With southwest monsoon withdrawing faster than expected, atmospheric conditions are slowly becoming favourable for the onset of northeast monsoon. However, it will take at least one more week to meet the criteria required for the declaration of the onset of northeast monsoon, also known as Thulavarsham, which is marked by evening thunderstorms and lightning.
According to the onset criteria of northeast monsoon, southwest monsoon should have withdrawn up to coastal Andhra Pradesh. Further, peep easterlies (up to 850 hPa) should have set in over Tamil Nadu or seasonal low should have been established in the South Bay of Bengal adjacent to Tamil Nadu coast. Now the line of withdrawal has almost crossed Maharashtra and parts of Karnataka, and conditions are favourable for the further withdrawal of southwest monsoon from the remaining parts of the country.
Though the eastern parts of Kerala, especially the Ghat region, started to receive isolated thundershowers, these rains are triggered by the remnants of the southwest monsoon and systems over the Bay of Bengal which are likely to continue till the easterlies begin trending towards the southern peninsula. Similarly, the scattered rain witnessed in parts of north India in the past few days was triggered by a western disturbance, said India Meteorological Department (IMD) sources.
According to the data available with the IMD, the average onset date of northwest monsoon is October 19 with a standard deviation of plus or minus eight days as per the long period average from 1901 to 2022. It means the onset can happen any time between October 11 and 27.
The highest number of onsets was on 15 October (11 times) followed by 19 October (9 times). The most delayed onset was on November 11 (1915) and the earlier onset was on October 4 (1952, 1966, 1984, and 1999). In 2021 and 2022, the onset was on October 25 and 29 respectively.
With Kerala witnessing the third-largest deficient southwest monsoon in 2023 with a 34% drop in rains, the State is pinning hopes on the northeast monsoon to bridge the shortfall. Farmers in central Kerala have asked the Irrigation department to convene an urgent meeting with farmers’ representatives in the wake of the poor storage in reservoirs and to avoid the situation of water being drained to the sea when farming activities commence there.
