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Monsoon onset dates: how the IMD forecast actually works

By Gagariya Editorial Desk•27 Jul 2026 IST•1 min read•Human Reviewed & Fact-Checked

One date moves sugar futures, sowing calendars and municipal drain-cleaning schedules. It's declared from Thiruvananthapuram.

How is onset declared?

The IMD uses fixed criteria: a minimum share of monitoring stations in Kerala reporting rain, westerly winds up to a set depth, and reduced outgoing longwave radiation — all sustained for two consecutive days.

What does the onset date not tell you?

When it rains in your city, unless you live in Kerala. The monsoon's northern limit advances in surges, sometimes stalling for a week. Delhi's typical arrival is around four weeks after Kerala's.

Why do forecasts differ from declarations?

The April forecast is a model estimate with a stated error margin. The declaration is an observation. Comparing the two each year is how you calibrate how much to trust the forecast.

What to watch next: the IMD's second-stage forecast, which updates regional rainfall distribution once onset is declared.

Questions people ask

What does monsoon onset over Kerala mean?
It is the IMD's formal declaration that sustained monsoon conditions — rainfall coverage, wind depth and radiation criteria — have been met over Kerala for two consecutive days.
How long after Kerala does the monsoon reach Delhi?
Typically around four weeks, though the advance happens in surges and can stall. The IMD tracks the northern limit of the monsoon as it progresses.

Sources

  1. India Meteorological Department (mausam.imd.gov.in)

Editorial Note: This explainer was researched using verified primary sources and official notifications, then independently fact-checked, reviewed, and finalized under human editorial guidance by the Gagariya desk.

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