It said quantitatively, the southwest (summer) monsoon rainfall is likely to be 99% of the Long Period Average (LPA) with a model error of +/- 5%. Northeast India, some parts of northwest India and southern parts of the South Peninsula will, however, receive ‘below normal’ rainfall. But it’ll not affect the agriculture operations as the entire rainfed areas of the country will get normal to above rainfall.
The LPA of the seasonal rainfall over the country as a whole for the period 1971-2020 is 87 cm. The monsoon is considered normal if it falls under 96 to 104% of LPA. The Met department predicted that there is 40% forecast probability of ‘normal’ rainfall, 15% probability of ‘above normal’ and 5% probability of ‘excess’ rainfall – which means there is 60% probability of good rainfall in the country during the summer season (June-September).
The spatial distribution suggests normal to above normal seasonal rainfall is most likely over many areas of northern parts of Peninsular India and adjoining Central India, over foothills of the Himalayas and some parts of Northwest India.
On Thursday, the IMD also introduced a new benchmark figure based on 1971-2020 data, showing marginal decline in monsoon rains and overall annual showers in the country over the long period of time. Certain parts, including northeast India, showed relatively higher decline which can be attributed to climate change.
Under the new ‘rainfall normal’ figure, all-India rainfall for the southwest (summer) monsoon is 8cm, based on 1971-2020 data, as against the earlier normal figure of 88cm based on 1961-2010 period. Till 2018, the IMD used 89cm, based on a 50-year average from 1951-2000, as the ‘normal’ rainfall benchmark for the monsoon season.
The revision is part of a periodic update on the normal rainfall figures. New rainfall normal has been computed using rainfall data of 4,132 rain gauge stations well distributed over the country, representing 703 districts. Though IMD director general M Mohapatra attributed the decrease in average normal rainfall to “natural multi-decadal epochal variability of dry and wet epochs” of allIndia rainfall, he did not rule out the possibility of impact of climate change.
“Presently the southwest monsoon is passing through a dry epoch which started in the decade of 1971-80. The decadal average of all India SW monsoon rainfall for the decade 2011-20 is -3.8% from the long-term mean. The next decade ie 2021-30 is expected to come closer to neutral and southwest monsoon would enter into the wet epoch from the decade 2031-40, ”said Mohapatra in an indication that wet spells may increase in the future.
“This rainfall normal is prepared based on the data for the period of 50 years and is updated periodically once in every decade. The new updated rainfall normal has been prepared based on data of 1971-2020 and will replace the existing rainfall normal based on 1961- 2010 with effect from the southwest monsoon season 2022, ”said Mohapatra. The IMD’s findings show that Gujarat region receives maximum ie 96% of its annual rainfall in SW monsoon while Tamil Nadu receives maximum ie 48% of annual rainfall in NE monsoon season (October – December) followed by 36% in southwest monsoon season.
The IMD will issue the updated forecasts for monsoon season rainfall in the last week of May. In addition to updates for the April forecast, forecasts for monsoon season (June-September) rainfall for four geographical regions, monsoon core zone, and forecast for the month of June will also be issued.