The Southwest monsoons start retreating in the first week of September from Pakistan border in Northwest India. October and November are the months of post (or retreating) monsoon season.
The retreat of these winds takes place due to weakening of low pressure area over the northwestern parts. This happens due to low temperatures caused by apparent shift of the sun towards the equator and also owing to the widespread rains bringing down temperatures perceptibly. Consequently, the air pressure starts decreasing. Such changes in the patterns of atmospheric pressure causes Southwest monsoons to withdraw.
Hence, this period is known as the season of retreating Southwest monsoons. By the end of October, these winds retreat from most of northern India. As a result, fair weather conditions prevail over this region.
The low pressure area lying over Northwest India is transferred to the middle of the Bay of Bengal by the end of October. As a result of these unstable conditions, severe cyclonic storms originate in the Bay of Bengal. These cyclonic storms strike along the eastern coast of India causing wide spread rain in the coastal regions.
Tamil Nadu coast receives maximum of its rainfall during October and November - the period of retreating monsoons.