IMD deficit forecast comes true

As the southwest monsoon enters its final leg this year, forecasts made by the much-maligned India Meteorological Department (IMD) now seem closer to reality.

The rains till Saturday have been 12 per cent less than normal. With only a month left for the rains to start withdrawing from major parts of the country, this deficit is unlikely to go down by much.

In fact, the Pune-based Indian Institute of Tropical Meteorology (IITM) has forecast large-scale withdrawal of the southwest monsoon from the country's northern parts in September. If it comes true, the overall deficit will widen.

CHASING RAINBOWS?

IMD

• April forecast: 95% of long period average
• June forecast: 88% of long period average
• Actual till Aug 30: 12% less than normal

SKYMET

• April forecast: 102% of long period average
• Early August: 98% of long period average
• Late August forecast: 90% of long period average

source: IMD & skymet

IMD classifies a meteorological drought as one where the overall rainfall deficiency is more than 10 per cent and 20-40 per cent of the country is under a drought-like condition.

September, last of the four-month southwest monsoon season, gets 173 mm of rain on an average, second lowest after June. So, even if there is a pickup in September's showers, it might not be strong enough to bridge the shortfall.

IMD's first forecast, in April, had said the rains would be 93 per cent of the long period average (LPA). In June, it revised this to 88 per cent of the LPA, signalling the possibility of a meteorological drought. The LPA for June-September is the average rain of 50 years from 1951, estimated at 886 mm.

Their June forecast was with an error margin of plus and minus four per cent. If the overall southwest monsoon stays at 88 per cent of LPA, which is where it now is, or goes down by two or three percentage points more, it would mean IMD's forecast was correct. And, that private weather forecasting agency Skymet erred in its first forecast, of rains at 102 per cent of the LPA. Skymet's first forecast for this year was also issued in April, days before IMD's official forecast.

Skymet had, in August, lowered its forecast to 98 per cent of the LPA, with a plus or minus four per cent margin error. Then, this Friday, it further lowered its 2015 forecast to 90 per cent of the LPA, much closer to IMD's June forecast of rains being 88 per cent of this figure.

"With two major months of the monsoon season recording significant rainfall deficiencies, a big recovery in September looks unlikely. Consequently, the revised monsoon forecast… This also means September will not see copious amounts of rainfall and will possibly end up with plus/minus five per cent of the monthly average rainfall of 173 mm," Skymet says on its website.

It also said that Madden Julian Oscillation (MJO), which has a bearing on India's southwest monsoon, made a brief appearance over the Indian Ocean in June. Rather, it remained for an extended period over the Pacific Ocean, which was of not much help to the Indian monsoon.

The Indian Ocean Diapole (IOD) retained the positive sign for a fairly long period, but it was never in consonance with MJO, which rendered it ineffective. On the contrary its positive effects were overshadowed by a strong El Niño.

It has now acknowledged that the El Niño weather phenomenon has remained very strong through the season and impacted rain over the country's southern and central parts. Initially, Skymet had discounted this impact. On the whole, then, it appears IMD did read the signs more accurately than the others.

The saving grace has been the better than expected spatial distribution of the rains, which ensured good kharif sowing.