At present, a low,mid-level circulation is seen over a Sea level LOW pressure system over S-W Bay near to East Srilanka coast.
Present pressure is around 1004mb
And location is 7.5N , 83.3E.
GFS model doesn't expect the system to deepen to Depression (or) move North or N-W before Monday morning. And after that, it'll deepen to a Depression or Well marked LOW and drift into central Tamilnadu coast on 30-Dec.
NAVGEM model almost similar to GFS, expects the system to become Well Marked LOW on Monday and drift into central Tamilnadu coast on 30-Dec.
On Tuesday, 30-Dec... an upper-level W.D trough is expected to drop upto 21N along S Pakistan, W,N-W Gujarat. This is expected to drift East into India in subsequent days, this can make the S-W Bay system to drift North initially and then N-E along S-E coast of India into N Bay.
According to GFS, Showers will reach central,N Tamilnadu coast and even Chennai after morning or noon of Sunday, 28-Dec.
On Monday, HEAVY rain expected along central,S-E,N Tamilnadu coast and moderate rain into Chennai.