Mumbai under water: Why monsoon came early to city this year, and why it is raining so much

Indian Express

26-05-2025

As showers intensified in Kerala over the past week, the India Meteorological Department (IMD) on Saturday announced the onset of monsoon in Kerala on Saturday — its earliest since 2009.
Within a day, the southwest monsoon advanced into parts of Maharashtra on Sunday and by Monday morning, the IMD also declared the onset of monsoon in Mumbai. As the early arrival of monsoon threw life out of gear in the maximum city as several pockets reeled with water logging, traffic and slow train services, this year's monsoon onset also shattered all time records. Data show that this is the earliest that monsoon has ever arrived in Mumbai with the previous earliest monsoon onset date logged at May 29 — in the years 1971, 1962 and 1956.
For perspective, the normal date of monsoon onset in Mumbai is June 11 — making this year's arrival over two weeks ahead of schedule.
Typically, monsoon onset in Mumbai happens nearly ten days after the onset of monsoon is announced in Kerala. The normal date of onset in Kerala is June 1, following which the southwest monsoon typically advances in Maharashtra by June 6 and then ushers into Mumbai coast by June 11.
This year, however, the early advancement of the southwest monsoon threw the normal patterns out of gear with the southwest monsoons traversing from Kerala to Maharashtra within a course of 24 hours, as against the typical timeline of six days.According to Shubhangi Bhute, director of IMD Mumbai, the early onset of monsoon across Kerala and fast progression was spurred by the 'very favourable' conditions for monsoon onset. 'We observed that the atmospheric conditions were satisfying all the parameters and criterias on the basis of which onset is declared,' Bhute told Express on Sunday.
Furthermore, senior meteorologists pointed to the Madden-Julian Oscillation (MJO), which is currently active, as another crucial contributing factor. A complex ocean-atmospheric phenomena, MJO is one the most important factors influencing Indian monsoons. It originated in the Indian Ocean and a key feature is the disturbance of clouds, wind and pressure moves eastward at the speed of 4-8 metres per second. Within 30 to 60 days, MJO wind bands can travel around the world and cause significant weather changes during their movement. In a favourable phase, it can enhance rainfall over India during the monsoon season.
In its extended range forecast issued on May 22, the IMD had said that the MJO was then in Phase 4 – with an amplitude > 1 – which is indicative of strong rainfall and storms in the region.
'While the favourable MJO is one contributing factor, the cross equatorial flow is also very strong at the moment, which further brings in a lot of moisture,' added an IMD official.
Meanwhile, IMD scientists said that a low pressure area in the Arabian Sea – that developed owing to a cyclonic circulation – also further aided the fast movement of the southwest monsoon. For the record, it was this low pressure area that had ushered in heavy pre-monsoon showers in Mumbai over the past few weeks.
Why has this May been one for the history books for Mumbai?
Besides the earliest monsoon onset ever recorded in Mumbai's history, the ongoing spells have broken several other records.
In what has made this month Mumbai's wettest May in the past 107 years, the IMD's Colaba observatory recorded 295 mm rain so far this month. Prior to this, the heaviest showers in the island city had been recorded in May 1918 at 279.4 mm. Meanwhile, having already received 197 mm rain this month, this is also Santacruz's rainiest May month since 2021.
Thanks to the heavy spells, heatwaves – a typical phenomenon during May – have also eluded the city this year. In fact, earlier on May 8, Mumbai woke up also to its coldest May morning ever as the minimum temperatures dipped to 22.2°C, as per the Colaba coastal observatory, which is the coldest day ever recorded in the island city since 1951.
The early onset has come at a time when Mumbai is already reeling under a surplus of over 700 percent from the actual normal rain for the month of May. Not only the metropolis but the entire state of Maharashtra is currently experiencing an unusual May with Buldhana district having logged a surplus of over 4000 percent from its actual long period average for the region, while coastal districts like Sindhudurg and Raigad have recorded surplus of over 2600 percent and 2000 percent respectively so far.
What can Mumbai expect this monsoon season?
Having now experienced an early arrival, Mumbai can expect above normal showers this monsoon season.
The IMD had said in its long-range forecast that this year the country is expected to experience above normal monsoon, which is forecast to be '105% of the Long Period Average (LPA) with a model error of ± 5%'.
The Met Department has pointed out that the neutral El Niño Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) conditions which are slated to prevail during the monsoon, will contribute to a bountiful season.