The disturbance over Bay of Bengal has moved to better conditions in South Central Bay and has picked up some intensity over the last 24 hours or so. Though it is still lacking in a proper structure yet it has moved into an area with better shear conditions which should aid it to build some organization & intensify into a possible depression shortly.
The Depression is expected to move in a Northerly direction along the coast of Tamil Nadu, this northward movement should give some decent rains to coastal Tamil Nadu aided partly by the moisture incursion from Western Pacific.
It is unlikely the depression would become a major system on account of the existence of the Dry Air Impact slightly north of Chennai latitude.
The more it comes closer to the coast the better the rain prospects for Chennai & Tamil Nadu. Tonight could see some decent rains for Chennai with the rains picking up from tomorrow and peaking early morning on 30th December. Isolated Heavy rains are possible for North Tamil Nadu starting tomorrow.