Thiruvananthapuram, May 16 (IANS): The onset of the Southwest Monsoon over Kerala is likely be slightly delayed this year from its normal date, which is June 1, according to the IMD.

The monsoon's onset over Kerala is now likely to be on June 4 with a model error of plus/minus 4 days, the Kerala State Disaster Management Authority quoting the IMD report.

The manner in which the IMD monitors the arrival of monsoon is by recording the rain data at the 14 weather stations located at Lakshadweep, Kerala and in Karnataka's Mangaluru.

The officials monitor the rains starting from May 10 and if 60 per cent of the stations or more record 2.5 mm of rain for two continuous days, then it satisfies the criterion that the monsoon has arrived.

Another yardstick that the IMD looks is the strength of the wind.

Cloud movements are also a key indicator for the weathermen when it comes to announcing the monsoon.

In the past one decade, the IMD operational forecasts have turned to be correct.