Southwest Monsoon 2025 Arrives Early: IMD Declares Earliest Onset in Seven Years

The India Meteorological Department (IMD) announced on Tuesday, May 13, the onset of the Southwest Monsoon 2025 over parts of the Bay of Bengal, South Andaman Sea, Nicobar Islands, and North Andaman Sea, marking the earliest recorded onset in the last seven years. The normal onset date over the South Andaman Sea and Port Blair is May 21, making this year's arrival at least a week early. This early onset is expected to be followed by further advancement of the monsoon into more parts of India over the next three to four days.

The IMD attributed this unusual advancement to several key meteorological factors: higher-than-normal minimum temperatures over North India, strengthening of westerly and easterly winds at different atmospheric levels, consistent pre-monsoon rainfall with thunderstorms over South Peninsular India for over 40 days, and higher-than-normal pressure over the northwest Pacific Ocean. These indicators collectively signaled a stronger and earlier monsoon flow, with forecasts suggesting the monsoon may hit Kerala by May 27, ahead of the typical June 1 onset date.

Advance of Southwest monsoon.

Understanding India’s Monsoon System: Southwest and Northeast Monsoons

India experiences two major types of monsoon systems:

1. Southwest Monsoon (June–September):

This is India’s principal rainy season, beginning in Kerala and moving across the country. It brings relief from intense summer heat and contributes significantly to India’s ecosystem and economy, particularly the Kharif crop season. Over 70% of the country's annual rainfall occurs during this period, with a Long Period Average (LPA) of 880 mm. The LPA, based on 50 years of data, serves as a reference benchmark to classify monsoon performance as normal, below normal, or above normal.

2. Northeast Monsoon (October–December):

Also known as the winter, retreating, or reverse monsoon, it primarily affects peninsular India. Though less intense, it is vital for Rabi crop cultivation, particularly in Tamil Nadu, which receives nearly 48% of its annual rainfall during this season. This monsoon benefits Kerala, Puducherry, Karaikal, Coastal Andhra Pradesh, Rayalaseema, and Yanam as well.

Monsoon Classification Based on LPA

The IMD classifies rainfall intensity using the mean (m) and standard deviation (d) of historical data:

• Normal rainfall: Between m - d and m + d

• Below-normal rainfall: Less than m - d

• Above-normal rainfall: More than m + d

This classification helps predict and compare seasonal rainfall behavior across different years and regions.

What is LPA?

‘Long period average‘ (LPA) is the average rainfall recorded from June to September, calculated during the 50-year period. It is the benchmark while forecasting the quantitative rainfall for the monsoon season every year.

Climate Change and Shifting Rainfall Patterns in India

A study titled “Decoding India’s Changing Monsoon Patterns: A Tehsil-level Assessment”, by Shravan Prabhu and Vishwas Chitale from the Council on Energy, Environment and Water (CEEW), highlights significant shifts in India’s monsoon behavior due to climate change. Key findings include:

1. Traditionally dry regions like Rajasthan, Gujarat, Konkan, central Maharashtra, and Tamil Nadu saw a 30%+ increase in southwest monsoon rainfall (compared to 1981–2011 data).

2. In contrast, Assam and Meghalaya, known for heavy rainfall, experienced a decline.

3. Overall, 55% of tehsils witnessed a rise in southwest monsoon rainfall, but in the form of short, intense spells, causing flash floods.

4. Wet extremes are now accounting for a greater portion of total seasonal rainfall.

5. Rainfall is now unevenly distributed across seasons and months.

6. Retreating monsoon rainfall increased by 10% in 80% of Tamil Nadu's tehsils, 44% in Telangana, and 39% in Andhra Pradesh.

7. These changes have implications for agricultural output and local ecosystems.

IMD’s Rainfall Alert System and Mission Mausam

To prepare the public for changing and unpredictable weather patterns, the IMD uses a colour-coded weather alert system:

• Green: No warning (Light to moderate rain: 15.6 – 64.4 mm)

• Yellow: Watch (64.5 – 115.5 mm, heavy rain)

• Orange: Alert (115.6 – 204.4 mm, very heavy rain)

• Red: Take action (Rainfall above 204.5 mm, extremely heavy)

Contrary to public interpretation, a Red alert does not imply “Red Alert” as in emergency protocols, but indicates that authorities must take immediate preventive action.

As part of India’s broader weather modernization effort, Prime Minister Narendra Modi launched "Mission Mausam" on January 14, 2025. The Union Cabinet approved a ₹2,000 crore budget in September 2024 to make India a weather-ready and climate-smart nation. Key implementation agencies include:

• India Meteorological Department (IMD)

• Indian Institute of Tropical Meteorology (IITM)

• National Centre for Medium-Range Weather Forecasting (NCMRWF)

Mission Mausam includes deployment of next-gen radar and satellite systems, advanced Earth system models, supercomputing power, and a GIS-based real-time decision support system.

Regional LPA Variations and Importance

While the national LPA stands at 880 mm, this figure varies regionally. For example:

• Northwest India (drier): ~61 cm

• East and Northeast India (wetter): ~143 cm

For Kerala, LPA by month is:

• June: 556 mm

• July: 659 mm

• August: 427 mm

• September: 252 mm

National monthly LPA figures (June–September) are:

• June: 16.36 cm

• July: 28.92 cm

• August: 26.13 cm

• September: 17.34 cm

With over 2,400 locations and 3,500 rain-gauge stations, IMD relies on LPAs to smooth year-to-year fluctuations caused by events like El Niño, La Niña, or extreme climate events.

Conclusion: Above Normal Monsoon Forecast for 2025

For 2025, the IMD forecasts an ‘above normal’ monsoon, with rainfall expected to be 105% of the LPA. This is encouraging news for India’s agricultural sector, water security, and rural economy, but the uneven distribution and intensity of rainfall events, likely intensified by climate change, pose ongoing challenges.

The early arrival of the monsoon over the South Bay of Bengal and Andaman Sea, the earliest in seven years, marks a pivotal moment in the 2025 monsoon timeline and reinforces the need for enhanced monitoring, climate resilience, and preparedness across all sectors.