Prediction leaves room for margin of error of four days
Monsoon will enter Kerala on June 1, India Meteorological Department said in a press statement on May 15. The southwest monsoon, which brings rain over most of the country, first sets foot in the southern sate, heralding its much anticipated arrival. Incidentally, June 1 is also the official normal onset date for the country, which is the average of monsoon arrival dates over several decades.
IMD uses an indigenous statistical model to predict monsoon arrival. The model is not cent percent accurate, and there is a chance of error of four days on either side of the date. So, even if the monsoon arrives four days ahead or late, it would still be considered within the correct range.
The onset date holds importance for the agricultural community, as they gear to cultivate the kharif crops. As monsoon progresses in a predicted manner, the onset dates gives a clue to when it will wash over other states.
The model takes into account parameters like minimum temperature over north-west India, pre-monsoon rainfall over south peninsula, radiation over south China Sea and Pacific Ocean, and wind conditions over the Indian Ocean.
IMD started offering operational forecast of onset in 2005, and has always been spot-on with the predictions. Interestingly, the monsoon onset dates have digressed sharply from the normal date on several occasions over the past 100 years. In 2005 itself, the monsoon arrived on June 7—a week's deviation from the normal date. The predicted date was June 10. These frequent deviations have propelled the IMD to reconsider the normal date.
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