India Meteorological Department (IMD) has said the probability for a normal monsoon (96-104 per cent of the long-period average, LPA) this year is 40 per cent against the 65 per cent predicted by leading private forecaster Skymet Weather.

Climatologically, the probability for a normal monsoon is 33 per cent. Climatological probability is based solely on the climatological statistics for a region rather than dynamical implications of current conditions. This is generally used as a baseline for evaluating the performance of weather and climate forecasts.

Below-normal probability

IMD’s first long-term forecast put out on Thursday also said there is an elevated 26 per cent chance of a below-normal monsoon (against a climatological probability of 17 per cent), which was on par with Skymet Weather’s 25 per cent.

The IMD said chances of “deficient rainfall” this year are at a slightly lower 14 per cent (climatologically 16 per cent) while “excess rainfall” has only a five per cent probability (climatologically 17 per cent) in a scenario clouded by statistical uncertainties. Quantitatively, the IMD said rainfall is expected to be 99 per cent of the LPA with a model error of +/- five per cent. Skymet had just the previous day put the quantum at 98 per cent with a margin error of +/- five per cent.

Bleak ‘excess’ rain outlook

One defining feature in both the outlooks was the poor or practically nil confidence in the probability of excess rainfall. IMD assessed this probability at only five per cent and Skymet at zero against the climatological probability of 17 per cent.

Mrutyunjay Mohapatra, Director-General, IMD, said in Delhi on Thursday that forecasts indicated normal to above normal rainfall for northern parts of Peninsular India and adjoining Central India, along the foothills of the Himalayas and parts of North-West India. Below normal rainfall is likely over many areas of North-East India, some areas of North-West India and southern parts of the South Peninsula.

Spatial, temporal scales

GP Sharma, President, Meteorology and Climate Change, told BusinessLine that the raised probability for a below-normal monsoon must be viewed against the fact that the seasonal weather system takes until August-end to run up the entire country. “During June, the monsoon would not have even touched parts of North-West India while by September 1, it would be time for it to start withdrawing from Rajasthan,” Sharma pointed out.

This effectively leaves only July and August as the most productive months. Skymet has forecast monsoon to be above-normal at 107 per cent of LPA in June; 100 per cent of LPA in July, the rainiest month; below-normal at 95 per cent of LPA in August, normally the second rainiest; and deficient at 90 per cent in September.

Intense wet, dry spells

Skymet sees rainfall deficit panning out from parts of Gujarat, central parts of Maharashtra, Madhya Pradesh and Chhattisgarh. The season will be marked out by periods of intense but short wet spells and hot and dry interludes. The IMD forecast statement said currently, helpful La Nina conditions are prevailing over the Equatorial Pacific region. The latest climate model forecast indicates that La Niña conditions are likely to continue during the monsoon season.

Closer home, ‘neutral’ IOD conditions prevail over the Indian Ocean and latest forecasts indicate that these conditions may continue until the beginning of monsoon. Thereafter, there is enhanced probability of a negative Indian Ocean Dipole (IOD) to evolve, with conventionally adverse implications for the monsoon.

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