The South-West Monsoon is crucial for Indian economy and agriculture. It accounts for almost all the annual rainfall in 75% of the geographical area and 78% of the gross cropped area in the country. Although the duration of the monsoon over various parts of the country varies from about two to four months, long range forecasts are made for the four-month period – June to September. The SW Monsoon on its return course becomes North-East Monsoon in September and feeds parts of South India for about two months or so.

The farmers of Maharashtra who reeling under drought are unable to buy the assurance for a normal monsoon extended by the India Meteorological Department (IMD).

The IMD’s forecast failed miserably last year. It revised its forecast twice. In the preliminary forecast in April 2012 it said that the countrywide rainfall will be normal at 99% of the long period average (LPA) of 89 cm, in June 22, 2012 it revised its forecast and said that the monsoon rains will be 96% of the LPA and in August 2, 2012 it scaled down its forecast to deficient monsoon at 90% of the LPA. Actually the monsoon season ended with deficient rainfall at 92% of the LPA. The IMD has explained it as “the unpredictable part of the variability is due to the natural variability (internal dynamics of the monsoon system”.

For this year the IMD’s recent forecast of a normal monsoon across the country has not totally removed apprehensions in different parts of the country. However, it has assured to issue and updated forecast in June alongwith forecasts for four broad homogenous regions, after taking into consideration the global weather data available till May. It will also come out with countrywide rain forecasts for July and August. Rain forecast for September will be done in August.

In its preliminary forecast issued in April, this year, the IMD has said that the countrywide rainfall in the four-month monsoon season in 2013 will be normal at 98% of the long period average (LPA) of 89 cm, subject to a model error of +-5%. The LPA is the average value of precipitation in the period 1951-2000.

The forecast done through a five-parameter statistical ensemble model using global weather data available till March says that the probability of a normal monsoon in the current year is 33% against a climatological value of 46%. The probability of the monsoon being below normal is 27% against a climatological value of 17%. The forecasters consider this climatological value of 17% quite high and hence the chances of monsoon rains slipping down to below normal level is not ruled out.

The monsoon is termed normal if the precipitation is in the range of 96% to 104% of the LPA. If the rainfall is between 90% to 96% of the LPA it is below normal and if it is below 90% of the LPA it is termed deficient. If the precipitation is in the range of 104% to 110% of the LPA it is termed above normal if it is above 110% of the LPA it is excess rainfall.

The 2013 monsoon can become below normal if the forecast model error of -5% occurs. The South-West Monsoon over the country is passing through 31-year cycle of depressed rainfall below LPA of 89 cm since 1985. Even if the monsoon turns out to be normal, the average precipitation will not be above the LPA.

Though global forecasts say that the monsoon’s spoilsport, El Nino, is unlikely to emerge in the Pacific Ocean, the possibility of development of a negative Indian Ocean dipole mode may weaken the South-West Monsoon in the country.

The snow cover in the northern hemisphere, this year, was excess and this is not judged as a favourable factor for Indian monsoon. Besides the rainfall distribution over the country is strongly related to the topography of the Western Ghats and hills in the north-eastern India. The IMD while making its forecast has taken inputs from several Indian and global meteorological institutions.

However, using a coupled dynamical forecast model under experimentation, the IMD says that the average precipitation over the country in the four-month monsoon season is likely to be 104% of the long period model average (LPMA), subject to a model error of +-5%.

The South Asian Climate Outlook Forum (SASCOF), representing weather bureaus of South Asian countries, has however said that the monsoon rainfall in the sub-continent “will most likely be within the normal range with a slight tendency towards the higher side of the normal range.” Regarding India, it has specified areas bordering Pakistan, Kerala and Tamil Nadu will receive poor rainfall, while that bordering the Himalayan range will receive excess rainfall.

The IMD is yet to come out with its forecast for the monsoon arrival at the Kerala coast. Last year the monsoon arrival was delayed by three days in the Andaman Seas and by four days at the Kerala coast. The normal date for monsoon arrival in Andaman Seas is May 20 and at Kerala coast is June 1.

Last year after reaching the Kerala coast, the advancement of the monsoon was subsequently delayed in many parts of the country. However the covered the entire country by July 11, four days earlier than its normal date. The withdrawal of monsoon from the Rajasthan-Pakistan border commenced only on September 24 as compared to its normal date of September 1.

With the monsoon suffering hiatus in its advancement last year, many parts of the country had poor rainfall in June. Rainfall in July too was poor in many parts of the country. In fact the pickup in the monsoon activity began from the last week of July and the countrywide rainfall in August recorded 101% of the LPA and in September it was 111% of the LPA.

In the four broad homogenous regions categorized by the IMD, Maharashtra has been placed in the stretch of the Central India category alongwith Gujarat, Goa, Madhya Pradesh, Chhattisgarh and Odisha. While Odisha, Madhya Pradesh, Chhattisgarh got good rains last year, Gujarat and major parts of Maharashtra reeled under drought. This shows that the forecast of a normal monsoon may not equally benefit all the states in a homogenous region. It raises another question – how homogenous is the region?

Historically there has been no case of the occurrence of three or more consecutive deficient or excess monsoon years in the country. There were three rare events of two consecutive years of deficient rainfall namely 1904 and 1905, 1965 and 1966, 1986 and 1987. Also there were two events of two consecutive years of excess monsoon rainfall namely 1916 and 1917, 1955 and 1956.

The lowest and the second lowest monsoon rainfall occurred in 1918 (75.1% of the LPA) and in 1972 (76.4% of the LPA). Countrywide highest and second highest monsoon rainfall occurred in 1917 (122.9% of the LPA) and in 1961 (121.8% of the LPA). (IPA Service)