Rainfall in the mountains could make flooding/landslide situation worse

Photo: Mike Jamieson

Slide and washout on Hwy 5 near Portia chain up area Sunday, Nov. 14, 2021

It’s not an ideal combination.

An "atmospheric river" moving over British Columbia is not just bringing heavy rain and snowfall. It’s also carrying warm air into higher elevations, and that could translate into rainfall causing rapid melt of the snowpack.

“The problem is that we’re adding, in addition to the heavy rain, we’re adding some runoff from the quickly melting snow, you know, at 1,000-1,500 metres and even up to 2,000. So that’s all adding to the runoff to lower elevations,” said Environment Canada Meteorologist Philippe-Alain Bergeron.

The atmospheric river, formerly known as the "pineapple express," is a large system of warm, moist air from the subtropical Pacific Ocean that clashes with colder air as it hits the coast of B.C. and moves inland. The forecast high in Kelowna, Kamloops and Vernon is 12 C Monday, and Penticton’s high is 14 C.

“Right now it’s stalled, for the time being, for the next 24 hours or so. But then we can see that it will finally start moving, initially quite slowly, and then it will accelerate towards the south. It will end up in Idaho and Washington and eventually Oregon late Monday. Then we will see our precipitation start to ease gradually pretty much from north to south,” explained Bergeron.

The system is especially concerning in areas of the Interior impacted by forest fires over the summer.

“That’s not meteorology per se, but people in forestry and geomorphology will tell you that rain on burn scars is very problematic as well. You have more likelihood for landslides.”

Bergeron points out that precipitation has been above-normal for most of the fall after we had record-setting heat and dry conditions over the summer.

“The climatology in November, typically, will get some of the heaviest precipitation of the year. This is compounded if you already have a little bit of snow on the ground at mid to higher elevations and then you get an event like this, where the freezing level is very high, then that’s more problematic.”

Flooding and landslides closed portions of Highway 1 and Highway 5 through the Fraser Canyon and near Hope Sunday, and Avalanche Canada issued avalanche bulletins in the mountain parks, including through the Rogers Pass.

In 2013, the devastating flood in Calgary and other parts of southern Alberta was attributed to heavy rain falling on the melting snowpack.