A map of an atmospheric river hitting Vancouver Island with cartoon boxing gloves lining up to punch the Island.

Photo Credit: Windy.com / VanIsle.News Staff

Ranking Atmospheric Rivers

Environment Canada is coming up with a new way to warn us when a bad one is coming

They aren’t a new thing, but climate change is making them worse

Atmospheric rivers have been in the news a lot this week. First, a huge one dumped a month’s worth of rain over November 14th–15th. It caused devastating flooding on Vancouver Island and the mainland.

Now, the NorthIsle and Haida Gwaii are supposed to get another one in the next day or two.

An atmospheric river is a long, narrow band in the atmosphere that sucks up large volumes of moisture from warm, tropical regions. It really is like a river in the sky.

Only when an atmospheric river gets going it can carry up to 70 times the amount of water flowing through the mouth of the Fraser River.

That’s why a big one can do so much damage.

They aren’t a new thing. It’s just that science only figured out how to describe them 20 years ago.

Ruping Mo is a senior research meteorologist with Environment Canada. He and a team of 9 other researchers have been working on a ranking system to help warn folks when a bad atmospheric river is coming. This new system is based on the approach used in  California. It ranks them from AR1 (weakest) to AR5 (strongest).

Think of it like the ranking system used for hurricanes or tornadoes—when the rank goes up; you can expect a worse storm.

Mo told the Times Colonist that the Canadian ranking system “is based on what is the impact, not just the strength of the atmospheric river.”

University of Victoria climatology professor Charles Curry told CTV News that Environment Canada could use the weather data it already collects to help decide on the rank.

“That information can be directly piped from weather forecasts and other data into an analysis which calculates that category.”

Then that data is mixed with information about specific regions to help figure out how much damage an AR could do.

So the same storm might be an AR3 on WestIsle where big rainforests can absorb tons of rain, but an AR4 in places like Hope or Abbotsford where dry forests and farms can’t take that much.

The new system is still being tested out. The research team is working out some bugs.

For example, Mo told the Times Colonist that last week’s colossal storm was ranked as an AR4 until the day before it hit. Then the day it hit, the system bumped it up to an AR5.

It could be because last week’s storm was off the charts, so to speak.

“We’ve never seen an event like that,” Mo said. “This event really caught us by surprise. We didn’t expect the impact would be so huge.”

Another problem is that climate change is making atmospheric rivers stronger. It’s getting harder to figure out what “normal” is these days.

Matthias Jakob, a UBC adjunct professor in the earth and ocean science department, told the Times Colonist that we could expect stronger AR storms in the future.

In fact, we could see storms that dump twice as much rain as this last big one.

“Atmospherically, it’s possible. It’s not out of the question,” Jakob said.

“The sky’s the limit for one of these superstorms.”