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Once again, IMD, Skymet differ on rain forecast

By Sanjeeb Mukherjee
August 06, 2018 15:53 IST
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Skymet lowered its full season monsoon forecast to ‘below normal’ from ‘normal’, while the IMD is sticking to its earlier forecast that rains in June to September of 2018 would be ‘normal’ at 97 per cent of the LPA with a model error of +/-5 per cent.

Over the past week, the country has seen two different predictions on how monsoon is going to pan out across the country.


While one expected monsoon to slow down, the other forecast a drastically better picture.

Two leading weather forecasting agencies - state-run India Meteorological Department (IMD) and privately-held Skymet - have come up with different views on the behaviour of southwest monsoon in the remainder of the season.

While Skymet said that monsoon in August and September would be ‘below normal’ because of a prolonged dry spell, particularly in rainiest regions of the Northeast and the west coast, the IMD said rainfall will be normal during the period.

For August and September, IMD said the definition of normal is different and it is counted from 94-106 of the LPA, while meteorologists in Skymet questioned the change.

“Of course, it is not wrong,” countered IMD DG KJ Ramesh.

“If you see the standard deviation or error margin of June-September forecast that the IMD makes in April, it is +/- five per cent, while two months forecast of August and September, the margin of error of +/- 8 per cent. Clearly, the normals are different and so are the error margins,” Ramesh said, adding that IMD has been following the same practice for years.

For August, Skymet predicted that the southwest monsoon would be deficient at 88 per cent of the long period average (LPA) with 70 per cent probability of rains being ‘deficient’, while for September, it predicted that monsoon would be below normal at 93 per cent of the LPA.

“Already, the cumulative monsoon deficit for the entire season as we speak is about 9 per cent, and with just 50-55 days to go for the monsoon season to end how will this cover up, when a big dry spell is being expected, which our models show will happen,” Jatin Singh, CEO of Skymet said.

He said in some quarters, authorities should start preparing for drought, else they could be caught on the wrong foot.

The IMD, on the other hand, said that rains in August would not only be normal but could even be better than its forecast in June and could be about 96 per cent of the LPA.

It is yet to majke predictions for September. “We are confident that monsoon 2018 would be normal. Yes, during the last week monsoon was passing through a weak phase and the weekly shortfall was more than 20 per cent, but a fresh depression is forming over Bay of Bengal, which will again push the monsoon into active phase around the middle of August and anything beyond that is unpredictable,” Ramesh said.

Skymet lowered its full season monsoon forecast to ‘below normal’ from ‘normal’ because of poor monsoon in August and September, coupled with insufficient rains in June and July, but the IMD is sticking to its earlier forecast that rains in June to September of 2018 would be ‘normal’ at 97 per cent of the LPA with a model error of +/-5 per cent.

“End of July, the El Nino is at 0.3, which isn’t harmful, while the Indian Ocean Dipole is positive.

"The Oceanic Nano Index (ONI) is also negative so far. All these are making us confident that monsoon over the next two months will not be normal, but its distribution will also be good, which would aid kharif crops,” Ramesh said.

He said so far in June and July, despite the so-called deficiency, the monsoon has been fairly distributed or else shortfall in kharif acreage won’t have come down to less than 2 per cent as compared to last year from almost 8 per cent few weeks back.

Skymet, however, said that the sea surface warming is continuing in the Pacific Ocean and is indicating towards an evolving El Nino, while IOD has not influenced Monsoon 2018 so far and has remained neutral since the season’s onset.

It is not the first time that IMD and Skymet have differed over their forecasts. In 2017, Skymet in its forecast had expected August rains to be 7 per cent below normal, while in September it said that rains would be 4 per cent below normal.

The IMD in its updated forecast said that August rains would be just 1 per cent below normal while in September too the monsoon would be normal. The forecast was with a model error of +/-9 per cent.

In the reality, the August rains were 13 per cent below the normal, while September showers were 12 per cent below normal.

Last year too, a big and extended break in monsoon in the last two months after fairly good beginning jeorpardised the cumulative average rainfall.

How it turns out in 2018 remains to be seen.

Photograph: Ajay Verma/Reuters

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Sanjeeb Mukherjee in New Delhi
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