The date when the monsoon arrives at a location, marking the transition from dry hot summer to the wet season. India's most-watched annual weather event.
PhenomenaWhat is Monsoon Onset?
Monsoon onset is the date when the southwest monsoon (the rainy season) first arrives at a given location, ending the long, hot pre-monsoon dry season and beginning four months of rainfall that supplies South Asia’s water, agriculture, hydropower and groundwater recharge.
For South Asians, no annual weather event is more important. The monsoon delivers roughly 75% of India’s annual rainfall in just four months. A timely, robust monsoon means abundant harvests, full reservoirs and reasonable food prices. A delayed or weak onset means drought, distress for farmers, food inflation and energy stress as hydropower output drops.
The India Meteorological Department (IMD) has officially declared monsoon onset over Kerala since 1901.
How IMD declares onset
The IMD uses a formal criterion for declaring monsoon onset over Kerala (MOK), introduced in 2006:
- Rainfall criterion: 60% of the 14 designated stations across Kerala, Lakshadweep and coastal Karnataka (Mangalore) must receive 2.5 mm or more rain for two consecutive days after 10 May.
- Wind criterion: Westerly winds at 925 hPa with speed ≥ 15-20 knots between latitudes 0–10°N and longitudes 55–80°E.
- OLR criterion: Outgoing Longwave Radiation (a satellite measure of cloud cover) below 200 W/m² in the box 5–10°N, 70–75°E — indicating thick, organised cloud.
When all three criteria are met, IMD declares onset and begins announcing the monsoon’s advance northward.
Average onset dates across South Asia
The southwest monsoon advances roughly north and northwest from Kerala, reaching the rest of the subcontinent over about 45 days:
| Location | Average Onset Date |
|---|---|
| Andaman & Nicobar | 22 May |
| Kerala (Trivandrum) | 1 June |
| Bengaluru, Chennai | 5 June |
| Hyderabad | 8 June |
| Mumbai | 10 June |
| Pune | 10 June |
| Kolkata | 11 June |
| Patna | 16 June |
| Ahmedabad | 20 June |
| Lucknow | 23 June |
| Delhi | 27 June |
| Chandigarh | 1 July |
| Jaipur | 1 July |
| Srinagar | 10 July |
In Bangladesh, onset typically occurs around 5 June at Chittagong and 10 June at Dhaka. In Pakistan, the monsoon arrives in the eastern plains (Lahore) by early July. Western Pakistan (Karachi, Balochistan) sees a weaker monsoon arriving 10–20 July.
What happens if onset is delayed?
Delayed onset is one of South Asia’s most consequential weather risks. Effects of a one to three week delay:
- Kharif crop sowing of rice, maize, soybean and pulses is pushed back, raising risk of insufficient growing days before October.
- Reservoir levels drop further before refilling, threatening summer hydropower and irrigation.
- Heatwaves continue longer into June, causing additional heat-stress deaths.
- Food inflation rises as markets anticipate poor harvests.
- Wholesale electricity prices rise as demand for air-conditioning continues while hydro generation lags.
Notable delayed-onset years:
- 2009: onset a week early (23 May), but the advance stalled during 8–20 June; monsoon 23% below normal — major drought.
- 2014: 5-day delay; weak monsoon.
- 2019: Onset on 8 June (one week late); slow advance — but eventually surplus.
- 2023: Onset on 8 June; slow advance northward.
By contrast, 2020 saw an on-time onset (1 June) and 2022 an early one (29 May); both produced near-normal to surplus rainfall and good harvests.
How onset is forecast
The IMD issues an official onset forecast in mid-May each year. Modern forecasts combine:
- Sea-surface temperatures in the Pacific (ENSO state — El Niño delays monsoon, La Niña accelerates).
- Indian Ocean Dipole (IOD) state — positive IOD aids South Asian monsoon.
- Eurasian snow cover — high winter snow can delay onset.
- Tropical convection indices from satellite OLR.
- Numerical weather models (ECMWF, GFS, IMD Global Forecasting System).
Forecast skill has improved dramatically — IMD’s mid-May onset prediction has been within 4 days of actual onset in most recent years.
Monsoon onset 2026
For 2026, ENSO-neutral conditions in the Pacific were transitioning towards El Niño by late May, and weak El Niño conditions developed in June. IMD forecast the onset over Kerala for 26 May (± 4 days), but the monsoon set in on 4 June 2026, 3 days after the normal date of 1 June.
Mausam Online publishes city-level forecasts that show onset and the subsequent rain pattern — see our Monsoon India 2026 article (Hindi) for state-wise expectations.
Frequently asked questions
When did the 2026 monsoon arrive at Kerala? On 4 June 2026, 3 days after the normal date of 1 June; IMD had forecast 26 May (± 4 days). The date is declared by IMD only after rainfall criteria are met.
Is “first rain” the same as monsoon onset? No. Pre-monsoon thunderstorms (“mango showers” in Karnataka, kal-baisakhi in Bengal) bring local rainfall before official onset. True monsoon onset requires sustained widespread rain, westerly winds and organised cloud bands — not just isolated thundershowers.
Why does Delhi get the monsoon a month later than Kerala? The monsoon advances geographically. Moist southwesterly winds from the Arabian Sea hit the Western Ghats first (Kerala), then progress north and east. It takes about 4 weeks for the system to penetrate inland to the Indo-Gangetic plain. Delhi’s onset is the result of the Bay of Bengal branch wrapping around and arriving from the east, while the Arabian Sea branch fades over arid northwest India.
What is the difference between “onset” and “monsoon arrival”? “Onset” is the formal IMD declaration. “Arrival” or “rain arrival” is informal. IMD uses scientific criteria; locals use the first heavy shower. They sometimes differ by a few days.
Where can I track the monsoon’s advance live? Mausam Online’s hourly precipitation forecast is available on every city page. See Thiruvananthapuram, Mumbai, Kolkata, Delhi, Patna, Dhaka.