Large mountain snowpack may pose threat with spring rain
Flooding concerns depend on forecast for rain through March and April
Snowpack levels in the Rocky Mountains are higher than normal after a flurry-filled winter, but flooding isn’t necessarily a concern as temperatures begin to warm up.
Snow samples in the mountains indicated snowpacks were 122 per cent of average in the Bow River basin and 132 per cent of average in the Oldman River basin as of March 1, according to Alberta Environment river forecast engineer Zahid Qureshi.
Those measurement levels are considered “above average to much above average” and “above normal to much above normal,” respectively, according to Alberta Environment.
“It’s higher than normal, for sure, but it’s not a record at this point,” said John Pomeroy, the director of the University of Saskatchewan’s Coldwater Laboratory in Canmore, who has more than 30 remote monitoring stations in the mountains.
At Fortress Mountain in Kananaskis, the snowpack is measuring at a water equivalent of 591 millime- tres at a high elevation monitoring site, compared with 451 millimetres last year at this time. At a lower elevation site, where less snow is usually observed, the snowpack is at about 525 millimetres, compared with 384 millimetres last year.
“That suggests that when snowmelt occurs later on in the spring and early summer, the river flows will be high, but snowmelt alone has never driven flooding in Calgary,” said Pomeroy. “For us to worry about flooding there would have to be a rain-on-snow event.”
That type of event, in which warm rain falls on snow accumulation, causes a rapid acceleration of the snowmelt, said Bob Sandford, the Epcor Water Security Research Chair at the United Nations University Institute for Water, Environment and Health.
Sandford said that’s what happened in 2013, leading to major flooding throughout southern Alberta.
But Qureshi pointed out that the snowpack in the mountains was still about average at this time of year back in 2013.
“(The flood) was caused because of the rainfall we received at that time and the intensity of the rainfall. Historically, the floods that are caused in Alberta, those are not caused because of the snowpack,” Qureshi said. “Because of the low temperature in the forecast, there is no expectation that this will cause any overland flood or river-related flooding.”
Precipitation throughout the remainder of March and April will be a key variable as to whether Calgarians have to worry about overly wet conditions this spring, according to Sandford.
“The warmer atmosphere can carry more precipitation,” he said. “We tend to get very heavy snowfall during March and sometimes well into April. Occasionally, you get a good storm in May, too. But it’s how warm and how fast the warming occurs that will determine the flooding hazard here. If a spring is slow and the temperature is just above freezing, that snow disappears gradually.”
It’s too early to say how much rain is coming, but Pomeroy described snowpack levels as “some of the highest numbers one would see.”
“There’s a lot of snow,” added Sandford. “We’ll be watching with great interest.”