The B.C. government declared a state of emergency on Nov. 17 as severe flooding caused by an atmospheric river cut off all access routes into cities and towns. Photo: B.C. Government / Flickr
The West Coast of Canada is known for its wet autumn weather, but the storm that British Columbia’s Fraser Valley experienced over the weekend was one for the record books.
A weather system called an “atmospheric river” flowed across the southwest corner of the province and, over a period of two days, brought strong winds and near-record amounts of rain, which caused widespread flooding and landslides. So far, one person has died.
Hope, Merritt and Princeton, which were particularly hard hit, received 100-200 millimetres (or more) of rainfall. And all of the highways connecting Vancouver to the rest of the province were closed due to washouts and landslides, isolating the city from the rest of Canada, at least by road, with significant economic impacts.
What is an atmospheric river?
An atmospheric river is a band of warm, moisture-laden air many hundreds of kilometres long and hundreds of kilometres wide that borders a large cyclonic low-pressure system.
The term has been applied to bands of moisture-associated weather systems that move inland from the Pacific Ocean. An atmospheric river can reach the coast anywhere between southeast Alaska and Northern California.
Because of the large amount of moisture conveyed in these weather systems, the term has become a metaphor for a terrestrial river. However, atmospheric rivers are not carried in a channel like a real river and can dump prodigious amounts of rain over large areas.
Atmospheric rivers from as condensed water vapour flows from the ocean onto land. Image: NOAA
Was it a record-breaking rainfall?
The weekend storm was noteworthy for both its duration and its intensity. It rained continuously over much of the storm track for more than 24 hours and the rate was higher than normal autumn rains.
In both cases, rapid runoff exceeded the carrying capacity of streams and rivers, causing them to spill out onto floodplains. In addition, flat areas like the Sumas Prairie area between Abbotsford and Chilliwack had already been saturated with October and early November rainfall, were unable to drain away the water and were consequently flooded.
Furthermore, snow at high elevations melted under the warm rainfall, exacerbating the flooding. Rainfall on Nov. 14 set records for many places in the region. For example, Abbotsford recorded 100 millimetres of rain that day, exceeding the previous record of 49 millimetres set in 1998. Hope had 174 millimetres, five times the amount in the record year of 2018.
Soils in forested landscapes are hydrophobic after severe fires — they repel water. Super-heated air during a wildfire disperses waxy compounds found in the uppermost forest litter layer. The compounds coat mineral grains in the underlying soil, making it hydrophobic. The water-repelling layer is typically found at or a few centimetres below the ground surface and is commonly covered by a layer of burned soil or ash.
Many areas burned during the 2021 wildfires, such as around Merritt, were deluged by rain the weekend of Nov. 13. It is possible that runoff from these burned grounds was greater and more rapid because of the hydrophobicity of the soil.
These events are so far “off scale” in comparison to past historic extreme events that climate modellers assert that they would not have happened or would not have been as severe were it not for climate change.
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The B.C. government declared a state of emergency on Nov. 17 as severe flooding caused by an atmospheric river cut off all access routes into cities and towns. Photo: B.C. Government / Flickr
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But the reality is this kind of digging takes lots of time and no small amount of money.
As many newsrooms cut staff, The Narwhal has doubled down on hiring reporters to do hard-hitting journalism — and we do it all as an independent, non-profit news organization that doesn’t run any advertising.
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Threats to our environment are often hidden from public view.
So we’ve embarked on a little experiment at The Narwhal: letting our investigative journalists loose to file as many freedom of information requests as their hearts desire.
They’ve filed more than 300 requests this year — and unearthed a veritable mountain of government documents to share with readers across Canada.
But the reality is this kind of digging takes lots of time and no small amount of money.
As many newsrooms cut staff, The Narwhal has doubled down on hiring reporters to do hard-hitting journalism — and we do it all as an independent, non-profit news organization that doesn’t run any advertising.
Will you join the growing chorus of readers who have stepped up to hold the powerful accountable?
Caving under the pressure of a mounting political crisis, Ontario Premier Doug Ford has reversed the decision to open the Greenbelt. Speaking at a podium...
Investigative reporting like The Narwhal’s is now blocked on Facebook — and soon Google will do the same. One way to make sure you still get the facts? Sign up for our free newsletter.
Investigative reporting like The Narwhal’s is now blocked on Facebook — and soon Google will do the same. One way to make sure you still get the facts? Sign up for our free newsletter.
