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IMD predicts more rain this monsoon

IMD said that rainfall will be ‘normal’ and likely to be 98% of the Long Period Average. Photo shows a sudden spell of rainfall that hit Chennai on June 6, 2017.

IMD said that rainfall will be ‘normal’ and likely to be 98% of the Long Period Average. Photo shows a sudden spell of rainfall that hit Chennai on June 6, 2017.   | Photo Credit: R. Ragu

Update says rainfall will be ‘normal’; private forecaster Skymet sticks to ‘below normal’ forecast.

The country is likely to get more rain than was originally forecast in April, the India Meteorological Department (IMD) has said. Rainfall would be ‘normal’ and around 98% of the Long Period Average (LPA), the IMD said in an update on Tuesday. This is 2% more than the 96% or ‘near normal’ rain it had forecast in April.

In the update, the IMD also said rains in July and August, the most important monsoon months for the kharif crops, would be 96% and 99% respectively, of what was normal.

Spatially too, the IMD expects a balanced geographical distribution. The season rainfall is likely to be 96% of the historical average in north-west India, 100% of the LPA over central India, 99% of the LPA over the south peninsula, and 96% of the LPA over north-east India, with a model error of ± 8 %. (The LPA is a 50-year average of the monsoon rains in India.)

El Nino is weaker than anticipated, says IMD

The IMD’s optimism about more rainfall is largely premised on hopes that a strong El Nino, which, as per its earlier forecast, was expected to surface in the later half of the monsoon, would now be much weaker than anticipated. “A weakened El Nino is largely why we expect better rains,” said Madhavan Rajeevan, Secretary, Earth Sciences Ministry.

In April, the IMD had said there was a 38% chance of near normal rains (96% of the LPA). Now the models showed a 50% chance.

The El Nino — characterised by surface waters of the equatorial Pacific warming up by more than half a degree — is known to dry up monsoon rain every six out of 10 years. A positive Indian Ocean Dipole (IOD) is said to buffer the impact of El Nino and contribute to better rains. (The IOD is a swing in surface temperatures that turns the western Indian Ocean alternately warmer and then colder than the eastern part of the ocean.)

New model

In April, the IMD shifted to using a new monsoon forecast system, called a dynamical model that works by supercomputers simulating the weather and extrapolating it.

It plans to make this as the base for all future forecasts, ranging from short-term weekly forecasts to the trajectory of the four-month- long monsoon. However, for its June update, the IMD chose to rely on its workhorse statistical model that forecasts the monsoon based on six meteorological parameters.

The dynamical model, according to the IMD statement, showed monsoon rains to be 89 cm or 100% of the LPA. “In April, both models showed the same. It’s good for computing the all-India figure but not yet good at capturing the regional spread,” said Mr. Rajeevan. “In the next few years, we hope to move entirely into the dynamical mode.”

Private weather forecaster Skymet said it was sticking to its “below normal” forecast at 95% (with an error margin of +/-5%) of the LPA. Rainfall for July stood at 94%, while for August it was 93% of the historical average.

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Printable version | Jul 11, 2020 11:31:53 AM | https://www.thehindu.com/news/national/india-likely-to-get-more-rain-says-imd/article18730851.ece

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