Five people were confirmed dead, four at sea and one on land, while dozens of fishermen were missing, police spokesman Buddhika Siriwardena said.
"The navy and the airforce have mounted a search for dozens of fishermen who have been reported missing in unusually rough seas," he said.
The Disaster Management Centre said that rains and strong winds forced authorities to open the sluice gates of four major hydroelectricity reservoirs in central Sri Lanka to prevent damage to dams.
Military spokesman Ruwan Wanigasooriya said ground troops were also deployed to help in relief and rescue operations.
Fallen trees blocked many roads, including the main highway to the Bandaranaike International Airport, just north of the capital, police said.
The bad weather was forecast to continue over the weekend with the South Western monsoon packing winds of up to 70 kilometres (43 miles) an hour and dumping heavy rain.
Seven people were killed in Sri Lanka last month when a tropical cyclone br