing good Easterlies to move into the Indian Sub Continent. What most parts of Southern Peninsular would be getting is Northerlies with some dry continental air moving in from Central India preventing the onset of Northeast Monsoon. The onset is likely to be delayed between 5 – 7 days as thing stand. 
In the meanwhile parts of South Tamil Nadu could get isolated showers today as well though the rains could slow down for most parts of Southern Peninsula from today.
Chennai Weather Update: Partly cloudy skies in the morning with day time temperature around 34°. Expect the day to be hotter than the last couple of days with high humidity making it a possibly uncomfortable afternoon.