The advisory, issued by the Natural Hazards Early Warning Centre at 6.30 a.m. today, is valid until 3.30 p.m.
Very heavy showers exceeding 200 mm are expected in the Western and Sabaragamuwa provinces and in the Puttalam, Galle and Matara districts.
Heavy rainfall of around 100 mm is also likely in parts of the Northern and North-Central provinces and in the Trincomalee and Kurunegala districts.
Authorities have urged the public to take necessary precautions to minimise damage caused by heavy rain, strong winds and lightning during thundershowers.
According to the Department of Meteorology, the highest rainfall recorded during the past 24 hours was 204 mm in Kesbewa, Colombo District.
