A moisture-laden system, which will be rated as a Category 3 atmospheric river, is expected to bring heavy rainfall starting Friday, targeting the South Coast. This will be the first of the season to impact the region. Rainfall warnings and flood watches have been issued to alert residents of prolonged heavy precipitation and strong winds through the weekend.

Atmospheric rivers transport huge amounts of moisture, like rivers in the sky. This moisture can fuel heavy rains when it arrives on the coast.

General rainfall totals of 40-70+ mm are expected, with up to 100 mm of rain possible along the coastal mountains. Localized totals of as much as 200+ mm are possible along western and inland portions of Vancouver Island.

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Strong southeasterly winds up to 70 km/h will also impact areas of Vancouver Island, the Sunshine Coast and Metro Vancouver on Saturday, which could result in power outages and travel impacts, including ferries. There's an increased risk of washouts, rockfall, and even landslides where the heavier rain falls.

Moisture from a low-pressure system will be funneled between a trough off the coast of B.C. and a ridge of high pressure across Oregon and California. A temperature clash will create favourable dynamics to draw up tropical moisture and steer it toward the South Coast.

The ridge to the south won’t allow the atmospheric river to slump south quickly, increasing rainfall totals across western Vancouver Island and the higher terrain across the Lower Mainland.

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Rain Across the Coast

Rain will start from the system Thursday overnight for the central and north coast, but by Friday morning, the entire coast will be engulfed in soggy conditions.

Rainfall warnings are in effect for western Vancouver Island ahead of this system’s arrival.

The rain will pick up in intensity across Vancouver Island on Friday afternoon, continuing steady through Saturday afternoon. Up to 150 mm may fall along the coast, with as much as 250 mm in the mountains.

The east side of Vancouver Island is slightly protected from the rain shadow effect, but could still expect 50-75 mm in places like Nanaimo. Meanwhile, Victoria is forecast to see 30-50 mm of rain, which could make for the rainiest event for the region since last February.

The Lower Mainland will see persistent rain on Friday and Saturday, with some heavier periods on Saturday, especially for Squamish, and North and West Vancouver. Rainfall amounts will vary greatly, from as little as 50 mm in Delta, to upwards to 100 mm in downtown Vancouver, and over 100 mm in North and West Vancouver.

Impact on Rivers, Roads and Travel

Rivers and watersheds aren't running particularly high, so the watershed will be able to take an influx of moisture.

Expect ponding on low-lying roads, however, and landslides could become a concern in areas like Highway 4 on Vancouver Island.

The winds will also be noticeable in the south, with gusts between 50-70 km/h expected on Saturday. This could impact travel, as well, and could result in localized power outages.

Be Prepared for the Storm

Be sure to check back for the latest weather updates across British Columbia this week. Residents should prepare for the possibility of flooding and downed trees and power lines. Those living in areas prone to flooding should take steps to protect their homes and belongings.

With files from Tyler Hamilton, a meteorologist at The Weather Network