The India Meteorological Department on Wednesday forecast that the onset of the South-west Monsoon over Kerala is likely on May 29, with a model error of plus or minus four days. In other words, the monsoon could enter the Indian landmass between May 25 and June 2. The normal date of onset over Kerala is June 1. Meanwhile, with the strengthening and deepening of the monsoon winds and widespread rainfall activity, the monsoon has already entered the waters surrounding the country. It advanced into the south-east Bay of Bengal, Andaman and Nicobar Islands and the north Andaman Sea on Monday, eight days in advance. Operational forecasts The IMD has been issuing operational forecasts for monsoon onset since 2005. Last year, the IMD had forecast that the monsoon was likely to set in on May 24 and the actual onset was four days later on May 28. In 2006, the forecast was for May 30, and it set in four days earlier on May 26. In 2005 also, there was a gap of four days between the predicted date (June 11) and the actual onset (June 7). During the last 50 years, the earliest onset over Kerala was in 1960, when the system set in on May 14 and the most delayed onset was in 1972, when the monsoon arrived only on June 18. Model error Speaking to The Hindu, a senior IMD official emphasised that the date of onset over Kerala had no one to one correspondence between the monsoon onset date over Kerala and the pattern and quantum of rainfall during the four-month season. The IMD, in its annual long term forecast issued on April 16, has predicted that the rainfall for the entire season and for the country as a whole was likely to be 99 per cent of the long period average, with a model error of plus or minus 5 per cent. Asked about the La Nina conditions over the equatorial pacific region, the official said the phenomenon is continuing to steadily weaken and as such there was not likely to be any La Nina or El Nino effect this year. The system, which developed during August last year, has been declining in strength. [The La Nina, which develops when the sea surface temperature over the equatorial Pacific becomes colder than normal, has been found to bring more rainfall over India.