Fewer weather stations reported extremely heavy rainfall this monsoon, as compared to last year. But the corresponding figure was higher than in 2017 and 2018, which is in keeping with the trend of a rise in extremely wet weather, India Meteorological Department (IMD) said in its analysis of the monsoon season on Thursday.
Officially, the monsoon season ended on Wednesday (September 30).
High variation in rainfall received in each of the four monsoon months since June 1 was unique this year, according to IMD.
The IMD data showed that 341 weather stations reported extremely heavy rainfall, measuring above 20 centimetres (cm) this year, as compared to 554 in 2019; and 321 and 261 in 2018 and 2017, respectively.
Very heavy rainfall, measuring between 11 and 20 cm, was recorded by 2,253 weather stations this year, as compared to 3,056 in 2019; and 2,181 and 1,824 in 2018 and 2017, respectively.
The total rainfall recorded this monsoon at 108.7% of the long period average (LPA) is the third-highest since 1990.
In 1994 and 2019, 112% and 110% of the LPA, respectively, were recorded.
This and the previous monsoon are the two consecutive years with above normal monsoon rainfall since 1958 and 1959, IMD said in its analysis.
In June, IMD had forecast normal monsoon rainfall of 102% of the LPA with error margin of +/-4% for the season. Rainfall range of 96% to 104% of the LPA is considered normal and 104% to 110% is considered above normal.
India had recorded above normal rainfall of 110% of the LPA last year.
“Considering month to month rainfall variation over India, the season is very uniquely placed in the historical record for its distinct and contrasting monthly variation. The rainfall over the country was 118%, 90%, 127% and 104% of the LPA during June, July, August and September, respectively,” IMD said.
IMD said that its region wise LPA forecast was off the mark.
In May, IMD had forecast 107% of the LPA for north-west India; central India (103%); east and north-east India (96%); and southern peninsula (102%).
But, the recorded rainfall over these regions was 84%, 115%, 107% and 129% of the LPA, respectively. “Thus, the forecasts of season rainfall over central, north-east and southern peninsula regions were an underestimate to the actual rainfall, while the forecast for north-west India was an overestimate. The forecast for the second half of the monsoon season (August – September) for the country was 104% against the actual rainfall of 118% of the LPA,” IMD said.
“Long range forecast for a four-month period and that, too, for various regions often leads to variation. It cannot be accurate. The longer the period, forecasting becomes trickier,” said K Sathi Devi, head, national weather forecasting centre (NWFC), IMD.
“The all-India average of monsoon rainfall was normal as per definition. However, that doesn’t give much insight into the regional manifestation of the monsoon. This year, the two cyclones during the onset, the low-pressure systems in the Bay of Bengal, and the occasional strong westerlies influenced the monsoon variability. The Arabian Sea temperatures were high throughout the season, enhancing the moisture supply, triggering some of the heavy rainfall events that led to regional floods,” said Roxy Mathew Koll, a climate scientist at Indian Institute of Tropical Meteorology in Pune.
Though officially the post-monsoon season has started from October 1, widespread and heavy rainfall is expected over Odisha, Jharkhand, Gangetic West Bengal during the next four-five days. Isolated heavy to very heavy rainfall is also likely in these states. This is mainly due to the influence of a low-pressure area over west central Bay of Bengal, off north Andhra Pradesh (AP) and southern Odisha coasts. Very heavy rainfall is also likely over Assam, Meghalaya and Tripura for the next five days.
“Monsoon withdrawal cannot be declared from these regions until rain stops completely. The withdrawal parameters have to be met. It’s difficult to say when the monsoon will withdraw completely,” added Devi.