Technical Analysis

What does weather forecast say

Maulik Tewari | Updated on May 04, 2013 Published on May 04, 2013


The India Meteorological Department (IMD) has predicted a normal monsoon - rainfall at 98 per cent of the long-term average, for the 2013 monsoon season of June–September, when the country receives about a three fourth of its annual rainfall.

Rainfall between 96-104 per cent of the Long Period Average (average rainfall received during the period 1951-2000) of 89 cm is considered normal or average.

This comes as good news with over half of the cultivated area in the country being rain fed and the agriculture sector employing over half of the Indian population.

There are in-fact five pre-defined rainfall categories — normal, above normal, below normal, deficient and excess rain with associated probabilities. For this year, the IMD has put the probability of normal monsoon (96-104 per cent of the LPA) at 47 per cent.

The likelihood of below normal rains (90-96 per cent of the LPA) is expected to be 24 per cent and that for above normal rains (104-110 per cent of the LPA) is 17 per cent. And, the chance of either of the extreme situations of deficient and excess rains is very low.

The above estimate given by the IMD is based on a statistical model. That is, it takes into account the historical relationship between the monsoon and certain predictor factors that have a bearing on the Indian monsoon.

These are the North Atlantic sea surface temperature, Equatorial South Indian Ocean sea surface temperature, East Asia mean sea level pressure, North West Europe land surface air temperature and Equatorial Pacific warm water volume, each of which is observed over specific months during a year.

However, note that the IMD’s forecast that we are talking about was issued last month and is the year’s first long range forecast.

The second one which is due in June will provide an update on the all-India forecast and also a regional rain distribution forecast covering the northwest, central, northeast and south peninsular parts of India.

There have also been no warnings so far on the El Nino and La Nina front. These can impact the performance of the South West monsoon in a significant way. El Nino, an abnormal warming of waters in the Eastern and Central Pacific is linked with poor monsoons.

This occurs every three to five years and can last up to 18 months. Conversely, La Nina, an abnormal cooling of waters in the Eastern and Central Pacific is associated with a heavy monsoon.

Last year, El Nino conditions were responsible for causing lower rainfall over large parts of northern, western and southern areas of the Indian subcontinent in the first half of the monsoon season.

Also, it needs to be mentioned that IMD’s forecasts have not always been very accurate. There was a rainfall deficit in 2009 which according to the official forecast was supposed to be a year of average rainfall.

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