Thunderstorm watch in effect for B.C. Interior; landslide warning issued

A severe thunderstorm watch across B.C.'s southern Interior has prompted the Lower Similkameen Indian Band (LSIB) to extend a landslide warning in parts of the Similkameen Valley already vulnerable due to past wildfires.

Environment Canada issued a thunderstorm watch on Sunday for the Okanagan, Shuswap and West Kootenay regions, including Kelowna, warning of possible hail, flash flooding and strong wind gusts. 

In response, the LSIB has extended its slope monitoring system warning for the Ashnola and Chopaka areas until 1 a.m. PT on June 26. The original alert was set to expire Sunday.

"We've had a slope monitoring program in effect here for a couple of months now," said Justin Smith, communications co-ordinator and emergency response manager for the band.

"Because of the number of recent fires within the Similkameen Valley and further west and north, we initiated this process."

The warning system, launched in partnership with Westrek Geotechnical Services, includes a static weather station set up on Keremeos Mountain in early April to provide real-time slope monitoring for the surrounding areas.

Smith says much of the landscape is still recovering from wildfires like the Snowy Mountain blaze in 2018, which left behind severe burn scars and damaged root systems, weakening the structural integrity of the soil.

"Some of the soil has been literally cooked to where it will not support life," he said. "Without the root systems of trees and native grasses, it causes the slope to lose strength.

"When you get 10 to 20 millimetres of precipitation over a short period, it can overwhelm that base and cause debris flows, or even a mudslide."

Though most of the Ashnola area is uninhabited, Smith says one LSIB village sits right at the confluence of the Similkameen and Ashnola rivers, making continuous monitoring essential.

"The Snowy Mountain fire burned significantly into that area. We later saw banks collapsing and flooding that caused serious infrastructure damage in Chopaka," he said.

He says it's not a question of if another slide will happen, but when.

"We want people ... to be safely looked after, or at least have a warning about it."

In its bulletin, Environment Canada warned that conditions are favourable for the development of severe thunderstorms that may be capable of producing strong wind gusts, large hail and heavy rain.

The agency also cautioned that heavy rain can cause flash flooding and significant reductions to visibility, while strong wind gusts can damage trees, buildings and overturn vehicles.