Mumbai: The southwest monsoon arrived in the city on Saturday, June 11, meeting its normal onset date. The India Meteorological Department (IMD)’s monitoring station in Santacruz (taken as representative of the city and suburbs) recorded 41.3mm of rain in the 24 hours ending 8:30am Saturday, as pre-monsoon showers intensified under unstable weather conditions.
The IMD’s Colaba observatory recorded 61.8mm of rain in the same duration. IMD classifies heavy rain between 64.5mm to 124.4mm, while moderate rainfall is in the range of 7.6mm to 35.5mm.
Generally cloudy conditions with light to moderate spells of rain or thundershowers are likely to prevail, as per forecasts. The IMD has sounded a yellow category alert for Mumbai on Sunday, indicating high chances of thunderstorms with lightning and gusty winds of around 35kmph at isolated places. Dried conditions are expected from June 15.
Experts and IMD officials had predicted a marginally delayed onset of the monsoon this year, with wind conditions being unfavourable for its northward progress after arriving in Kerala on May 29. The monsoon’s progress had stalled since May 20. It reached Andaman and Nicobar Islands a week ahead of its usual date on May 16 but slowed down after that, arriving in Maharashtra only on June 10.
“Wind conditions have changed since then. The progress of the monsoon in the last 24 hours has become rapid, but this is not uncommon,” said KS Hosalikar, head of the IMD’s surface instrument division in Pune.
“Westerly and south-westerly winds, which are essential for transporting rain-bearing clouds over the Konkan coast, were very weak. They have picked up speed since Tuesday. There is also an offshore trough, or low-pressure area, quite close to Mumbai, which creates a favourable gradient for moisture to travel from over the Arabian Sea, though we are not seeing the typical monsoon surge,” Hosalikar added.
Despite seeing its most widespread and intense monsoon showers of the season on Friday night, Mumbai is still to overcome its rainfall deficit for the month, which stands at -54.2mm in Santacruz and -47.4mm in Colaba. Officials and experts have said that despite the normal onset, deficit rains are likely till at least June 20.
The monsoon’s arrival will not be robust this time, emphasised Akshay Deoras, an independent meteorologist and PhD candidate at the University of Reading, UK, who also expressed scepticism at the IMD’s decision to declare Saturday as the onset date. “The monsoon has certainly arrived in South Konkan, which is indicated by the presence of southwesterly monsoon winds in the lower part of the atmosphere. However, as indicated by the doppler weather radar in Mumbai, monsoon winds are yet to be established over Mumbai-MMR. This looks likely on June 13,” he said.
Moreover, rainfall activities in Mumbai-MMR are still driven by thunderstorms, which are occurring during late evening hours, another indicator of the pre-monsoon weather regime.
“The offshore trough forms when monsoon winds hit the western ghats, creating an elongated low-pressure region off the western coast of India in the lower troposphere. If this trough is active, there will be a lot of cloud cover off the western coast. Satellite images confirm that the offshore trough has formed up to the latitude of south Konkan, so I don’t see any clear indication of the monsoon’s arrival in Mumbai-MMR on 11 June,” Deoras explained.
By June 9 last year, the city had already recorded over 100mm of rain, as the monsoon had also arrived earlier than usual. Mumbai witnessed its most delayed onset in the last 45 years in 2019, when the southwest monsoon was declared over the city on June 25. In 2020, it was on June 14, in 2018 on June 9, and June 12 in 2017.