IMD’s Monsoon Forecasting

Why in news?

• India Meteorological Department (IMD) has released its first Long Range Forecast(LRF) for 2022.

More about the news:

• Every year, before the onset of monsoon, the India Meteorological Department (IMD) releases two forecasts.
• The first forecast, which is usually more preliminary, comes around mid-April and the second, a more detailed assessment, is released around late May.
• Recently, IMD released its first forecast for monsoon 2022.

What is the Long Range Forecast (LRF) for 2022?

• According to IMD, the monsoon rains are likely to be ‘normal’ – that is, it will likely be about 99 per cent of the long period average (LPA) rain value.
• Rains are categorised as normal if they are 96 to 104 per cent of the LPA
• Therefore, India is likely to receive a normal monsoon for the fourth consecutive year.
• This forecast is based on the Long Period Average (LPA) of the 1971-2020 period.

What is the Long Period Average (LPA)?

• The IMD predicts a “normal”, “below normal”, or “above normal” monsoon in relation to a benchmark Long Period Average (LPA).
• According to the IMD, the LPA of rainfall is the rainfall recorded over a particular region for a given interval (like month or season) averaged over a long period like 30 years, 50-years etc.
• The IMD maintains LPAs for every meteorological region of the country.
  • This number ranges from around 61 cm for the drier Northwest India to more than 143 cm for the wetter East and Northeast India.
• Hence, LPAs act as a benchmark while forecasting the quantitative rainfall for that region for a specific month or season.
• As per the norms of the World Meteorological Organization to which India is a signatory, should be updated every 10 years
• The LPA value used till 2021 was 88 cm, which was the average rainfall received by India in monsoon months between 1961 and 2010.
• Now, IMD revised the LPA value to 87 cm (average for 1971 to 2020). This reduction of about 1 cm of LPA rains, indicates two things:
  • (i) Indian monsoon rains have been shrinking over time
  • (ii) Measured against the previous LPA, prediction for monsoon 2022 would have looked grimmer

What is the range of a Normal Monsoon?

• The IMD maintains five rainfall distribution categories on an all-India scale. These are:
  • Normal or Near Normal: When the percentage departure of actual rainfall is +/-10% of LPA, that is, between 96-104% of LPA
  • Below Normal: When the departure of actual rainfall is less than 10% of LPA, that is 90-96% of LPA
  • Above normal: When actual rainfall is 104-110% of LPA
  • Deficient: When the departure of actual rainfall is less than 90% of LPA
  • Excess: When the departure of actual rainfall is more than 110% of LPA

Why LPA is needed?

• Smooth out trends of rainfall: An LPA is needed to smooth out trends so that a reasonably accurate prediction can be made because the IMD records rainfall data at more than 2,400 locations and 3,500 rain-gauge stations.
• Covers large variations in either direction: A 50-year LPA covers large variations in either direction caused by freak years of unusually high or low rainfall (as a result of events such as El Nino or La Nina), as well as for the periodic drought years and the increasingly common extreme weather events caused by climate change.

PRACTICE QUESTION:

Which of the following statements is/are correct regarding monsoon forecasting in India?

1. Long Period Averages (LPAs) act as a benchmark while forecasting the quantitative rainfall for that region for a specific month or season.

2. India Meteorological Department maintains Long Period Averages (LPAs) for every meteorological region of the country

Select the correct answer using the code given below:

(a) 1 only

(b) 2 only

(c) Both 1 and 2

(d) Neither 1 nor 2

Answer