BC’s River Forecast Centre is predicting an earlier start to the freshet this year, as March weather was warmer than normal in most parts of the province, including the East Kootenay.
The East Kootenay snowpack is at 99 percent of normal, however due to warm weather throughout the winter, low to mid elevation snow packs across the province are greatly diminished this season. While the provincial average for all April 1 surveys is 91 per cent of normal, the average for sites below 1200 m elevation is 62 per cent of normal, and just 44 per cent of normal for sites below 1000m.
Warm weather towards the end of March and in early April has led to the onset of the melt season across the province. All of the provincial automated snow weather stations have recorded melt over the past week, as well as a number of manual surveys which experienced a loss of snow water equivalent between the March 1 and April 1 surveys. The transition from snow accumulation to snow melt is two to three weeks earlier than usual this season.
Temperatures across British Columbia continued to be well above normal through the month of March, with daily temperatures being 1-3˚C above normal through southern BC, and 2‑4˚C above normal through the Kootenays, Central, and Northern BC. These warm temperatures have persisted throughout the 2015-16 winter.
With warm temperatures, mid-season melt, and precipitation as rain, most rivers across British Columbia have experienced well above normal streamflow (150% to 200% of median value) over the past several months. Snow melt runoff that typically flows later in the season has already passed through their watersheds. As of early April, virtually all of the rivers in the province were flowing well above normal for the time of year.
This advance in runoff timing is expected to continue to lead to an earlier freshet this season, both in terms of timing of peak flows and the recession to the low-flow season.
The forecast of warmer weather through the spring, combined with warm temperatures already experienced this winter, is likely to be an important factor in this year’s freshet season. With the advanced melt of low to mid-elevation snow that has already occurred, continued warmer than normal temperatures are expected to continue to drive an advance in the freshet season. Typically peak flows in most snow-dominated rivers in the province reach their peak from mid-May through late-June. With the advance in melt already observed, and forecast for ongoing warm weather, the peak season is likely to be advanced to the late-April to early-June period for most rivers in the province.
For both spring flood risk and summer low flows, snow pack is just one of the important elements that determine whether or not extreme conditions will emerge. Weather, through the spring and summer, is also a key driver on whether or not flooding or low stream flows will occur. May and June are climatologically the wet season for the BC Interior. While indicators, like El Niño, have stronger linkages to seasonal temperature, precipitation is difficult to forecast beyond a one to two week horizon. Extreme wet or dry weather can significantly impact risks for peak flows and low flows, the latest report says.