What Is an Atmospheric River? The Weather Phenomenon Flooding BC
Atmospheric rivers dump enormous amounts of rain on BC in very short periods. The 2021 event caused catastrophic flooding that cut off Greater Vancouver from the rest of Canada.
Rivers in the Sky
Atmospheric rivers are long, narrow corridors of concentrated water vapour in the atmosphere — essentially rivers of moisture flowing through the sky. A single atmospheric river can carry 7-15 times the water volume of the St. Lawrence River. When one of these moisture plumes makes landfall against BC's mountains, it can dump months of rain in just a few days.
The November 2021 Disaster
On November 14-15, 2021, an exceptional atmospheric river made landfall in southern BC. The Coquihalla Highway received over 100mm of rain in 24 hours. The resulting floods and landslides were catastrophic: the Coquihalla Highway — the main route between Vancouver and the interior — was destroyed in multiple locations. The Trans-Canada Highway was also blocked. Greater Vancouver was briefly cut off from the rest of Canada by road. Thousands were evacuated. Agriculture in the Fraser Valley — Canada's most productive agricultural region — was devastated, with hundreds of thousands of livestock drowned.
Why BC Is So Vulnerable
BC's geography makes it particularly susceptible. The Coast Mountains act as a barrier to moisture-laden Pacific air, forcing it to rise rapidly. Rising air cools and releases its moisture as precipitation — often in extraordinary quantities. Climate change is intensifying this pattern, making extreme atmospheric river events more frequent and intense.
Improved Forecasting
Atmospheric rivers can now be detected and forecast up to 10 days in advance using satellite imagery and numerical weather prediction models. Environment Canada and BC's Ministry of Transportation use this information to prepare emergency responses ahead of major events.
