Late or early? Forecast agencies differ on monsoon’s arrival in Kerala

A well-marked low pressure area is seen over southeast Bay of Bengal on the morning of May 15, 2020. Photo: Twitter/@Indiametdept  

The India Meteorological Department (IMD) has forecast a delay in the arrival of the monsoon over Kerala while Skymet, a private weather forecaster, expects it to arrive earlier. The difference in the arrival dates by both agencies is as much a week.

The normal date of onset for the State is June 1. In a statement on Friday, the IMD set a date of June 5 whereas Skymet, on its website, has forecast May 28. The time of the monsoon’s arrival in Kerala does not influence its overall progress, distribution or quantum of rain over the ensuing monsoon months of June-September.

Developing cyclone

The discrepancy appears largely on the agencies’ interpretation of the influence of a developing cyclone in the Bay of Bengal as well as the prevailing summer temperatures in north India.

This storm, which is a normal feature during May, is likely to burgeon into a cyclone by the weekend, and aid the advent of the monsoon into the Andaman & Nicobar Islands. Hereon, the monsoon typically takes 10-11 days to reach the Kerala coast. G.P. Sharma, vice president, meteorology, Skymet, said that the cyclone will not hamper this routine progress and that the early onset of monsoon in the A&N Islands will therefore bring the monsoon early over Kerala.

The IMD too expects the monsoon to reach the islands by the weekend. Below-normal summer temperatures in north India and prevailing rainfall from Western Disturbances (WD) will delay the monsoon’s further progress to Kerala. WD are rainbearing systems that originate in the Mediterranean regions and bring rain to north India.

“In the last two months, WD activity has been heightened. This has lowered temperatures and isn’t helping the monsoon to establish,” D.S. Pai, Head, Climate Services, National Climate Centre, IMD Pune, told The Hindu. “The developing storm will further pull some of the monsoon winds into the Andamans but it will take more time for conditions to develop enough for it to gain strength and move into Kerala.”

Error margins

Both agencies use different models to predict the forecast date and have different error margins in their dates. The IMD’s model has an error margin of four days, while Skymet has a two day error margin. That means, were the monsoon to arrive on June 1, it would still be “accurate” by the IMD’s definition, said Mr. Sharma.

Other than factors such as windspeeds, the monsoon is considered to have arrived over Kerala only if it meets several criteria, the most important being 14 meteorological stations in Kerala and Karnataka receiving a certain minimum amount of rainfall over two consecutive days.

Last year, the monsoon came over a week late on June 8, two days after the IMD’s forecast of June 6. The last time the IMD’s forecast was out of its model’s error window was in 2015, when the IMD had said the monsoon would arrive on May 30 but it actually arrived on June 5.

Revised onset

This year, the IMD has revised the monsoon’s onset dates over several regions. The normal date for the monsoon’s entry into the A&N Islands has been May 20, which is now May 22. The date for Kerala remains unchanged at June 1. “In general, there is a delay in the new monsoon onset/progress normal dates compared to the existing normal dates over most parts of the Indian monsoon region except Lakshadweep Islands, parts of northeast India and western parts of south Peninsula, and some areas of north and extreme northwest India where monsoon advance is relatively faster in the new onset dates than the existing normal dates,” it said in a report on Friday.

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Printable version | Oct 18, 2021 9:29:37 PM |

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