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How Forecasters Got Their Monsoon Predictions Wrong, Again


India got its monsoon forecast wrong this year, consistently.

For starters, the Indian Meteorological Department’s (IMD) May 31 forecast for the onset of the monsoon in Kerala had raised hopes for early rains across the country. But this was revised, at the last minute on May 30, to declare the start of the monsoon for June 3.

IMD, on June 11, forecast that the monsoon would advance to the remaining parts of the country outside of southern Rajasthan and Kutch for the next six to seven days. But the northern limit of the monsoon has been stationary, passing through Diu, Surat, Nandurbar, Bhopal, Nowgong, Hamirpur, Barabanki, Bareilly, Saharanpur, Ambala and Amritsar since June 13.

Last week, IMD had forecast that the monsoon would start in Delhi on June 15, around 12 days before its normal start date. But on Tuesday June 15, IMD scientists said the start in Delhi could take another seven to 10 days due to a slump in the westerly winds that had weakened monsoon flow over northwestern India.

Read also | Unfavorable conditions for monsoon progress in parts of northwestern India

Forecasters, both government and private, know that any update on the monsoon can lift or dampen public sentiment, shape economic policy, and have recognized gaps in the forecasts. The fact that the monsoon itself is a very complex system due to its interactions with various land and ocean features makes the overall forecast during the season challenging, according to forecasters.

The explanation, the criticism

On Wednesday, IMD, in its bulletin, said that a western unrest was affecting the western Himalayan region and a cyclonic circulation was spreading over eastern Uttar Pradesh. These large-scale weather conditions are not conducive to further monsoon advance in Rajasthan, remaining parts of Gujarat, Punjab, Haryana, and Delhi. However, he said there could be slow monsoon progress in parts of Uttar Pradesh.

Experts said that meteorologists should have identified an impending slump in the westerly winds well in advance based on models and should have retained the forecast of the onset of the monsoon for Delhi. Monsoon onset dates are particularly important for northwestern India due to the relatively low average rainfall received in the region after a grueling summer.

“It is not a challenge to forecast or see a Western shock coming. They (IMD) probably did not anticipate the possibility. I’m not sure what exactly could have happened, ”said M. Rajeevan, secretary of the Ministry of Earth Sciences, who had said that the monsoon could slow its progress towards the end of the month.

But forecasting heavy rain events during the monsoon season is challenging. “Heavy or extremely heavy rain events during the monsoon are less predictable. Monsoon lows (low pressure area), monsoon depressions are relatively difficult to predict due to the complex nature of the monsoon. It is not impossible. The shorter the forecast period, the greater the precision, ”explained Rajeevan.

IMD forecasters may not have anticipated the change in the monsoon track due to strengthening westerly winds. “The area of ​​low pressure that had formed over the Bay of Bengal last week and is responsible for the rapid advance of the monsoon into northwestern India suddenly changed course. Its effect remained more concentrated towards Uttar Pradesh, Jharkhand, Bihar, etc. and did not progress towards Rajasthan. These changes can be monitored two to three days in advance and the IMD has alerted people accordingly about the change in start dates, ”said DS Pai, scientist and head of climate research and services at IMD Pune.

Also, while there is a set of statistical parameters that are followed for the onset of the monsoon in Kerala, in other parts of the country, the onset of the monsoon is driven by rain. “We see if there has been rain and if there is a chance that the rain will continue for a couple of days,” Pai added.

An inappropriate focus on wind patterns in the monsoon forecast may be a factor in forecasting faults, said Akshay Deoras, an independent meteorologist and PhD student at the University of Reading in the UK.

“There is a specific set of criteria for declaring the start of the monsoon over Kerala, which includes a combination of observed winds, cloud intensity and rainfall. However, very often, rain is given more importance than winds to declare the progress of the monsoon in other parts of India. Traditionally, great importance was placed on rain as observations at altitude were limited. However, we now have better upper-air observations and wind forecasts. While the monsoon progresses in June, it may still rain due to non-monsoon systems, such as western shocks. In such cases, wind patterns help to identify such events, resulting in better predictions of the arrival of the monsoon to a region, ”he said.

