Monsoon veers off forecast
The Southwest monsoon stuck to the theme of 2020 — deviating significantly from the geographical distribution forecast by India Meteorological Department (IMD) in June.
Peninsular India has recorded 26.3% excess rain this monsoon since June 1 and 78.6% excess rain in September (till September 14). It is expecting more rain due to a well-marked low-pressure area that has developed over the Bay of Bengal.
Northwest India has seen 13.5% lower rainfall since June 1 -- this isn’t a deficiency though because rainfall that is between -19% and +19% of the long period average is termed normal. However, there are parts within this region that have done worse -- west Uttar Pradesh has seen 30% lower rains; Himachal Pradesh, 18% lower; Uttarakhand, 15% lower; Jammu and Kashmir 27% lower; and Haryana, Chandigarh and Delhi, 6% lower. Rainfall that is between -20% and -59% of the average is termed deficient. West Rajasthan, also in the same region has seen a 30% surplus. Scientists said the north-western region is unlikely to record heavy rain between now and the end of the monsoon.
IMD in its long-range forecast in June predicted above normal monsoon rain over northwest India at 107% of the average, and normal rains over peninsular India at about 102% of long period average (LPA; monsoon average for 50 years), central India (103%), and east and northeast India (96%). Central India has recorded 14.5% excess rain so far (it is still in the normal category, though).
This monsoon has been characterised by several unique features , such as no low-pressure areas forming in July, resulting in the monsoon trough (line of low pressure) oscillating to the Himalayan foothills very frequently bringing rains mainly to the peninsular region and northeast India in July while leaving northwest India dry.
Five low pressure systems formed in August which brought excess rains to the central and western region but not much to the north-western region again.
“Long range forecasts have a 9% error margin. They are only indicative because they are forecast several days in advance,” said M Mohapatra, director general, IMD, explaining the divergence.
“In the first half, particularly up to July northwest India had a very large rain deficiency. Low pressure areas didn’t form and a couple that formed did not move up to northwest India. We are not expecting any improvement in rain deficiency over northwest India now because we are heading towards monsoon withdrawal. The long-range forecast doesn’t capture these peculiar patterns that can develop during the monsoon,” said DS Pai, senior scientist, IMD Pune.
The divergence highlights the challenge of forecasting weather, one expert said.
“The monsoon models usually consider parameters such as La Nina or El Nino (cold and warm currents in the southern Pacific) and other weather parameters which may develop during the course of the monsoon. But there are years when the complete forecast goes wrong. The models also consider the impact of climate change. One of the possible impacts of climate change is the path of the low-pressure systems. They are moving west wards but are not impacting the north-western region. Forecasting geographic distribution is a bit tricky,” said Mahesh Palawat, vice president, climate and meteorology, Skymet Weather, a private forecaster.
Both 2018 and 2019 also saw divergences. The long-range forecast for monsoon 2019 was 96% of LPA with a model error of ± 4% . The actual rainfall recorded was 110% of LPA. The forecasts issued in May for season rainfall over northwest India, Central India, northeast India and South Peninsula were 94%, 100%, 91% and 97% of the LPA respectively all with model errors of ± 8%. But the actual rainfall recorded over these regions were 98%, 129%, 88% and 116% of the LPA respectively. Central India and Peninsular India didn’t meet the forecast. In 2018, the forecast for northeast India was 93% but only 76% rain was recorded.
Normally, the withdrawal of monsoon is supposed to begin from September 17 when rains begin to reduce across the country until completely withdrawing on October 15.
But this year, IMD’s extended range forecast shows extensive and heavy rains along the west coast between September 11 and 24 and then until October 1 over several parts of central India.
“We haven’t got any indication as yet of monsoon withdrawal from northwest India. Another low-pressure area is likely to develop over Bay of Bengal around August 17 but we need to assess the models further to be certain. Monsoon withdrawal can begin only once anti-cyclonic flow is established and moisture reduces significantly over the region,” said K Sathi Devi, head, national weather forecasting centre.