Central Idea: The monsoon is likely to set in over Kerala with a “slight delay” on June 4, the India Meteorological Department (IMD) said. The usual onset date over Kerala is June 1, within a seven-day window.

What does the “Onset of Monsoon” mean?

• The onset of the monsoon over Kerala marks the beginning of the four-month, June to September southwest monsoon season over India.
• It brings more than 70 per cent of the country’s annual rainfall.
• It marks a significant transition in the large-scale atmospheric and ocean circulations in the Indo-Pacific region.
• The IMD announces it only after certain newly defined and measurable parameters, adopted in 2016, are met.
• The onset is a significant day in India’s economic calendar.

How does IMD predict the monsoon?

• Broadly, the IMD checks for the consistency of rainfall over a defined geography, its intensity, and wind speed:

1. Rainfall: The IMD declares the onset of the monsoon if at least 60% of 14 designated meteorological stations in Kerala and Lakshadweep record at least 2.5 mm of rain for two consecutive days at any time after May 10.
2. Wind field: The depth of westerlies should be upto 600 hectopascal (1 hPa is equal to 1 millibar of pressure) in the area bound by the equator to 10ºN latitude, and from longitude 55ºE to 80ºE. The zonal wind speed over the area bound by 5-10ºN latitude and 70-80ºE longitude should be of the order of 15-20 knots (28-37 kph) at 925 hPa.
3. Heat: The INSAT-derived Outgoing Longwave Radiation (OLR) value (a measure of the energy emitted to space by the Earth’s surface, oceans, and atmosphere) should be below 200 watt per sq m (wm2) in the box confined by 5-10ºN latitude and 70-75ºE latitude.

• The onset is not officially declared until the prescribed conditions (above) are met.

Factors considered by IMD

• The IMD uses a specialised model that forecasts the arrival dates within a four-day window.
• It uses six predictors:

1. Minimum temperatures over northwest India
2. Pre-monsoon rainfall peak over south Peninsula
3. Outgoing long-wave radiation (OLR) over the South China Sea
4. Lower tropospheric zonal wind over the southeast Indian Ocean
5. Upper tropospheric zonal wind over the east equatorial Indian Ocean, and
6. OLR over the southwest Pacific region

Back2Basics: Long Period Average (LPA)

• The IMD predicts a “normal”, “below normal”, or “above normal” monsoon in relation to a benchmark “long period average” (LPA).
• The LPA of rainfall is the rainfall recorded over a particular region for a given interval (like month or season) average over a long period like 30 years, 50 years, etc.
• LPA refers to the average rainfall recorded from June to September for the entire country, the amount of rain that falls every year varies from region to region and from month to month.
• The IMD’s prediction of a normal monsoon is based on the LPA of the 1971-2020 period, during which India received 87 cm of rain for the entire country on average.
• It has in the past calculated the LPA at 88 cm for the 1961-2010 period, and at 89 cm for the period 1951-2000.

Why LPA is needed?

• The IMD records rainfall data at more than 2,400 locations and 3,500 rain-gauge stations.
• Because annual rainfall can vary greatly not just from region to region and from month to month, but also from year to year within a particular region or month.
• An LPA is needed to smooth out trends so that a reasonably accurate prediction can be made.
• A 50-year LPA covers for large variations in either direction caused by freak years of unusually high or low rainfall, as well as for the periodic drought years.
• It also takes into account the increasingly common extreme weather events caused by climate change.

Range of normal rainfall

The IMD maintains five rainfall distribution categories on an all-India scale. These are:

1. Normal or near normal, when the percentage departure of actual rainfall is +/-10% of LPA, that is, between 96-104% of LPA;
2. Below normal, when 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 departure of actual rainfall is less than 90% of LPA; and
5. Excess, when the departure of actual rainfall is more than 110% of LPA.

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