IMD, Skymet differ on monsoon arrival: A look at reasons behind divergent statements by two agencies

The India Meteorological Department and private weather forecasting agency Skymet Weather on Sunday gave divergent statements on whether the monsoon has arrived in Kerala.

While the IMD said that the arrival of the southwest monsoon over Kerala is likely to be delayed by two days, and is expected to make an onset over the state by 3 June, Skymet said that the monsoon has already arrived.

In recent years, the IMD and Skymet have often made differing predictions about the weather ahead, particularly the all-important monsoon. Last year as well, the official forecasting agency had said on 31 May that the monsoon had not arrived yet, while the private agency had said that it had arrived earlier than expected, as noted by Hindustan Times.

Here is a brief look at what lies beneath these differing opinions:

Official status of IMD, Skymet

The IMD is an agency under the Union Ministry of Earth Sciences. It is the principal agency responsible for meteorological observations, weather forecasting and seismology. Skymet Weather, on the other hand, is a private agency headquartered in Noida. It began giving forecasts in 2012.

While there have been controversies in the past over differing predictions, the two agencies' forecasts have often been largely similar. For example, in 2014, both of them said that India would get deficient rains and the country did end with a 'meteorological' drought, as noted by The Economic Times. In 2013, both agencies correctly predicted a good monsoon, although Skymet's prediction was more accurate.

Criteria to define 'onset'

Part of the reason for differing statements about whether the monsoon has arrived is the definition of the 'onset.'

According to the IMD, the onset of the southwest monsoon over Kerala depends on three parameters, as noted by PTI.

If after 10 May, 60 percent of 14 stations — Minicoy, Amini, Thiruvananthapuram, Punalur, Kollam, Allapuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery, Kannur, Kudulu and Mangalore — report rainfall of 2.5 millimetres or more for two consecutive days, the onset over Kerala can be declared on the second day, provided other two criteria are also in concurrence.

This has to be supplemented by the wind speed. The depth of westerlies should be maintained up to 600 hectopascal (hPa), in the box equator to Latitude 10-degrees north and Longitude 55 degrees to 80-degrees east. The zonal wind speed over the area bounded by Latitude 5 to 10-degrees north, Longitude 70-80-degrees east should be of the order of 15 20 knots at 925 hPa.

The Outgoing Longwave Radiation (OLR) should be below 200 watt per square meter (wm-2) in the box confined by Latitude 5-10 degrees north and Longitude 70-75 degrees east.

Skymet has said all the IMD parameters are fulfilled to declare the onset of Southwest Monsoon over Kerala. The IMD, however, said the depth of westerlies is not as much as it is expected, and the rainfall parameters remain unfulfilled to declare onset of monsoon over Kerala.

With inputs from PTI

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