This is a real-time Chennai rain map and will be updated every 15 minutes.

Here is a map I always use before I start out on a rainy day. I pull my predictions from this map. We can predict at least an hour ahead if it will rain or not.

Chennai MAX (dBZ) - 250 Kms radius

(Image source: http://www.imd.gov.in/)

The mid point of this image where the cross is marked is from where this radar image is taken. This is the Doppler radar next to the RBI building when you take the sub-way to join secretariat road. The foot ball over the building where we all would wonder while passing this place.

This is a 250 KM radius image. You also will see the 50 km, 100 Km, 150 KM and 200 KM markings. From here you can find where you are, on this map. You can relate this with Google Map if you want to.

Now how can you predict. For South West monsoon, you will see the rain clouds moving from land to the sea. For North East monsoon, you will see the rain clouds moving from the sea to the land.

The color on the map shows if it is raining heavily or just a mere drizzle. You can see the scale on the right. Here is a quick reference.

Dark Blue - Very light rain (Drizzling)
White - Moderate rain (It is raining)
Magenta - Heavy rain (It is pouring)

I also use this Animation to see how fast and in which direction the rain clouds are moving.
http://www.imd.gov.in/section/dwr/img/Animation/dop_ani_main.htm

Here set the time interval in milliseconds and the frames you want to see. This will stitch the frames to an animation which will show you the movement.

This is a crude way of finding. If you are really interested in knowing more, there is an active armature weatherman community. You can catch up with them here,
http://www.kea.metsite.com
http://forum.keaweather.org/
http://blog.keaweather.in/
https://www.facebook.com/groups/156539441183253/