How India’s monsoon forecast works

From June to September, the south-west monsoon brings India more than 70% of its rain for the year. It fills reservoirs, recharges groundwater and waters the kharif crops — rice, pulses, cotton, soybean — on which millions of farmers depend. That is why its forecast is national news.

What “normal” means

The India Meteorological Department (IMD) measures a season against the long period average (LPA): the average rainfall over the whole country in June–September. Based on the years 1971 to 2020, the LPA is 87 centimetres. A season is then put in one of five categories:

• Deficient — less than 90% of the LPA
• Below normal — 90% to 95%
• Normal — 96% to 104%
• Above normal — 105% to 110%
• Excess — more than 110%

A national figure can hide a lot: a “normal” season can still bring floods in one state and drought in another, or a dry July followed by a very wet September.

The forecasts, step by step

• Mid-April: the first long-range forecast for the season as a whole, with its probabilities for each category. IMD gives its model error, typically plus or minus 5% of the LPA.
• End of May: an updated forecast, with figures for regions of the country and for June.
• Through the season: forecasts for each month, and the extended- and short-range forecasts and warnings issued every day.

The forecasts combine statistical models, built on past relations between the monsoon and conditions around the world, with dynamical models: computer simulations of the ocean and the atmosphere.

Onset and withdrawal

The monsoon’s normal date of onset over Kerala is 1 June, give or take about a week. IMD declares it only when set conditions are met — widespread rain at a list of stations in Kerala and coastal Karnataka, the right winds, and deep cloud. The monsoon normally covers the whole country by 8 July, and begins to withdraw from north-west India around 17 September. From October to December, the north-east monsoon brings Tamil Nadu, Puducherry and parts of Andhra Pradesh much of their rain.

What pushes it either way

• El Niño — unusually warm water in the central and eastern Pacific — tends to weaken the Indian monsoon; La Niña, the cool phase, tends to help it.
• The Indian Ocean Dipole: when the western Indian Ocean is warmer than the eastern (a “positive” dipole), the monsoon often does better.
• Snow cover over Eurasia in winter and spring has also been linked to the strength of the monsoon.

None of these decides a season on its own, which is why forecasts are given as probabilities. For the weather where you are, see our weather pages; for official warnings, always go to IMD.