VANCOUVER - Environment Canada issued a rainfall warning for much of Vancouver Island and B.C.'s Central Coast Saturday morning.

The agency said it expects rainfall amounts of 100 to 120 millimetres to fall in coastal areas of north and west Vancouver Island before rain tapers off on Sunday morning.

"We're looking at heavy rain," said Louis Kohani, a meteorologist with Environment Canada. "This is due to an atmospheric river aimed at Vancouver Island."

Environment Canada had previously issued special weather statements for the island, but upgraded to a rainfall warning Saturday morning.

Because the atmospheric river is tropical in origin, rain is expected at almost all elevations, with freezing levels upwards of 3,000 metres for the South Coast and near 2,500 metres for the North Coast, Environment Canada said.

High runoff and rising river levels are likely, the agency said, adding that localized flooding in low-lying areas is possible.

Relatedly, the provincial River Forecast Centre has issued a "high streamflow advisory" for coastal areas across B.C., including on Vancouver Island.

"River flows are expected to rise through Saturday, with peak levels expected through Sunday and into Monday," the centre said in its advisory.

"The public is advised to stay clear of the fast-flowing rivers and potentially unstable riverbanks during the period of rapid-rising river levels," the centre said.