A season of change: On IMD forecast system

It is time for the India Meteorological Department to incorporate lessons from the new normal

Updated - April 18, 2020 12:30 am IST

Published - April 18, 2020 12:02 am IST

In the season of the abnormal, the IMD has announced that the monsoon this year would likely be normal . The agency follows a two-stage forecast system : indicating in April whether there are chances of drought or any other anomaly and then a second update, in late June, with a more granular look at how the monsoon will likely distribute over the country and whether danger signs are imminent. ‘Normal’ means India will get 100% of its long period average, with a potential 5% error margin. The IMD’s April forecast, experience suggests, is not much to go by especially if the agency declares it ‘normal’ as rarely, if ever, do weather models catch signs of an impending shortfall or a large excess in April. Also being a part of a hierarchical government set-up, the agency defaults to being conservative. In April last year, it said the monsoon would be ‘near normal’, an arbitrary category. Private forecasters expected a shortfall, predicated on the development of a future El Niño. The IMD did account for this but said it was unlikely El Niño would strengthen enough to dampen the monsoon. It however kept its estimate on the lower side of ‘normal.’ In the end, India received excess rains, the highest in a quarter century.

The April forecast is a vestige of the agency’s reliance on the ‘ statistical forecast system ’ where values of selected meteorological parameters are recorded until March 31 and permutations of these are computed and compared to the IMD’s archive of weather data. It is also reflective of an era when landline telephones were the state-of-the-art in personal communication. Along with connectedness, weather forecasting has metamorphosed. Climate, as well as technological change, allows new weather variables — such as surface temperatures from as remote as the southern Indian Ocean and regular updates from the Pacific Ocean — to be mapped. Powerful computers mathematically simulate the weather based on these variables and extrapolate onto desired time frames. Using these dynamical models is a change the IMD has incorporated and experimented with for years. It made two key changes this year: reducing the definition of ‘normal’ rainfall by 1 cm, to 88 cm and, officially updating monsoon onset and arrival dates for many States. This was long due and constituted acknowledgement of the accumulated impact that global warming has been having on monsoon patterns, particularly for cities and States. The monsoon was arriving later in many places, had long weak spells, and lingered longer. This has already heralded thinking, in the agency, on whether India should move to a new monsoon-accounting calendar instead of the century-long tradition of June-September. This would signal a truly momentous break from the past. Just as COVID-19 is forcing introspection on the links that tie people, trade and ecology, it is also time for the IMD to incorporate the lessons from the new normals.

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