IMD Monsoon Forecast, April 2019

Why in news?

The India Meteorological Department (IMD) recently released the monsoon forecast for 2019, and expects a "normal" monsoon rains.

What are the highlights?

• Monsoon rains in India is forecast to be 96% of the Long Period Average (LPA) and thus likely to be “normal” this year.
• This is a more optimistic assessment from the one by private weather forecasting agency, Skymet, that warned of ‘below normal’ rains in June-September.
• However, uncertainty prevails as last year the IMD's April forecast indicated normal monsoon rains but India saw below normal rainfall (91% of LPA).
• The IMD issues its first monsoon forecast in April and then updates it in June.

What is the rationale?

• The IMD’s optimism is based on global climate models projecting a ‘weakening El Nino.’
• The El Nino, a cyclic warming of the Central and Eastern Pacific region, has historically been linked to a weakening of monsoon rain.
• A temperature rise greater than 1°C for 3 months is considered a ‘strong’ El Nino and a threat to the monsoon.
• A 0.5°C-1°C rise is called weak El Nino conditions; currently the El Nino is 0.9 C.
• The IMD’s models in March, expect the El Nino to peak around May and then recede for the rest of the monsoon months.
• Globally too, other models expect El Nino to recede after June or July.
• All these suggest the possibilities of a normal as opposed to below normal monsoon rains in India this year.
• Another factor, called a positive Indian Ocean Dipole (IOD) (warming in the western Arabian ocean) could neutralise the potential negative impact from the El Nino.

How does IMD calculate monsoon level?

• The India Meteorological Department (IMD) expresses the projected rainfall in terms of Long Period Average (LPA).
• The LPA is the average rainfall recorded during the months from June to September, calculated during the 50-year period from 1951 to 2000.
• LPA is kept as a benchmark while forecasting the quantitative rainfall for the monsoon season every year.
• The LPA of the monsoon season over the entire country is 89 cm.
• Like the countrywide figure, IMD maintains an independent LPA for every homogeneous region of the country.
• E.g. 143.83 cm, 97.55 cm, 71.61 cm, and 61.5 cm for East and Northeast India, Central India, South Peninsular India, and Northwest India respectively.
• Going by this, IMD maintains five rainfall distribution categories on an all-India scale. These are:

1. Normal or Near Normal - when there is +/-10% departure of actual rainfall i.e. between 96-104% of LPA
2. Below normal - when departure of actual rainfall is less than 10% of LPA i.e. 90-96% of LPA
3. Above normal - when actual rainfall is 104-110% of LPA
4. Deficient - when departure of actual rainfall is less than 90% of LPA
5. Excess - when departure of actual rainfall is more than 110% of LPA

Source: The Hindu, Indian Express