This is rare indeed. Rains are going to continue for another 1 hour or so. The last time September got day rains was on Sept 11, 2013 and 29th September, 2011.
The rains are convective. It was triggered at mid-night and smashed Bangalore and is sustaining so long due good conditions over Chennai.
Enjoy the awesome weather. The radar pic will explain the strati-form and rain clouds.
Courtesy: Tamil Nadu Weatherman