Monsoon 'likely to set in' over Kerala around June 4: IMD

📌 Summary:

• IMD on June 2, 2026 revised its forecast: Southwest monsoon now "likely to set in" over Kerala around Thursday, June 4
• Original May 15 forecast had predicted onset on May 26 with a ±4-day model error — June 4 arrival overshoots even the upper bound (May 30)
• First operational onset-forecast miss for IMD since 2015 (when May 30 forecast slipped to June 5); model has been accurate every year 2005-2025 barring that single lapse
• Normal onset date for Kerala = June 1; June 4 is a marginal delay
• Official onset criteria: ≥60% of 14 designated Kerala/Karnataka stations must record ≥2.5 mm rainfall on two consecutive days, plus specific wind (westerly at 600 hPa) & outgoing longwave radiation (OLR) thresholds
• Delayed onset can compress kharif sowing window for paddy, pulses, oilseeds; affects reservoir filling and hydro-power generation

🎯 UPSC Relevance: GS1 — Indian monsoon system, climatology, weather forecasting; GS3 — Agriculture's dependence on monsoon; food and water security.

📝 Prelims Facts:

• IMD = India Meteorological Department, under Ministry of Earth Sciences (MoES); founded 1875; HQ: New Delhi
• Normal monsoon onset date for Kerala: June 1
• Onset criteria parameters: rainfall over 14 enlisted stations, westerly winds at 600 hPa over 925-700 hPa layer, OLR
• IMD onset-forecast model in operation since 2005
• Southwest monsoon contributes ~75% of India's annual rainfall

🔑 Key Term: Outgoing Longwave Radiation (OLR) — Energy radiated by the Earth back to space in the infrared band; low OLR over the Bay of Bengal/Arabian Sea indicates deep convective cloud cover (a key monsoon-onset signal).