This is a big question for IMD(Indian Meteorological Department) that why we are not so accurate about there forecasts. There are a lot of variations and the Graph by IMD shown itself the same.
Here is a part of an article sourced from ‘The Wall Street Journal.’ shows that how our predictions varies from actual.
“…..India’s weather department this week unveiled its annual early forecast of how much rain the monsoon will likely dump on the Subcontinent this year. A precise prediction is crucial for the hundreds of millions of farmers who depend on the annual rains but history shows that these early forecasts are usually wrong.
Last year the India Meteorological Department predicted a slightly weak monsoon, but India got a drought. The year before it forecast below-average rains, and the subcontinent got well above average. In 2009 when India had its worst drought in decades, the IMD had predicted a normal monsoon.
The government’s weather watchers use the average rainfall for the last 50 years to define what is normal. If the rainfall for the June-through-September rainy season falls between 96% and 104% of the average, then that is a normal or good monsoon.
Rainfall of less than 90% is considered a drought, rainfall of more than 110% of the average is considered an excess.
The department has been monitoring the moody monsoon for more than a century and over the years has had to repeatedly tweak its forecasting model and acquire new technology to better predict the rains. But its earliest predictions still leave a lot to be desired.
While the IMD allows for a wide margin of error of 5% above or below its April predictions, it still rarely gets it right. Even within that 10 percentage point margin of error, its early prediction has been right in only six of the last 21 years.
This year the department predicted below-normal rains again which could be particularly bad news after last year’s drought and some freak weather earlier this year that has flattened winter crops….” Sourced : http://blogs.wsj.com/indiarealtime/
This is a big WHY??