Rain has broken out along the southern Deccan and neighboring west coast of southern India during the last few days. Rainfall of 23 mm was observed as of Monday at Bangalore whereas sea-side Mangalore picked up about 6 cm since Sunday. This would be unusually late in the season for rain this far north.
If the lateness of the south India rain were not enough, there is more of it on the way. For help in understanding this, the reader may wish to go here to look at the IMD infrared satellite imagery.
The COLA site compiles GFS forecast rainfall ("QPF") in a fortnightly outlook:
During the first forecast week, forecast rainfall is shown for most of the west coast/Western Ghats,
even reaching into northern India and Nepal. The underlying trigger for this forecast of rain would be two separate troughs in the Westerlies forecast to cross the Subcontinent from the west.
First of these western disturbances will swing quickly across the Subcontinent during the next 24-48 hours, triggering showers and thunderstorms in the recurving stream of tropical moisture. The idea that a few spots will get heavy falls--say, 10 cm or so--seems valid to me.
Look for another western disturbance to dip through the Khaleej of South West Asia before transiting the Subcontinent at week`s end. It is this second western disturbance that could trigger a little New Year`s Day rain at Mumbai.
I do not yet know what to make of it, but the second week of the forecast is shown to be unusually wet over eastern India. It is something to watch during the coming days.
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