Weather and snowpack conditions through March have increased the risk of seasonal flooding this year, and snowpacks may continue to build in April.
The snow basin indices for Okanagan and Similkameen have both reached 152 per cent of normal as of April 1, according to a bulletin released this week by the B.C. River Forecast Centre.
“Given this year’s La Niña conditions, it is unlikely that the [flood] risk will ease much prior to the melt season,” said the bulletin.
Although snowpacks are one element of seasonal flood risk during the freshet season, there are other important factors.
Extreme temperatures can cause rapid melting and intense or prolonged rainfalls can also add to the risk.
Temperatures in March were relatively cool and precipitation was well above normal for most locations in the B.C. Interior, the River Forecast Centre said.
La Niña – equatorial Pacific Ocean currents – began easing in February. Nonetheless, it is not uncommon for the effects of La Niña to persist several months beyond the defined La Niña period.
In previous La Niña events, snowpacks in B.C. tended to grow more rapidly than normal through April.
Province-wide snow basin indices in La Niña years tend to increase by two to five per cent through April.
“While there is still uncertainty over how weather patterns will play out over the next few months, continued increases in snow basin indices into May are likely to occur, given this year’s La Niña context,” the bulletin said.
At the same time, temperatures were expected to continue to be below normal and rains in early April were expected to increase mountain snowpacks.
Typically, 95 per cent of the annual B.C. snowpack has accumulated by April 1. The transition from snow accumulation to net snow melt normally occurs around the middle of April.
The snowpack accumulations in the Okanagan and Similkameen basins are the highest in the province, though the Boundary and Skagit aren’t far behind.
Only in the north of the province are snowpacks below normal for this time of year.
Last year at this time snowpacks in the Okanagan basin were only at 105 per cent of normal. A warm spring and rapid melt caused flooding in May and June throughout the region.
Forest fires burned the largest area in B.C.’s history last summer and many watersheds were affected.
These include large areas in the Cariboo-Chilcotin, Thompson-Okanagan, West Coast, and Kootenay-Boundary regions.
Disturbances by fire can affect the hydrologic responses of streams, rivers and lakes to potential flooding, the River Forecast Centre said.
Flows from snowmelt in watersheds impacted by fires tend to be greater and to peak earlier than in undisturbed areas, even in normal weather conditions.
Most of the areas susceptible to earlier and high flows due to fire are to the north of this region.
The next release of the snow conditions and water supply bulletin is scheduled for May 7 and will report on conditions as of May 1. A second May report will be released on May 21.
RICHARD McGUIRE
Osoyoos Times
