The announcement came after three days of widespread rainfall across Mumbai and surrounding areas, which led to waterlogging, traffic snarls and an orange alert from the weather department.

The southwest monsoon typically reaches Kerala around June 1 before advancing northwards across the country. So, how does IMD declare the arrival of the monsoon? How do pre-monsoon showers differ from monsoon rains?

What are pre-monsoon showers?

Pre-monsoon showers in Mumbai refer to rain or thundershowers before IMD officially declares the southwest monsoon over the city. IMD’s seasonal calendar defines the pre-monsoon period as March-May, while for Mumbai reporting, early-June rain before the onset date is often described as pre-monsoon activity.

IMD’s formal onset criteria are framed first for Kerala, and then the monsoon’s further advance is tracked through the Northern Limit of Monsoon (NLM). The criteria include three broad signals:

Rainfall: After May 10, at least 60 per cent of 14 specified stations in and around Kerala, Lakshadweep and coastal Karnataka must record 2.5 mm or more rainfall for two consecutive days.

Wind field: Westerly winds must be established deep enough, up to 600 hPa, with lower-level zonal winds of around 15-20 knots at 925 hPa over the specified Arabian Sea region.

Clouding/convection: INSAT-derived Outgoing Longwave Radiation (OLR) should be below 200 W/m², indicating deep convective clouding associated with organised monsoon conditions.

When did monsoon arrive in Mumbai over the last five years?

According to the IMD, Mumbai’s normal monsoon onset date is around June 11, but the actual arrival has varied sharply in recent years, ranging from May 26 to June 25.

How much rain has Mumbai received in the last 48 hours?

On June 23, the Colaba observatory recorded 49.0 mm of rainfall, while Santacruz received 12.3 mm. On June 22, Colaba recorded 12.8 mm of rainfall, while Santacruz received 18.8 mm.

Is heavy rain alone the criterion for monsoon onset?

In a telephonic conversation with Business Standard, IMD Director General of Meteorology Dr Mrutyunjaya Mohapatra said the weather department examines three key indicators before announcing the onset of the southwest monsoon over Mumbai. These include:

• 
  Rise in rainfall that persists for at least two days
• 
  Presence of southwesterly winds
• 
  Sufficient depth in the southwesterly wind flow

In other words, heavy rainfall alone is not enough for the weather department to declare monsoon onset.

With the monsoon now officially over Mumbai, weather officials expect rainfall activity to increase across the city and adjoining parts of Maharashtra in the coming days. The onset also comes as a relief after an unusually dry June that had raised concerns over reservoir levels and water availability in the financial capital.