Deoras added that in the case of the Delhi forecast, the wind pattern should have been considered well in advance. “Wind patterns are considered to be more predictable than rainfall. Over the past ten days, various weather models showed a western disturbance that blocked the monsoon progress over northwestern India in mid-June. In such cases, the judgment of a forecaster is very important. “

Why did others get it wrong?

Skymet Weather, a private weather forecasting company, announced the start of the monsoon in Kerala on May 30, while IMD announced the start on June 3. How could there be such a great disparity?

“When we announced the start over Kerala, our criteria for monsoon rain and winds were met, but not the criteria for outgoing long-wave radiation (OLR). We still declare the onset of the monsoon because a notebook-style start is often not possible, ”said Mahesh Palawat, vice president of climate change and meteorology at Skymet Weather. OLR represents the total radiation that goes into space emitted by the atmosphere or the degree of cloudiness.

IMD had said on May 28 that the monsoon was likely to start in Kerala on June 31, one day before its normal date. But on May 30, IMD said the monsoon parameters are not yet being met. According to the IMD monsoon start parameters, after May 10, 60% of the 14 available stations listed (Minicoy, Amini, Thiruvananthapuram, Punalur, Kollam, Allapuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery, Kannur, Kudulu and Mangalore) would have To report rainfall of 2.5 mm or more for two consecutive days, the depth of the westerly winds must be maintained up to 600 hPa. The outgoing long wave radiation (OLR) value should be less than 200 wm-2 (watts per square meter).

“@Indiametdept will never manipulate data or make statements to justify its forecasts. They humbly admit anticipated failures. In weather and monsoon forecasts, no one can be perfect. We are also accountable to Indian taxpayers. We show respect to our country, ”M Rajeevan, secretary of the earth sciences ministry tweeted on May 30 after IMD’s sudden retraction of May 31 in Kerala.

Skymet Weather also did not anticipate the sudden pause of the monsoon before it starts in Delhi. “The models are most accurate for three or four days. Thereafter, its accuracy is reduced, which may be the case this time when forecasting the start in Delhi. The western component of the winds was not prominent before. People in North India are anxiously waiting for the monsoon rains, desperately waiting for the rain to cool the region, so the start dates are important to the people, ”added Palawat.

The next challenge

The real test for the prognosis is yet to come. During the monsoon season, there are sudden events of extremely heavy rains that are difficult to forecast and cause localized disasters. Summer monsoon rain over India has decreased by about six percent between 1951 and 2015, with notable declines in the Indo-Gangetic Plains and Western Ghats according to the Climate Change Assessment in the Indian Region, a report by the Ministry of Science of the land posted. last year.

There has been a shift in recent years towards more frequent dry spells and more intense wet spells during the monsoon. In central India, the frequency of extremely heavy rains with intensities above 150mm per day increased by about 75% during 1950-2015.

“Forecasting heavy rain events during the monsoon season remains a challenge despite recent advances in numerical weather prediction. Things get more challenging when the monsoon flow interacts with the mountains and with the low pressure systems of the monsoon, ”Deoras said. He noted the challenges in regional weather predictions for a region like Mumbai; the recent heavy rain event in the city on June 12 was not detected by weather models well in advance.

India receives about 70% of its annual rainfall during the four-month monsoon season; This is crucial for the country’s economy dependent on agriculture and for the cultivation of rice, soybeans and cotton. A normal monsoon this year will help the agricultural sector significantly. Good rains have been one of the main reasons for the resilience of the agricultural sector for two years despite the pandemic. India has more than 150 million farmers and almost half of the Indians depend on farm income. Up to 60% of the net sown area in India does not have access to irrigation.

And that’s why the stakes are high in getting the weather forecasts correct. 2021, once again, offers lessons.

How Forecasters Got Their Monsoon Predictions Wrong, Again

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