The onset of monsoon marks the beginning of the four-month rainfall season. (Reuters)

The sluggish pace of monsoon has further delayed its onset over Kerala to June 7, private weather forecaster Skymet has said, revising its earlier forecast date.

Skymet previously said the monsoon would reach Kerala, commencing the four-month rainfall season in the country, on June 4 with an error margin of plus or minus two days.

"(But) present weather conditions are indicating monsoon onset would be now around June 7, with an error margin of plus or minus 2 days," Skymet said.

The India Meteorological Department has said the expected day of monsoon arrival over Kerala could happen on June 6, with an error margin of plus or minus four days.

"Dynamics of monsoon keep changing. So far not even one out of the three criteria needed for declaring the onset of monsoon has been met. In fact, it would take some more days for conditions to become favourable for the same," said G P Sharma, president, meteorology and climate change, Skymet Weather.

Mr Sharma pointed out three main reasons for the sluggish pace of monsoon.

There has been a low pressure area off the Somalia coast on the Horn of Africa, which is governing the wind pattern. The system has taken away the moisture and restricting the stream of westerly winds over western Arabian Sea from reaching Kerala, he said.

An anti-cyclone - a high pressure system making the air more dense - over central Arabian Sea is pushing northerly winds that too are blowing along the west Coast. These winds are not instrumental in giving rains over the coast, Sharma said.

The Somali jet phenomenon too is very important for the onset of monsoon surge, he added.

Somali Jet is the core of strong winds that originate from Kenya, cross equator as a part of cross-equatorial flow while blowing along the Somali coast and finally entering the Indian Ocean Region and moving towards Kerala.

This pattern has not been established so far, Sharma said.

Monsoon made an appearance over the Bay Islands in Andaman and Nicobar on May 18, little early than its normal onset date of May 20.

However, its progress has remained extremely sluggish, Sharma said.

By May 27, the monsoon surge covered some more parts of Andaman and Nicobar Islands and by May 30, it covered a significant portion of Bay of Bengal and Islands including Port Blair.

Usually, by May 25, the Northern Limit of Monsoon (NLM) covers both Port Blair as well as Sri Lanka, Skymet said.