India’ Expects Normal Rains from SW Monsoon

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April 25, 2022SANDRP

India Meteorological Department (IMD) has in their first forecast for India’s crucial South West Monsoon (June-Sept) 2022 has forecast that India will receive 99% of Long Period Average (LPA) rain with a model error of +/- 5%. This means that IMD has forecast on April 14, 2022 that India is likely to get normal rainfall (96% to 104% of LPA) rains for the fourth consecutive year. IMD says that currently La Nina conditions are prevailing and are likely to continue during the monsoon.

The forecast says that June-Sept 2022 rainfall is likely to be normal (96-104% of LPA) with 40% probability, below normal (90-96% of LPA) with 26% probability, Deficient (below 90% of LPA) with 14% probability, above normal (104-110% of LPA) with 15% probability and surplus (above 110% of LPA) with 5% probability.

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. Below normal rainfall is likely over many areas of Northeast India, some areas of Northwest India and southern parts of the South Peninsula.

IMD will issue the updated forecasts for monsoon season rainfall in the last week of May 2022. In addition to update 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 also will be issued.

IMD’s reduced LPA However, IMD has reassessed the LPA rainfall and revised it 12 mm downwards from 880.6 mm (based on rainfall during 1961-2010) to 868.6 mm (based on rainfall in 1971-2020). Till 2018, the IMD used 89cm, based on a 50-year average from 1951-2000, as the LPA for the SW monsoon.  The New All-India ANNUAL rainfall normal based on rainfall of 1971-2020 is 1160.1 mm, 16.8 mm lower compared to earlier normal of 1176.9mm based on data of 1961-2010.

The IMD says this decrease is part of natural multidecadal epochal variability of dry and wet epochs of all India rainfall. Presently the SW monsoon is passing through dry epoch which started since 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 i.e. 2021-30 is expected to come closer to neutral and SW monsoon would enter into the wet epoch from the decade 2031-40.

Based on data of 1971-2020, the SW monsoon rainfall over India contributes 74.9% to the Annual Rainfall. June, July, August, and September contribute 19.1%, 32.3% (280.4 mm), 29.4% (254.9 mm) & 19.3% respectively to the total SW monsoon seasonal rainfall. IMD says these figures remained unchanged as compared to those based on data of 1961-2010. All India pre-monsoon season (March-May) rainfall (130.6mm) and post-monsoon season rainfall (121.0mm) contribute around 11% and 10% respectively to the annual rainfall over India.

Gujarat region receives maximum rainfall i.e. 96% of it’s annual rainfall in SW monsoon season. Tamil Nadu receives 48% of annual rainfall in northeast monsoon season (Oct-Dec) followed by 36% in southwest monsoon season.

The new LPA has been computed using rainfall data 4132 rain-gauge stations “well distributed over the country representing 703 districts of India”, according to IMD. This means IMD uses data from on an average less then six rain-gauge station in a district. That is clearly highly inadequate considering the diversity of meteorological, topographic and other aspects within most districts in India.

Skymet Forecast Earlier on April 12, 2022, the private weather forecasting agency, Skymet, said that the southwest monsoon in 2022 is expected to be ‘normal’ at 98 percent of the LPA (880.5 mm) with an error margin of 5 %. Skymet said the first half of the SW monsoon (June-July) is expected to fare better than the latter half (Aug-Sept).

– Rajasthan and Gujarat along with Nagaland, Manipur, Mizoram, and Tripura are at risk of being rain deficient throughout the season. Kerala and North Interior Karnataka will witness scanty rains in July-August. But, Punjab, Haryana, and Uttar Pradesh, in North India, and rainfed areas of Maharashtra and Madhya Pradesh will witness above normal rainfall.

– Rainfall in June is expected to be 107 of the LPA of that month, while rainfall in July is expected to be 100 per cent of the LPA. In August, rainfall across India is expected to be around 95 per cent of the LPA while in September, rains are expected to be 90 per cent of the LPA.

IMD’s forecast performance in recent decades Past data reveals that IMD’s monsoon predictions are more often than not off the mark although, its long-range forecasting skills are improving. IMD’s first long range forecasts (LRFs) of the monsoon are made in April and carry a 5 percentage point margin of error (plus/minus). In 2021, the agency’s first LRF of 98% seasonal rainfall was bang on, with the monsoon ending up at 99% of the LPA. In the decade (2011-2020), monsoon rainfall was outside the forecasts’ margin of error in eight of the 10 years. The two years when the forecasts came true were 2011 and 2017.

The average deviation of the forecasts from actual rainfall during 2011-2020 was 7.1 %, higher than the error margin of the forecasts. This was an improvement over the previous decade (2001-2010), when the forecasts deviated from actual by 8.6 % on average. During that decade, monsoon rainfall was within the forecast’s 5 percentage-point error margin on three years (another year, 2003, had a 6 % deviation but both forecast and actual rainfall were in the normal range of 96%-104%). However, the 3-4 accurate forecasts in the 2001-2010 decade were offset by some spectacular failures. IMD’s forecasts were off the mark by 10 percentage points or more in four years, as opposed to one in 2011-2020. These included years 2002 and 2009, when the forecasts were off by 20 and 19 percentage points, respectively, and 2004, when the forecast deviated by 13 percentage points. This led to a major overhaul of the forecast methodology in 2007.

Madhavan Nair Rajeevan, a veteran meteorologist who recently retired as secretary, Union earth sciences ministry pointed out that the accuracy of forecasts needs to be seen in the light of variability of monsoon rainfall. For instance, in the 1991-2000 decade, the average deviation of IMD forecasts from actual rainfall was 6.7 %, better than the corresponding figures in the next two decades. But the 1990s was a decade when the monsoon was normal in most years, with no drought year. Thus, monsoon variability was low and the probability of forecasts coming true that much higher.

Taking into account rainfall variability (by dividing the average forecast deviation with variability, that is, standard deviation), the accuracy of IMD forecasts during 1991-2000 comes to 1.08 – where 1 is the climatological probability and values higher than 1 are worse than the climatological probability. The lower this value is below 1, the higher the accuracy of the forecast. For the 2001-2010 decade, the forecast accuracy comes to nearly 0.9 and for 2011-2020 it’s 0.82. This means, despite higher variability in the monsoons, IMD’s forecasts have been improving down the decades.

SANDRP (ht.sandrp@gmail.com)