IMD monsoon rainfall categories

Topic: Climate of India and the Monsoon · NCERT: Beyond NCERT

Meaning

IMD monsoon rainfall categories are five labels that the India Meteorological Department (IMD) uses to describe one season's all-India June–September rainfall, measured as a percentage of the Long Period Average (LPA). The five labels are deficient (below 90%), below normal (90–95%), normal (96–104%), above normal (105–110%) and excess (above 110%) [2].

These labels turn a rainfall figure into one simple word that farmers, the government and the RBI can act on. IMD uses the same words in its seasonal forecasts and in its end-of-season reports.

Explanation

The yardstick: Long Period Average (LPA)

• LPA (the average all-India rainfall for June to September over a long base period) is the measuring stick. Every season is compared with it.
• Current LPA = 87 cm (868.6 mm), base period 1971–2020 [1][2].
• Old LPA = 88 cm, base period 1961–2010.
• How a season gets its label:
• IMD adds up the all-India rainfall from June to September.
• It divides this total by the LPA and turns it into a percentage.
• The percentage then falls into one of the five bands below.

The five categories and how often each one happens

• Climatological probability means how often each category has happened in the past [2].

Category	% of LPA	Climatological probability (%)
Deficient	below 90	16
Below normal	90–95	17
Normal	96–104	33
Above normal	105–110	16
Excess	above 110	17

• Normal is the most common category. It has happened in about one season out of three [2].
• The scale is balanced around normal:
• There are two dry bands (deficient, below normal) and two wet bands (above normal, excess).
• Each of these four bands has a probability of about 16–17% [2].

How the categories are used in forecasts

• Two-stage long-range forecast (LRF), used since 2003: IMD issues a first forecast in mid-April and an update by the end of May [2].
• What the April forecast contains:
• A number with an error margin.
• The chance of each of the five categories.

• A map showing which areas are likely to get above-normal, normal or below-normal rain [2].

• Forecast method since 2021: a Multi-Model Ensemble (MME), which combines many coupled global climate models (computer models in which the ocean and the air exchange heat and moisture). One of these models is IMD's own Monsoon Mission Climate Forecast System (MMCFS) [2].

• Signals that push the forecast towards a category [2]:
• El Niño (warming of the equatorial Pacific Ocean) usually weakens the monsoon, which points towards the dry categories.
• A positive IOD (Indian Ocean Dipole: the western Indian Ocean is warmer than the eastern side) usually helps the monsoon.

• More Eurasian snow cover in winter and spring usually means less monsoon rain. This is an inverse relationship.

• Example: the 2026 forecast (issued 13 April 2026) was 92% of LPA ± 5%, which falls in the below normal band [2].

• Why: El Niño was expected to develop during the season, a positive IOD was likely only near the end of the season, and January–March snow cover was slightly below normal [2].

What the categories do not show

• A category describes only the all-India seasonal total. It does not show how the rain is spread over places or weeks.
• A "normal" season can still fail farmers. A long break in rain at sowing time can ruin crops even when the seasonal total looks normal.

Where it occurs

• All-India scale: the five labels describe the whole country's June–September rainfall. IMD (founded in 1875, now under the Ministry of Earth Sciences, MoES) runs this system.
• Homogeneous regions (regions with similar rain patterns) have their own LPAs on the 1971–2020 base [1]:
• East & Northeast India: 1,367.3 mm
• Central India: 978.0 mm
• South Peninsula: 716.2 mm
• Northwest India: 587.6 mm

• IMD gives separate forecasts for these four regions at the end-of-May stage [2].

• Monsoon core zone (MCZ) is the central belt that covers most of India's rain-fed farm regions. Its LPA is 832.2 mm (1971–2020) [1].

• Recent seasons by category:
• 2023: 94% of LPA, below normal (an El Niño year).
• 2024: 108%, above normal.
• 2025: 108% (937.2 mm), above normal, the 5th highest since 2001 [1].

• 2026: forecast of 92 ± 5%, below normal [2]. The final figure should be checked against IMD's end-of-season report.

• 2025 shows how the all-India label can hide regional failure [1]:

• Northwest India got 127% of its LPA and East & Northeast India got only 80%.
• East & Northeast India's total of 1,089.9 mm was the 2nd lowest since 1901.
• Three subdivisions were deficient: Arunachal Pradesh, Assam & Meghalaya, and Bihar (9% of India's area).

Don't confuse with

• Above normal vs excess: above normal is 105–110% of LPA, and excess is above 110%. A 108% season, like 2024 or 2025, is "above normal", not "excess" [2].
• Deficient season vs drought year: a deficient season depends only on rainfall (below 90% of LPA). An IMD drought year also needs more than 20% of the country's area to be hit by drought, along with all-India rain more than 10% below LPA (verify current IMD definition).
• Normal category vs a good monsoon for farmers: "normal" (96–104%) describes only the seasonal total. Monsoon failure can still happen through a long break at sowing time or through regional shortfalls.
• Current LPA vs old LPA: the current LPA is 87 cm (868.6 mm, 1971–2020). The old LPA was 88 cm (1961–2010) [1][2]. The percentages and categories are now calculated against the new base.

Prelims Hooks

• Normal monsoon = 96–104% of LPA. Deficient is below 90%, below normal is 90–95%, above normal is 105–110% and excess is above 110% [2].
• Trap: 105–110% is "above normal", not "excess" [2].
• Normal has the highest climatological probability, 33%. Each of the other four categories has about 16–17% [2].
• LPA = 87 cm (868.6 mm), base period 1971–2020. The old LPA was 88 cm, base period 1961–2010 [1][2].
• 2026 forecast: 92% of LPA ± 5%, which is below normal, with El Niño expected [2].
• 2025: all-India rain was 108% (above normal), but East & Northeast India got only 80% [1].

Mains Points

• Why the categories matter for the economy (GS-III):
• About half of India's net sown area is rainfed, and about 46% of workers are in farming (PLFS 2023-24).
• So a forecast of "below normal" or "deficient" is an early warning of lower kharif output, low reservoir storage, weak rural demand and food inflation. The RBI considers the monsoon when it makes its inflation projections.

• Policy responses: contingency crop plans, more micro-irrigation, drought-tolerant crop varieties, crop insurance and buffer stocks.

• Distribution matters more than the all-India label:

• In 2025 India as a whole was "above normal" at 108%, yet East & Northeast India got 80% and three subdivisions were deficient [1].

• Category labels should be read together with regional, monthly and district-level data. This is why IMD now issues forecasts for each region and each month [2].

• Better forecasts make the categories more useful:

• IMD moved from the 16-parameter statistical model (used 1988–2002) to dynamical MME forecasts that give the chance of each category [2].
• Mission Mausam aims to bring forecasts down to the Panchayat level, using 6 km models and hourly nowcasts [3].
• This helps disaster management and climate adaptation, because climate change is making rainfall more erratic.

Related concepts

• Monsoon and the Indian economy
• Monsoon failure
• Monsoon forecasting by IMD
• National Monsoon Mission
• Long Period Average of monsoon rainfall

Read more

• The monsoon, the economy and forecasting