The India Meteorological Department on Tuesday said the southwest monsoon, which has advanced further into the south Arabian sea and south-west Bay of Bengal, is expected to hit the of Kerala by 9 June.
The IMD had earlier forecast that the June-September monsoon will have a delayed onset in Kerala on 7 June, with an error margin of four days.
''In view of strengthening of cross equatorial flow and deepening of westerlies over south Arabian sea, onset of monsoon over Kerala is very likely by 9 June,'' an IMD bulletin said.
The second half of June is expected to receive more rainfall than the first half, and due to a La Nina situation, more rains are likely during September.
''Pre-monsoon to monsoon transition is to occur, there should be a change in circulation pattern,'' said D S Pai, head of IMD's long range forecasting division.
''There could be several reasons for the delay in monsoon onset, such as the El Nino and Indian Ocean warming. Pre-monsoon also came in the last week of May which is a significant delay for pre-monsoon,'' he added.
In its fortnightly outlook on Tuesday, the Australian Bureau of Meteorology said that the tropical Pacific Ocean remains in a neutral El Nino-Southern Oscillation state - that is, neither El Nino nor La Nina conditions prevail. But the agency maintained that there is around a 50-per cent likelihood of La Nina forming later in 2016, a phenomenon associated with above-average rainfall in India.
In its second forecast, the IMD said the odds overwhelmingly favoured an above-normal monsoon - 106 per cent of the long period average (LPA). The monsoon is considered normal when the rainfall is 96-104 per cent of the LPA and is considered above normal when it is 105-110 per cent of the LPA. IMD has also forecast 107 per cent rainfall in July and 104 per cent in August.
Meanwhile, heat wave conditions continue in Rajasthan, west Madhya Pradesh, Gujarat and east Madhya Pradesh. On Wednesday, heat wave conditions are expected to prevail in Rajasthan, Madhya Pradesh, Punjab, Haryana, Chandigarh, Delhi and south Uttar Pradesh.
Once the monsoon marks its onset over Kerala, the IMD expects it to rapidly advance over the rest of the country.
''There may be a slight delay in the advance till central India, but then, further progress will be normal,'' Pai said.
IMD declares monsoon onset if 60 per cent of the 14 stations enlisted in Kerala and Karnataka report rainfall of 2.5 mm or more for two consecutive days, and other criteria such as wind field are fulfilled.