Explained IMD Long Range Forecast for Southwest Monsoon 2026
India Meteorological Department issue the operational Long Range Forecast (LRF) for the southwest monsoon seasonal rainfall averaged over the country as a whole in two stages. The first stage forecast is issued in April and the second stage or updated forecast is issued by the end of May.
On 13th April 2026, IMD declared first stage LRF for southwest monsoon seasonal rainfall during 2026. The dynamical and statistical models are used to derive the LRF. The salient features of the LRF are as follows:
1) The monsoon rainfall is likely to be 92% of long Period Average (LPA) with model error of ± 5%.
2) The forecast and climatological probabilities for different categories of rainfall is given below:
| Category | Rainfall Range (% of LPA) | Forecast Probability (%) | Climatological Probability (%) |
| Deficient | < 90 | 46 | 16 |
| Below Normal | 90-95 | 31 | 17 |
| Normal | 96-104 | 27 | 33 |
| Above Normal | 105-110 | 6 | 16 |
| Excess | >110 | 1 | 17 |
3) The spatial distribution suggests that the below normal seasonal rainfall is most likely over many parts of the country except some areas over Northeast, Northwest and Southern Peninsular India, where above-normal rainfall is likely.
The Global Parameters
A) Weak La-Nina conditions are transitioning to ENSO-neutral conditions over the equatorial Pacific and likely to continue during the April to June 2026 season.
B) Present Neutral IOD conditions are likely to be developed into Positive IOD towards end of the Southwest Monsoon season.
C) The Northern hemisphere snow cover extent during January to March 2026 was slightly below normal.
Impact
I) Weak La Nina / El Nina – Not favourable for Monsoon over India
II) Positive IOD – favourable for Monsoon over India
III) Snow Cover over Northern hemisphere inversely with southwest monsoon rainfall in subsequent season.
Notes :Dynamical Models (The Modern Approach)
• Model: The primary tool is the Monsoon Mission Climate Forecast System (MMCFS).
• Method: This is a coupled ocean-atmosphere-land model that simulates real-time physical processes of the atmosphere. IMD uses a Multi-Model Ensemble (MME) system, which averages outputs from several global climate models, including the MMCFS, to reduce individual model errors.
• Contribution: It provides detailed spatial distribution (map) of rainfall probability (above, below, or normal) over India, the Monsoon Core Zone, and for specific months.
Statistical Models (The Traditional Approach)
• Models: Multiple Regression (MR) models and Reduced Space Regression (RSR) models are used.
• Method: These models calculate the statistical relationship between monsoon rainfall and antecedent (pre-existing) global atmospheric/oceanic parameters (like sea surface temperatures).
• Contribution: They provide robust, simple, and quick estimates of the overall monsoon rainfall for the country as a whole.
