Monsoon to be normal, says IMD
The country is likely to see “normal” monsoon rains, the India Meteorological Department (IMD) said in its forecast on Monday. However ocean temperatures in the Pacific and Indian oceans, around August, pose concerns.
India is likely to get 97% rainfall, defined as between 96% and 104% of the Long Period Average (LPA) of 89 cm it typically gets between June and September. The IMD has ruled out chances of a “drought,” ascribing only a 14% chance of ‘deficient’ rains and given a 42% likelihood of ‘normal’ monsoon rains.
In any given year, there’s a 16% chance of deficient and 33% odds of normal rains.
Not always accurate
The IMD’s forecasts in April aren’t always accurate. In April 2016, for instance, it said India would get 106% of the LPA — or ‘above normal’ rains in Met parlance. India ended up with 97% or ‘normal’ rains.
It also under-estimated rains in 2015 and 2014, when India ended up with deficits greater than 10%, or drought-like conditions.
Last year’s forecast was more accurate, with India getting 95% as opposed to the IMD’s 96% estimate.
Details, such as how the monsoon will pan out over the country and the quantum of rainfall in July and August — the key monsoon months — would be made available in the department’s June update, said IMD chief K.J. Ramesh at a press conference in New Delhi.
The Met department used a statistical model for arriving at its 97% estimate, which has an in-built error margin of 5%. Another model, called the dynamical model, forecasts 99% rainfall. This however is still a work-in-progress and the IMD hopes that this model will be the mainstay of monsoon forecasting in the coming years. The IMD bases its optimism of a normal monsoon this year on the absence of an El Nino, a relative warming of the surface-temperatures in the Central Pacific, and associated with a weak monsoon.
The converse phenomenon, of a La Nina and cooling of these waters, generally brings good rains. Currently La Nina conditions prevail and weather models suggest a slight warming or so-called “neutral” conditions to prevail around August.
“Our own outlook and weather models from various international agencies suggest that an El Nino is unlikely during the monsoon and neutral conditions are to prevail at that time,” said Mr. Ramesh.
However there’s a mass of warm water that lurks beneath the Western Pacific and “…if during the monsoon months that mass rises up and warms the surrounding cold water, it could be a problem,” said a senior meteorologist intimately familiar with the IMD forecast-preparation process. “Historically a transition from La Nina to neutral conditions has generally meant normal monsoons.”
Another worry lurks closer home. A positive temperature gradient in the West and Eastern Indian oceans, called a ‘positive Indian Ocean dipole’, is associated with neutering warm-water hostilities from the Pacific. Such a positive dipole is unlikely this year.
“The IOD conditions are among the reasons why we have forecast 97% and not 100% monsoon,” said D.S. Pai, Chief Forecaster, IMD.
Rainy days: The IMD bases its optimism of a normal monsoon on the absence of an El Nino.