IMD and Weather Forecast in India

Table of Contents

IMD and Weather Forecast in India- Context

Recently, India Meteorological Department (IMD) said that while rain in September would be “above normal”, the overall June-September rainfall would be at the “lower end” of normal.
The factors that caused the August deficit were a “negative” Indian Ocean Dipole.
- “Negative” Indian Ocean Dipole: it is characterized by warmer eastern Indian Ocean and colder closer to the Indian coast, which is considered unhelpful for the monsoon.

About: IMD, also referred to as the meteorological department was established in 1875.
- Headquarter: situated at Pune with regional offices at Mumbai, Kolkata, Nagpur, and Delhi.
Parent Ministry: IMD is an agency of the Ministry of Earth Sciences of the Government of India.
Mandate: It is the principal government agency responsible for meteorological observations, weather forecasting, and seismology.

IMD uses Long Period Average (LPA) as a base to predict the expected amount of Monsoon rainfall in a particular year.
- Long Period Average (LPA): It is the average rainfall recorded during the months from June to September which is calculated during the 50-year period.
- Role of LPA: LPA acts as a benchmark in forecasting quantitative rainfall for the monsoon season every year.
Categorization of Monsoon Rainfall: Based on LPA, IMD categorizes yearly monsoon rainfall on an all-India basis in below five categories-

1. Normal or Near Normal: percentage departure of actual rainfall is +/-10% of LPA (rainfall between 96-104% of LPA).
2. Below Normal: When the departure of actual rainfall is less than 10% of LPA, that is 90-96% of LPA.
3. Above Normal: When actual rainfall is 104-110% of LPA.
4. Deficient: When the departure of actual rainfall is less than 90% of LPA.
5. Excess: When the departure of actual rainfall is more than 110% of LPA.

Dynamical Monsoon Forecast Model: It was recently adopted by IMD which uses the evolving weather patterns to predict monsoon.
- In the Dynamical Monsoon Forecast Model, weather conditions are simulated and projected into the future by the supercomputer.
- Unlike IMD’s previous statistical forecasting method, this model is useful for predicting rainfall over smaller spatial and temporal scales.
Performance of the Dynamic Model: It is more accurate at forecasting impending changes in weather a week or two in advance, it has not proved reliable at long-range forecasting in the past three years.
- For instance, in 2019, the IMD forecast reduced monsoon rainfall at 96% of the LPA but India got record-breaking rainfall at 110% of the average.
- In 2020, it first forecast 100% of the LPA and updated it to 102%, but India again got a torrential 109% which was outside the 5% error window.