Met dept: Monsoon onset over Kerala delayed by a week

In a statement, the IMD said the statistical model forecast it uses suggests that the monsoon onset over Kerala is to be delayed slightly and is likely to set over Kerala on June 7, with a model error of plus or minus four days.

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S Pai, Deputy Director General (Climate division), Pune IMD, said: “Of the six predictors, five predictors that are used in the statistical model indicate delayed monsoon over Kerala. These predictors indicate the seasonal transition. Pre-monsoon showers to monsoon showers show lagging in these predictors, so we say that there is delay of onset over Kerala. However, we cannot rule out the possibility of a depression forming in the Bay of Bengal which will help monsoon to set over south Andaman Sea around May 17 and over north Andaman Sea on May 20. This will cause increased rainfall activity in the southern peninsula.”

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The onset of southwest monsoon over Kerala represents the start of the rainy season. The normal date for monsoon onset over Kerala is June 1. Since 2005, IMD has been issuing operational forecast for the onset using an indigenously developed statistical model with a model error of plus or minus 4 days.

The model uses the following six predictors: minimum temperatures over north-west India, pre-monsoon rainfall peak over south peninsula, outgoing long wave radiation (OLR) over South China Sea, lower tropospheric zonal wind over southeast Indian Ocean, upper tropospheric zonal wind over east equatorial Indian Ocean, and outgoing long wave radiation (OLR) over the south-west Pacific region.

The forecast of the monsoon onset issued in the last 11 years (2005-2015) except 2015 proved to be correct. The verification of the forecast for the last five years is given below (see box).

“We have had that one exception as there was a system in the Arabian Sea last year so the monsoon got delayed,’’ Pai said.

The verification of the forecast for the last 5 years is given below.

The southwest monsoon advances over the Andaman Sea usually around May 20 with a standard deviation of about one week. The IMD has observed strong convective activity over the equatorial Indian Ocean for the last several days. A well-marked low pressure area has formed over the Indian Ocean which will concentrate into a depression and move north-westward.

The IMD said that the conditions are becoming favourable for the onset of southwest monsoon over Nicobar Islands, south Andaman Sea and parts of south Bay of Bengal around May 17 and advancement over the entire Andaman Sea close to its normal date.

It also said that past data suggests that there is no association of the date of monsoon advance over the Andaman Sea either with the date of monsoon onset over Kerala or with the seasonal monsoon rainfall over the country.
Meanwhile, private weather forecasters issued an early forecast date of May 29. Last week, Skymet issued the onset date as May 29. Other forecasters said a delay in monsoon is noticed in the years after El Nino year.

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