Why Is It Raining So Heavily in May and What Triggered Early Onset of Monsoon 2025?
The India Meteorological Department (IMD) declared the onset of the southwest monsoon over Kerala on May 24, and entry into Maharashtra the very next day, i.e. May 25 2025, marking an early arrival—eight days ahead of the usual June 1 schedule. This early monsoon onset, the first since 2009, brings significant implications for India’s agriculture and economy, as the monsoon contributes over 70% of the country’s annual rainfall.
How is the Monsoon Onset Declared?
The IMD follows strict criteria to declare the monsoon onset, including:
- Rainfall: At least 60% of 14 designated stations in southern India must record ≥2.5 mm rainfall for two consecutive days.
- Wind Field: Westerly winds (15-20 knots) must extend up to 600 hPa (hectoPascals).
- Outgoing Longwave Radiation (OLR): Satellite data must show OLR values below 200 W/m², indicating favorable atmospheric conditions.
This year, the monsoon arrived simultaneously over Kerala, Lakshadweep, Mahe, parts of the Arabian Sea, the Bay of Bengal, southern Karnataka, and even Mizoram in the northeast.
What Caused the Early Monsoon in 2025?
Several large-scale atmospheric and oceanic factors contributed to the early onset:
- Madden-Julian Oscillation (MJO): A weather-enhancing phenomenon that amplified rainfall.
- Mascarene High: A high-pressure system near the Mascarene Islands intensified monsoon winds.
- Convection: Increased vertical heat and moisture transport triggered early rains.
- Somali Jet: Strong cross-equatorial winds accelerated monsoon flow.
- Heat-Low Over Pakistan: A low-pressure zone pulled moist monsoon winds inland.
- Monsoon Trough: An elongated low-pressure system extended from Pakistan to the Bay of Bengal, boosting rainfall.
Monsoon Progress: How Far Has It Advanced?
By May 25, the monsoon had already covered:
- Kerala, Karnataka, Goa, Parts of Maharashtra, Tamil Nadu (partial), Lakshadweep, and Mizoram.
- Arabian Sea & Bay of Bengal: Early advancement due to strong wind systems.
The Northern Limit of Monsoon now stretches from Devgad (Maharashtra) to Kohima (Nagaland), indicating rapid progress.
Why Does This Matter?
An early monsoon onset can benefit agriculture but also raises concerns about extreme weather events. With climate patterns shifting, understanding these factors helps policymakers and farmers prepare better.
