The onset of Northeast Monsoon is closely linked with the prevailing oceanic conditions over the Indian Ocean (IOD) and Pacific Ocean (El Nino / La Nina). A set of conditions need to be in place for declaring the arrival of Northeast Monsoon:
- Withdrawal of Southwest Monsoon up to latitude 15°N
- Onset of persistent surface Easterly winds over Tamil Nadu coast
- Depth of Easterly winds up to 5000 feet over Tamil Nadu coast
- Fairly Wide Spread rainfall over Coastal Tamil Nadu, South Coastal Andhra Pradesh and adjoining area
Northeast Monsoon onset is not declared before 10th October even if conditions described exist. The change over of winds over the Bay of Bengal off Tamil Nadu and Andhra Pradesh coast commence around 14th Oct and steady flow of Easterly/ Southeasterly winds sets in thereafter. The normal date for onset of Northeast Monsoon is 20th October. However, there are aberrations and the onset invariably gets delayed as it is happening this season also. In the last 15 years, only thrice the onset was before date: 19 October (2012), 18 October (2014), and 16 October (2019). The latest onset was on 01st November in 2018.
The frequent formation of weather systems in the Bay of Bengal is delaying the retreat of Southwest Monsoon. Consequentially the onset of Northeast Monsoon also waits in the wings pending withdrawal of the former one. The appearance of a fresh low pressure over the Bay of Bengal is further going to stall the withdrawal and accordingly, the arrival of Northeast Monsoon may roll over towards last few days of this month or even later.