Forecasters have elevated their warnings about an atmospheric river system that is expected to hit coastal British Columbia on Friday, bringing potential flooding, heavy rain and high winds.

B.C.鈥檚 River Forecast Centre has issued a flood watch for the south and central coasts, while Environment Canada has upgraded its special weather statement over the region to a rainfall warning, with mountainous areas of Vancouver Island expected to get more than 200 millimetres of precipitation.

The weather agency says the atmospheric river system will arrive early Friday and persist through provincial election day on Saturday in places including Metro Vancouver, Whistler and almost all of Vancouver Island.

The north and central coasts from Bella Bella to Haida Gwaii are also under a wind warning, with gusts expected to reach 110 kilometres an hour.

The River Forecast Centre says a high streamflow advisory is in effect for the north coast, upper Fraser and the Thompson regions.

Environment Canada issued the first snowfall warnings of the season along the British Columbia and Yukon border, with accumulations up to 20 centimetres expected in some areas.

The weather office says the snow will spread through southwestern Yukon until Saturday.

It says 10 centimetres of snow is expected in most regions, but predicts up to 25 centimetres in Swift River.

It says an arctic ridge of high pressure will clear the skies on Sunday and temperatures will fall to about -20 degrees Celsius by Monday.

Environment Canada says the 鈥渇irst substantial snow鈥� is also expected south of the border in Fort Nelson, B.C., starting Friday.

It says about 10 centimetres is expected in most regions, but there could be more than 20 centimetres close to the border.

The weather office is warning drivers about low visibility Friday night due to drifting snow.

It says the Trans-Canada Highway near Rogers Pass may also 鈥渟ee wet snow Friday afternoon before it quickly changes to rain as the weather system brings in mild air.鈥