The India Meteorological Department (IMD) recently published its first Long Range Forecast (LRF) for 2022. According to the forecast, India is likely to experience a normal monsoon for the year, marking the fourth consecutive year of such predictions. The IMD also updated the definition of average rainfall during this time. It typically issues two primary forecasts each year: one in April and another, more detailed forecast, in the last week of May, illustrating the expected spread of the monsoon across the country.
Introduction to India Meteorological Department (IMD)
Established in 1875, the IMD is an agency operating under the Ministry of Earth Sciences. It is chiefly responsible for monitoring meteorological observations, predicting weather patterns, and studying seismology.
Key Highlights from the Forecast
The forecast suggests that India will receive 99% of the Long Period Average (LPA) rainfall, as the previous benchmark of 89 cm was decreased to 88 cm in 2018 and revised further to 87 cm in 2022. Normal monsoon is when the rainfall is between 96% and 104% of the LPA. Interestingly, El Nino, associated with Central Pacific warming and causes dryer conditions over northwest India, is not expected this year. Instead, existing La Nina conditions are likely to persist during the monsoon. These periods denote below-average sea surface temperatures across the east-central Equatorial Pacific. Furthermore, regions including northern parts of peninsular India, central India, and the Himalayan foothills can anticipate “normal” to “above normal” rainfall, whereas some areas in northeast India and southern parts of South India may see a subdued monsoon.
Understanding the Long Period Average (LPA)
The LPA refers to rainfall recorded over a specific interval (monthly or seasonal), averaged over an extended period like 30 or 50 years. The IMD classifies monsoon as “normal”, “below normal”, or “above normal” with respect to LPA. For instance, the LPA was calculated at 88 cm for the 1961-2010 period, 89 cm for 1951-2000, and 87 cm for 1971-2020. Since the annual rainfall can vary drastically across regions and months, the IMD maintains LPAs for every meteorological region in the country.
The Necessity of LPA
LPA is essential to normalize trends for accurate prediction. This is crucial because the IMD captures rainfall data from over 2,400 locations and 3,500 rain-gauge stations. It takes into account variations in annual rainfall not just from region to region and month to month, but also year to year within a particular region or month. A 50-year LPA covers for substantial variations in either direction caused by unusually high or low rainfall years, periodic drought years, and extreme weather events due to climate change.
Defining a Normal Monsoon Range
The LPA for season rainfall across the country from 1971-2020 is 87 cm. The IMD maintains five categories of rainfall distribution on a national level:
* Normal or Near Normal: Actual rainfall is +/-10% of LPA (between 96-104% of LPA)
* Below Normal: Actual rainfall is less than 10% of LPA (90-96% of LPA)
* Above Normal: Actual rainfall is 104-110% of LPA
* Deficient: Actual rainfall is less than 90% of LPA
* Excess: Actual rainfall is more than 110% of LPA
Monsoon Pattern and Duration in India
The Indian summer monsoon typically lasts from June to September and begins receding from North India at the start of October. The southwest monsoon winds first reach south India and are more active there compared to the interior north India, explaining the longer monsoon duration in the south. The amount of rainfall decreases from east to west in Northern India due to the progressive decrease in the humidity of the winds. As these moisture-bearing winds move further inland, they exhaust their moisture, leading to a gradual decrease in rainfall from east to west.
