What caused the highest-ever single-day rainfall during December in Mumbai?

Mumbai witnessed its highest-ever December rainfall as heavy, unseasonal showers lashed the city on Wednesday. The India Meteorological Department’s (IMD) Santacruz observatory recorded 91.2 mm rainfall in 24 hours. Mumbai also witnessed its second-coldest day in a decade at 24.8°C — 8°C below normal for the period. Other Maharashtra districts such as Pune (75.4 mm) and Nashik (63.8 mm) also witnessed their highest-ever December rainfall.

Unseasonal rainfall
Rainfall during November-December is unusual in Maharashtra. Wednesday’s rainfall was caused by a cyclonic circulation over southeast and east-central Arabian Sea. Its interaction with a western disturbance — a cyclonic circulation originating in the Mediterranean, traversing across Central Asia to bring rain and snow over north India — brought a lot of moisture.

Shubhangi Bhute, scientist at the weather agency’s Mumbai office told The Indian Express that the rainfall in the winter was caused by a cyclonic circulation over the south-east Arabian Sea and adjoining Lakshadweep area that was lying over south-east and east-central Arabian Sea as on Wednesday. Most parts of Maharashtra witnessed a wet spell under the influence of these systems from November 30 to December 2.

Past record
Till date, the record for the highest-ever December rainfall in Mumbai was on December 6, 2017, when Cyclone Ockhi passed off the city’s coast. At that time, the country’s financial capital had recorded 53.8 mm rainfall in 24 hours. The city has rarely experienced heavy rainfall during December in the past decade. Mumbai recorded 1.2 mm rainfall on December 5, 2019, and 1.5 mm on December 13, 2014 — both of which are categorised as ‘light rainfall’.

Forecast
According to the Met department’s seven-day forecast, Mumbai will witness a partly cloudy sky with a spell or two of rain or thundershowers till the weekend. The IMD has forecast dry weather for the state starting next week.