On 26-Dec, a weak upper-level WD trough is expected to reach N Pakistan ... https://d2jhuj1whasmze.cloudfront.net/photos/normal/ftSai.jpg
Now, a WD upper,mid-level trough seen along Nepal and Uttarpradesh.
And, this is expected to drift east into NE States of India in 24hrs.

Till 8:30am of 24-Dec, many zones of N,NW,N-central India witnessed COLD day and night conditions.
COLD Christmas day morning expected over many zones of N,N-central and central (Madhyapradesh) India ... https://d2jhuj1whasmze.cloudfront.net/photos/normal/ftStr.jpg
Due to next WD, Saturday morning temp can be +1 or 2C than today over N,NW India., but Madhyapradesh will have COLD day, night.

From noon, evening of 26-Dec, scattered snow / rain for Kashmir and into N Himachal ... https://d2jhuj1whasmze.cloudfront.net/photos/normal/ftSG8.jpg

26-Dec, light / moderate rain expected for S Andhra, N-coast Tamilnadu, Chennai

Analysis show that the easterlies has reached upto S,S-central Bay with subdued moisture content .. https://d2jhuj1whasmze.cloudfront.net/photos/normal/ftOBE.jpg
This easterlies is expected to reach Tamilnadu coast on 26-Dec..
With less moisture over N,central coast and better content over SE,S TN.
Due to easterlies, moderate to heavy rain expected for SE,S Tamilnadu and some zones of central-coast on 27-Dec ... https://d2jhuj1whasmze.cloudfront.net/photos/normal/ftP49.jpg

Today, at low-levels, a LWD and a weak circulation seen over S Chatisgarh, S Odisha and adjoining NE Andhra.. https://d2jhuj1whasmze.cloudfront.net/photos/normal/ftQ4c.jpg
Due to this, good low-level moisture seen along Odisha, NNW Bay, NE Andhra and S Chatisgarh ... https://d2jhuj1whasmze.cloudfront.net/photos/normal/ftQda.jpg
11:40pm, 24-Dec.
S,central Odisha is cloudy with light rain and cloudy over S Bengal ... https://d2jhuj1whasmze.cloudfront.net/photos/normal/ftQj7.jpg
In 36hrs, this moisture along Odisha coast is expected to be pushed S,SSW along SE coast India by anti cyclone over W India.
GFS expects this low-level moisture to be pushed upto S-coast Andhra and N-coast Tamilnadu, #Chennai today evening.. https://d2jhuj1whasmze.cloudfront.net/photos/normal/ftQEO.jpg

This moisture along with easterlies is expected to pop low intensity showers along S,central coast Andhra from midnight of 25-Dec.
On 26-Dec, 1 or 2 light / moderate scattered rain expected over S-coast Andhra, N-coast Tamilnadu, #Chennai .. https://d2jhuj1whasmze.cloudfront.net/photos/normal/ftQSA.jpg
In the meantime, 26-Dec, easterlies will start giving showers along central-coast Tamilnadu.
Rain will be scattered, 1 or 2 will be heavy !
Shower activity along #Chennai and N-coast Tamilnadu is expected to persist only till mid-morning or noon of Sunday, 27-Dec !
Due to presence of real conditions in upper-levels over S-coast Andhra, N-coast TN, rains will be of less intensity ... https://d2jhuj1whasmze.cloudfront.net/photos/normal/ftRgn.jpg