Slow snowmelt continues but rain brings flood warnings

·4 min read

An atmospheric river of rain passing through the Southern Interior this week swelled rivers and streams, prompting weather forecasters and emergency officials to warn residents across the region to be ready for flooding.

The system, which was expected to drop 30-50 millimetres of rain on the Kootenays over Monday and Tuesday, pushed the region into a higher danger rating for spring flooding.

A flood warning was issued for the Elk River area in East Kootenay, and flood watches for the East and West Kootenay. The flood watch was extended at press time to include the Slocan River, Kaslo River, and the Upper Columbia, including Golden and Revelstoke areas. People were warned to stay away from rushing waters along streams and tributaries.

On Monday, residents living in the community of Six Mile east of Nelson were put on evacuation alert, after concerns were raised about a swollen Duhamel Creek.

“Current hydrologic modelling is indicating risks for flooding into the Tuesday period, particularly in the East Kootenay region,” said an updated report from the River Forecast Centre on Monday. “Flows in the 5-year to 10-year return period range are likely, with flows in the 20-year range or higher being possible.

“Given the uncertainty in the position of the weather pattern and heaviest rainfall, it is possible that adjacent areas in the West Kootenay and Upper Columbia also experience significant flows (e.g. exceeding 5-year to 10-year flows).”

Despite the short-term impact of the rains, longer-term forecasts are for only minor problems for people living on most major waterways in the region.

Slow snowpack melt

While snowpack levels remain high in the Valley Voice readership area – automated stations were recording an average of 170% of normal snowpack levels in the West Kootenay at the start of the month – the cool spring weather and less-than-normal rainfall had reduced the concern about flooding.

“The mean monthly temperature was more than two degrees below normal, and was the coolest mean monthly temperature since 1996,” says a report by weather forecaster Jesse Ellis at the Southeast Fire Centre. “Not since 1977 did temperatures fail to reach 25 degrees at some point during the month of May.”

Long-range weather forecasts predict the cooler weather will continue, until at least mid-June. And longer-term flood forecasts parallel that, predicting a slow, regular melt on West Kootenay rivers and tributaries.

The Province’s CLEVER prediction model, which forecasts river flows 10 days into the future, shows river and stream levels on the Arrow Lakes, North Kootenay Lake and the Slocan Valley to barely reaching the level they reach every two years (after the spike caused by this week’s rains).

Rain the wild card

Rain remains the wild card during the freshet. Thunderstorms or ‘rain bombs’ can quickly overwhelm rivers and their feeder streams. And that’s what residents should be prepared for, says Dave Campbell, the head of BC’s River Forecast Centre.

“The big thing is it’s another source of water. We could have 20-30 millimetres of snowmelt every day, then we get 20-30 millimetres of rain, and that’s when we see the rivers really responding,” he says.

“It’s the rain that’s concerning; it’s much less predictable.”

Another concern is a sudden return to hot weather, or even another ‘heat dome,’ which can spike temperatures up to the mid-40s.

Campbell said last year the Province “dodged a bullet” when the heat dome arrived, as most of the annual snowpack had already melted. But as summer 2022 approaches, the possibility of a heat event combined with freshet increases.

“Once it starts, it’s like a train with its momentum,” says Campbell. “We can get events where we get warm air in a high-pressure ridge. That can really accelerate the melt process. We can get quite a range of melt…

“[In 2021] we were seeing record snowmelt rates, of 80-100 millimetres a day. Typically, we think of 30 millimetres as a fast melt,” he said. “Even the deepest snowpack is maybe 1,000-2,000 millimetres of snow. You’d melt that off within a week.

“And if we had an event like that emerge in the next two to three weeks, we’d see wide-scale flooding.”

Be prepared

And that’s why officials are encouraging residents to be prepared – have an emergency travel bag prepped, and gather important documents and family keepsakes in case a sudden evacuation is called for. They also suggest people contact friends or family to plan emergency shelter if possible, as increased tourism may put more pressure on public evacuation facilities.

How-to guides on preparing for emergency evacuation can be found online.

John Boivin, Local Journalism Initiative Reporter, Valley Voice

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