Rain over the western coast, especially in Mumbai, is likely to peak between June 8 and 10, private agency Skymet said.
However, the government said as monsoon is strengthening, increased rains were expected from June 7 in Kerala, coastal Karnataka, Konkan belt and Goa. It also warned of "possible flooding" from June 10 onwards in these areas.
Monsoon, which made an onset in Kerala on May 29, three days before its scheduled arrival, has advanced in some parts of Tamil Nadu, and southwest, west-central, east-central, northeast of Bay of Bengal, and in most of the northeastern states, the weather office said.
"Conditions are likely to become favourable for further advance of southwest monsoon into some more parts of south peninsula, Bay of Bengal, Assam, Arunachal Pradesh and remaining parts of Tripura and some parts of Meghalaya, and sub-Himalayan West Bengal during next 2-3 days with the likely development of favourable circulation features and strengthening of cross equatorial winds.
"Rainfall activity is likely to increase over parts of Maharashtra and Goa states from June 6, leading to advance of southwest monsoon over these areas during June 6 to June 8," the IMD said in its bulletin.
Secretary, Ministry of Earth Sciences, M Rajeevan, said monsoon is "strengthening" from June 7 and Kerala, coastal Karnataka, the Konkan belt, including Mumbai, and Goa are likely to witness "very heavy rains" and "possible flooding" from June 10 onwards.
"Monsoon is strengthening. Increased rains expected from 7 June Kerala Coastal Karnataka Konkan & Goa coasts Very heavy rains & possible flooding from 10th onwards especially coastal Karnataka Konkan & Goa including Mumbai Details of IMD forecasts .Watch out," he said in a tweet.
"Sustained heavy raining coming for a week starting on the 6th of June. (June) 8/9/10 can be extremely heavy. Warnings should be issued now. Just stay indoors," Skymet CEO Jatin Singh said on Twitter.
A cyclonic circulation is forming over Konkan and Goa, and it will gradually move north along the Maharahstra coast, Mahesh Palawat, vice-president, Skymet, said.
"Mumbai, Dahanu, Thane, Ratnagiri and Sindhudurg could witness heavy to very heavy rains and extremely heavy rains from June 6-10. From June 10-11, Surat, Valsad and adjoining districts of south Gujarat will also receive heavy to very rains," Palwat said.
On Saturday, three people, including two children, died due to electrocution as pre-monsoon showers hit Mumbai.
The mega-city and the western coast are prone to very heavy rains.
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