The Monsoon Economy
How the Monsoon Is Forecast
How the India Meteorological Department forecasts monsoon rainfall, what "normal" and "long period average" mean, and why forecasts matter for farmers and markets.
Every April, the India Meteorological Department, or IMD, issues its first long-range forecast for the coming southwest monsoon, followed by updates in late May or June and forecasts for each month.
The long period average
Forecasts compare expected rainfall with the long period average, or LPA: the average rainfall for June to September over a base period. For the period 1971 to 2020, the national LPA is about 87 centimetres.
Categories
IMD describes seasonal rainfall as:
- Deficient: below 90 percent of LPA.
- Below normal: 90 to 95 percent.
- Normal: 96 to 104 percent.
- Above normal: 105 to 110 percent.
- Excess: above 110 percent.
How forecasts are made
IMD uses:
- Statistical models based on relationships between the monsoon and indicators such as ocean temperatures.
- Dynamical models: computer simulations of the atmosphere and oceans, run on supercomputers.
Key signals include El Niño or La Niña conditions in the Pacific and the Indian Ocean Dipole.
Beyond the seasonal total
IMD also provides:
- Onset dates, such as the monsoon’s arrival over Kerala, usually around 1 June.
- Extended-range forecasts for the next few weeks.
- District-level and block-level forecasts and agricultural advisories.
Why forecasts matter economically
- Farmers decide what and when to plant.
- Governments plan food stocks, seed supplies and contingency plans.
- Companies plan production and inventory for rural markets.
- Investors and the RBI anticipate inflation and demand.
Accuracy
Seasonal forecasts have improved but remain uncertain. A national forecast can’t capture regional variation or the timing of dry spells, which often matter most for farmers.
When IMD forecasts an "above normal" monsoon in April, tractor makers' share prices rise and newspapers predict lower food inflation. If June rains then disappoint, those expectations quickly reverse.
A normal national total can include floods in some regions and drought in others. Timing and distribution matter.
- IMD issues its first monsoon forecast in April, with updates later.
- Rainfall is compared with a long period average of about 87 cm (1971-2020).
- Normal is 96 to 104 percent of the LPA.
- Forecasts guide farmers, governments, companies and investors, but uncertainty remains.
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