IMD predicts normal monsoon at 96% LPA

India's Meteorological Department will update its forecast twice -- once in May and then in June.

India Meteorological Department (IMD), the nodal body for weather forecasts in the country, has forecast a “normal” monsoon with rainfall at 96% (with an error margin of +/-5%) of the long period average (LPA), the lower bound of the category, reflecting the weather phenomena at play this year.

There is a 35% forecast probability of a “normal” monsoon this year. (REUTERS)

LPA for monsoon season between June to September is 87 cm , based on the average from 1971 to 2020.IMD’s forecast, its first for this year’s monsoon comes a day after private met forecaster, Skymet Weather forecast “below normal” rainfall during the monsoon season.

IMD will update its forecast twice -- once in May and then in June.

There is a 35% forecast probability of a “normal” monsoon this year followed by a 29% probability of a “below normal” monsoon; a 22% probability of a “deficient” monsoon and only 11% probability of an “above normal” monsoon and only 3% of an “excess” monsoon year, IMD said.

There are 5 categories of monsoon rainfall: normal (96 to 104%); below normal (90 to 95%); deficient (less than 90%); above normal (105 to 110%) and excess (more than 110%). “There is good consensus among various models that it will be a normal monsoon year,” said M Ravichandran, secretary, ministry of earth sciences while announcing the monsoon forecast.

Below normal to normal rainfall is likely over some areas of Northwest India and parts of west-central India and some pockets of Northeast India; normal rainfall is likely over many areas of peninsular India and adjoining east-central India, east India, Northeast India and over some parts of Northwest India, IMD said on Tuesday.

If IMD’s forecast is accurate, this will be the fifth year with “normal” rainfall .

According to India’s agriculture ministry, 51% of India’s farmed area, accounting for 40% of production, is rain-fed, making the monsoon critical. With 47% of the country’s population dependant on agriculture for their livelihood (according to this year’s Economic Survey), a bountiful monsoon has a direct correlation with a healthy rural economy.

IMD’s forecat seeks to factor in three factors that could affect the monsoon.

The first is El Nino, a condition likely to develop during the monsoon season around July. The impact of El Nino, which refers to warmer than normal ocean temperatures in the east equatorial Pacific, may be felt during second half of monsoon season. El Nino has a negative effect on the monsoon.

“El Nino conditions if at all are likely to impact the monsoon during August and September. All El Nino years are not bad monsoon years. About 40% of the El Nino years in the past (1951-2022) were years with normal or above normal monsoon rainfall,” said M Mohapatra, director general, IMD.

The second is the Indian Ocean Dipole (IOD) , which is presently neutral over the Indian Ocean . The latest climate models indicate that positive IOD conditions are likely to develop during the monsoon.

Positive IOD is good for southwest monsoon over India. IOD refers to the temperature differential between the western and eastern Indian Oceans. A positive IOD has a direct correlation with a good monsoon.

The third is the extent of snow-covered area over the northern hemisphere as well as Eurasia. This was below normal between December 2022 and March 2023. Lower snow cover is favourable for the subsequent monsoon rainfall over India, Mohapatra said.

“We are expecting these two positive factors to counter the adverse impacts of El Nino, if at all adverse effects develop during monsoon,” added Mohapatra. According to IMD there is a 48% probability of El Nino conditions during June, July, August and 55% probability of El Nino during July, August and September period.

The 2023 El Nino is expected to develop following a triple dip La Nina event (2020-22). La Nina is the opposite of El Nino and is characterised by cooler currents in the equatorial eastern Pacific. Based on data from 1951 to 2022 when there were 15 El Nino years, Mohapatra said there is no one-to -ne relation between El Nino and Indian summer monsoon rainfall.

Out of total 15 El Nino years, there were six years with normal to above normal rainfall. He further added that a majority of models indicate positive that IOD is likely to develop during monsoon season.

IMD also allayed fears of any impact on crops due to a poor monsoon. “(The) Overall message to the farming community is not to worry or panic. We are issuing weekly advise and forecasts for agriculture community which will continue. For El Nino and monsoon there is no specific forecast for them since these are large systems that will cover the entire country. The forecast is normal and hence nothing to worry about,” said Mohapatra.

To be sure, the evenness of the monsoon and the timing of the rains is critical. Some of the previous monsoons have been patchy, with a few days of extreme rainfall events making up for long dry spells.

But IMD is convinced this year’s monsoon will be alright.

“Things will become clearer in May, but overall, the message is that (while) it will not be a great monsoon as it has been in the past three years, there is nothing to panic as yet. It may be a subdued monsoon,” explained M Rajeevan, climate scientist and former secretary, ministry of earth sciences.

“It’s also important to reiterate that the El Nino and the Indian Summer Monsoon do not have a master-slave relationship. They have a mutual and selective interaction. Both are very important global climate systems that interact with each other,” added Rajeevan.

IMD will issue the updated forecast for monsoon season rainfall in the last week of May . Part of the reason why early forecasts get it wrong at times is the distance from the time for which the forecast is made. Small errors in measurements fed into forecast models can become large errors for periods further away from the day on which the model is being run. All forecasts are more accurate closer to the event.

Monsoon rainfall, the lifeline of agriculture in many states, is expected to be “below normal“ to the tune of 94% (with an error margin of +/-5%) of the long-period average (LPA) of 86.86 cm for the 4-month period, Skymet Weather said on Monday.

There is a 40% chance of “below normal” rainfall (seasonal rainfall that is between 90 to 95% of LPA) during the season; 25% chance of normal rainfall (seasonal rainfall that is between 96 to 104% of LPA); 15% chance of above normal (seasonal rainfall that is between 105 to 110% of LPA); and 0% chance of excess (seasonal rainfall that is more than 110% of LPA) according to Skymet Weather.

Both Skymet and IMD have erred with their forecasts in the past.

To be sure, Skymet’s first monsoon forecasts of monsoon rainfall for the past five years have differed from the actual rainfall by 4-16% of LPA. For instance, the forecaster said monsoon rainfall would be 98% of LPA in 2022 when the actual rainfall was 106%. IMD’s recent first forecasts of monsoon have roughly the same accuracy as Skymet’s, different from actual rain by 7-13% of LPA since 2018.