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At 109% of LPA, all-India seasonal rainfall this year third-highest, after 1994 and 2019
New Delhi, October 1, 2020
At 109% of the long period average (LPA), the all-India seasonal rainfall this year was the third highest, after 1994 and 2019, the India Meteorological Department (IMD) said today.
Quantitatively, the 2020 all-India monsoon seasonal rainfall during June 1 to September 30 has been 95.8 cm against the LPA of 88.0 cm based on data of 1961-2010.
Considering the recent years since 1990, the all-India seasonal rainfall this year was third highest, after 112% of LPA in 1994 and 110% of LPA in 2019.
Thus 2019 and 2020 are two consecutive years with above normal monsoon rainfall, after 1958 (110% of LPA) and 1959 (114% of LPA ), IMD said.
Considering the four broad homogenous regions of India, the monsoon seasonal rainfall during 2020 has been 106%, 84%, 115% and 129% of LPA respectively over East and North-East (NE), North- West (NW), Central and South India.
Therefore, seasonal rainfall has been in excess over Central and South India, normal over East and NE India and deficient over NW India, a press release from the IMD said.
Out of 36 meteorological sub-divisions, two sub-divisions received large excess, 13 received excess and 16 sub-divisions received normal monsoon rainfall during 2020 while only five sub-divisions received deficient rainfall.
These five Met sub-divisions which got deficient rains are Nagaland, Manipur, Mizoram & Tripura (- 32%), West Uttar Pradesh (-37%), Uttarakhand (-20%), Himachal Pradesh (- 26%), Jammu & Kashmir and Ladakh (-34%).
Considering month-to-month rainfall variation over India as a whole, the season is uniquely placed in the historical record for its distinct and contrasting month-to-month variation. The rainfall over country, as a whole, was 118%, 90%, 127% and 104% of LPA during June, July, August and September respectively.
It was a good beginning for the season in terms of rainfall with formation and movement of the cyclone Nisarga, over the Arabian Sea. It helped the monsoon to advance into mainland along the west coast.
Subsequent features favoured timely advance and monsoon covered entire country by June 26 against normal date of July 8 (about 12 days ahead of normal date). The country, as a whole, experienced excess rainfall (118% of LPA) during June.
In July, many unfavourable features of monsoon appeared resulting in deficient rainfall for the country (90% of LPA).
The weak monsoon in July was mainly due to the absence of any major monsoon disturbance over Bay of Bengal. Absence of such major systems in July also caused weakening of the monsoon trough.
The monsoon trough lay to the north of the normal position or closed to the foothills of the Himalayas on many days. It resulted in frequent and prolonged floods over north-eastern India, Bihar and adjoining areas of east Uttar Pradesh.
At the same time, major parts of central and northwest India received deficient rainfall.
During August, there was back to back formation of low-pressure systems over the north Bay of Bengal and movement towards Gujarat and south Rajasthan.
The monsoon trough was mostly south of the normal position and remained active. The Arabian Sea was very active with stronger winds reaching up to 50-60 kmph in lower levels during a few days in the month.
Five low pressure systems formed during August 4-10 , 9-11, 13-18 , 19-26 and 24-31 which caused higher than normal rainfall over central and western part of India.
The total number of low pressure days was 28 against the normal of about 17. It caused 2-3 spells of riverine floods over Odisha, Telangana, Madhya Pradesh, south Gujarat and south Rajasthan.
It was a record rainfall in August 2020, when all India rainfall was 127% of LPA. It has been the record highest in the last 44 years, after August 1976 (128.4% of LPA).
It is also fourth highest in last 120 years. The highest ever percentage departure for the month of August during 1901-2020 had been 33% above LPA during 1926.
During August 2020, consecutively for 4 weeks, India got excess rain with 13% to 41% above LPA during week ending August 12 to the week ending September 2. Similarly, the most deficient monsoon conditions prevailed in the second fortnight of July.
Monsoon in 2020 withdrew from western parts of northwest India on September 2 against the normal date of September 17 with a delay of around 11 days. The withdrawal has been delayed mainly due to an active monsoon trough in association with the formation of two low pressure systems in September.
As on October 1, southwest monsoon has withdrawn from Punjab, western Himalayan region, Haryana, Chandigarh, Delhi and many parts for Rajasthan and some parts of Uttar Pradesh.
The withdrawal line of the Southwest Monsoon passes through Lat. 29°N/ Long.81°E, Lakhimpur Kheri, Shahjhanpur, Alwar, Nagaur and Lat. 26°N/ Long.70°E.
The forecast for the date of monsoon onset over Kerala issued on May 15 was June 5 with a model error of +- 4 days. The actual monsoon onset over Kerala was on June 1 and therefore the forecast was correct.
The first stage forecast for the season (June-September) rainfall over the country as a whole issued in April was 100% of LPA with a model error of +- 5% of LPA.
The forecast was upgraded to 102% of LPA with a model error of +/- 4% of LPA in the updated forecast issued in May. IMD also predicted a probability of 65% of monsoon rainfall to be normal to above normal.
However, the actual seasonal rainfall for the country as a whole was 109% of LPA, which is more than the predicted value.
Considering the four broad geographical regions of India, the forecasts issued in May for the season 3 rainfall over Northwest India, Central India, Northeast India and South Peninsula were 107%, 103%, 96% & 102% of the LPA respectively all with model errors of +/- 8%.
The actual rainfall over Northwest India, Central India, Northeast India and South Peninsula was 84%, 115%, 107% and 129% of the LPA respectively.
Thus, the forecasts of season rainfall over the Central India, Northeast India and South Peninsula regions were underestimate to the actual rainfall, while the forecast for Northwest India was an overestimate.
The forecast for the second half of the monsoon season (August – September) for the country, as a whole, was 104% with a model error of 8% of LPA against the actual rainfall of 118% of LPA.
This year, IMD had predicted possibility of the development of weak La Nina conditions in the second half of the season in its forecasts issued in April and May. The cool ENSO neutral conditions observed over the equatorial Pacific Ocean in the beginning of the year turned in to weak La Niña conditions by the end of August 2020 as predicted by IMD.