The parameters are way off track all of a sudden and this shows how difficult it is to predict the dynamics of the monsoon which can change on an hourly basis
Representational image. Reuters
The onset of monsoon over Kerala, which was earlier expected to be by 27 May, has got delayed. Now, the Indian Meteorological Department has officially announced that the monsoon will make an entry into Kerala by 29 May or Sunday which is actually three days earlier than the normal arrival date of 1 June.
The parameters are way off track all of a sudden and this shows how difficult it is to predict the dynamics of the monsoon which can change on an hourly basis.
What are the set criteria for the declaration of South West Monsoon over Kerala?
Following are the IMD benchmark for defining the monsoon weather:
1) Rainfall: If after 10 May, 60 per cent of the available 14 stations that are viz. Minicoy, Amini, Thiruvananthapuram, Punalur, Kollam, Allapuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery, Kannur, Kudulu and Mangalore report rainfall of 2.5 mm or more for two consecutive days, the onset over Kerala be declared on the second day, provided the following criteria are also in concurrence.
2) Wind field: The depth of westerlies should be maintained up to 600 hPa, in the box equator to Lat. 10?N and Long. 55?E to 80?E. The zonal wind speed over the area bound by Lat. 5-10?N, Long. 70-80?E should be of the order of 15 – 20 Knots at 925 hPa. The source of data can be RSMC wind analysis/satellite.
3) Outgoing Longwave Radiation (OLR): INSAT derived OLR value should be below 200 wm-2 in the box confined by Lat. 5-10?N and Long. 70-75?E.
All of the criteria should be taking place for the declaration of the southwest monsoon over Kerala and even then further advancement in other parts of India is based on NLM(Northern Limit of Monsoon).
Why the monsoons didn’t make an onset over Kerala on the earlier predicted date of 27 May?
Previously the Indian Meteorological Department suggested that the monsoon will arrive over Kerala earlier than normal by 27 May with a model error of +/- 4 days but till now there is no further advancement of monsoon in the Indian mainland till 28 May.
What caused the delay in monsoon onset
1) Too Much Pre Monsoon Rains: The Southern Peninsula is experiencing excess pre-monsoon rains this season. Till 28 May combined southern states have experienced 192.1mm of rainfall against the normal of 112.8mm. The departure from normal stands at +70%.
Pre-monsoon rains are convection based hence there are short spells of intense thundershowers from the high-level clouds. They form over land while in the case of monsoon, it’s the opposite. The Southwest Monsoon features long-lasting rains from low to mid-level clouds they generally originated in the sea and push into Kerala.
So far the typical monsoon rain features are missing. Thunderstorm based rains can’t justify monsoon onset in the case of Kerala.
2) Active Western Disturbance to be blamed:
The Western Disturbance in India is derived from the East Moving SubTropical Jet streams which are at the highest levels of the atmosphere.
Usually, by the end of May or early June, it starts shifting to the North of the Himalayas and Tropical Easterly Jet(TEJ) comes into existence. They flow East to West over peninsular India, its formation leads to a reversal in upper-level atmosphere changes like high-pressure areas start nurturing into low-pressure areas which leads to quick onset and progress of monsoon.
Since the tropical jet streams are diving deep south and Western Disturbances are impacting North India, the conditions for monsoon onset in Kerala are slow-paced.
IMD in the latest press release mentioned the southwest monsoon is expected to make an onset in Kerala during the next 2-3 days i.e anytime in the coming week but as per the precipitation models of IMD and many agencies, it is clear that even if monsoon hits Kerala the rainfall is likely to be on the weaker side at least till 10 June. Simultaneously other parts of India may have to wait slightly longer for the monsoon to arrive.
The author, better known as the Rohtak Weatherman, interprets and explains complex weather patterns. His impact-based forecasts @navdeepdahiya55 are very popular in north India.
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