May 29, 2021, 3:56 p.m.

By TWC India Edit Team

Saturday, May 29: This year, monsoon rains are likely to arrive slightly earlier than usual over Kerala, as they are expected to begin drenching the South Indian state starting next Monday, May 31. According to the India Meteorological Department’s (IMD) daily update, the southwest monsoon has now advanced into some more parts of the Maldives-Comorin area, Sri Lanka and southwest and the east-central Bay of Bengal. Its northern limit continues to approach closer to the Indian mainland’s southernmost tip, and therefore, meteorologists expect that the season’s onset over Kerala is likely to occur on May 31, 2021. This would mean the monsoon rains will commence over Kerala a day earlier than normal, as the southwest monsoon season typically sets in over God's Own Country on June 1. This latest prediction is in line with the monsoon onset forecast the IMD had released on May 14, 2021, which indicated the likelihood of southwest monsoon’s arrival over Kerala on May 31, with a model error of ± 4 days.

Wet June and a dry September in Kerala

In June, the climate models have forecast a wet start, with above-normal rainfall across the western coast of India, including Kerala. However, the rains may not drench all parts of the state evenly. Due to inland convection, the Western Ghats section is likely to receive near-normal rains during the first two weeks of June, while the coastal districts may remain drier-than-normal. Forecasts suggest that most parts of Southern India may remain dry for a while during mid-June. But, the latter half of June would be much wetter, as the Madden–Julian Oscillation (MJO) returns to a favourable phase over the Indian Ocean and the monsoon speeds up northwards, explains Leon Brown, Head of Global Forecasting Operations at The Weather Company. Both July and August are also expected to remain normal or slightly wetter for Kerala. However, in September, the state may witness a precipitation shortfall of up to 5%, as the monsoon begins retreating from the country. And despite their minute differences throughout the season, most model projections agree that Kerala will receive above-average rainfall in most places for the next four months. Meanwhile, this wetter-than-normal monsoon season in Kerala will follow a wetter-than-usual pre-monsoon season. Since the latter’s beginning on March 1, 2021, the state has collectively recorded precipitation worth 733.9 mm—a whopping 125% higher than its long-term average for this period, which stands at 326.2 mm.

Impact of Cyclone Yaas

The fact that there has been no difference in the onset date predicted by long-range and short-range forecasts may indicate that Cyclones Tauktae and Yaas had no significant impact on the monsoon winds. However, experts suggest that Cyclone Yaas indeed helped monsoon to arrive faster, and in the absence of the system, the onset would have been delayed to beyond June 2, as the MJO had been progressing eastwards. “Cyclone Yaas was most likely a result of the MJO over the Bay of Bengal last week, in addition to the high sea surface temperatures. Conversely, high sea temperatures can delay the monsoon onset since the land/sea temperature contrast drops. This cyclone reduced the temperatures by 3 to 4°C over the mid to northwest Bay of Bengal and helped the monsoon progress northwards, plus drew a lot of moisture northwards,” clarifies Leon. Last year, Cyclones Amphan and Nisarga had helped pull the monsoon trough, which resulted in earlier-than-expected monsoon onset. Therefore, in 2020, the monsoon had begun in Kerala on its normal date of June 1, despite the forecast predicting a slightly delayed arrival on June 4. The last time an early arrival was recorded in Kerala was back in 2018 when the season began on May 29.

Normal rainfall across India

As per the IMD’s first long-range forecast for the 2021 southwest monsoon season, which was released on April 16, India is likely to experience monsoon rainfall that is 98% of the Long Period Average (LPA), with a model error of ±5%. The LPA refers to the average monsoon rainfall from 1961-2010, which stands at 88 cm (880.6 mm to be precise). Accordingly, this forecast of 98% indicates that a total of around 86.2 cm rainfall is expected across the country during the entirety of this year's monsoon season.

On the other hand, IBM’s The Weather Company (TWC) has forecast a marginally wetter-than-normal season this year. As per TWC's fully-calibrated probabilistic seasonal forecast model, the total rainfall from June to September could accumulate up to 101% of the seasonal norm. While the average rainfall figures across the country are likely to remain normal, eastern and northeastern states are forecast to witness drier-than-normal conditions consistent with the trends in recent years. August is likely to remain dry for most parts of the country, except the Southern Peninsula, while the trend reverses in September, leaving southern states drier in the last month of monsoon. From June to September, all the seven northeastern states and West Bengal are likely to witness only 90 to 95% of normal monsoon rainfall as per the TWC forecast. Meanwhile, northern, western and southern regions are likely to mark a wetter-than-normal monsoon with 100 to 105% of normal rainfall.