The B.C. River Forecast Centre increases the frequency of its Snow Survey and Water Supply Bulletins to twice a month as the freshet swings into gear.
The most recent measurements were taken on May 15, and reported on May 21, 2020.
The report says that most snow basin indices for May 15th dropped relative to May 1st values. The overall average of province-wide measurements decreased from 106% of normal on May 1st to 97% of normal for May 1st. However the East Kootenay snow indices are at 115% of normal, the same as they were on May 1. That is considered moderately high.
The May 15th Snow Survey is less comprehensive than May 1st, with only 17 manual snow surveys conducted for May 15th compared to 108 manual snow courses for May 1st. Thus, the May 15 bulletin is most useful in determining if the snowmelt is early, delayed or seasonal. Spring snowmelt for 2020 is considered seasonal in nature. Larger river systems have yet to reach their peak levels for the year and are still at risk for flooding this Spring due to snowmelt and/or heavy rainfall. These include areas in the Upper Fraser, Cariboo Mountains, North Thompson, South Thompson, West Kootenay, East Kootenay, Upper Columbia and Lower Fraser Valley. Rivers in other areas of the province remain high and are sensitive to flooding from heavy rainfall.
Seasonal temperatures in the Interior and several storm systems in the first two weeks of May contributed to minor snow accumulation at higher elevations in the Columbia, Kootenay, and Boundary.
Generally, 15-25% of the accumulated snow pack has melted by mid-May. Most sites have melted at seasonal rates this year. Most lower elevation areas are now snow-free (approximately below 800-1000m in coastal BC, below 1600m in southern BC and below 1200-1300m in northern BC), while higher elevation snow pack has experienced limited melt.
Most rivers in British Columbia were flowing at normal or above normal for early-May. There have been no Flood Watches or Flood Warnings issued by the River Forecast Centre in May; however, many regions have been under High Streamflow Advisories for several weeks. Regions with rivers and creeks under advisory include: the Cariboo, North Thompson, Bonaparte, Okanagan, Nicola, Boundary, Shuswap, & Salmon River. The larger river systems in the Interior (Upper Fraser, Quesnel, North Thompson, South Thompson, Kootenay, Columbia) are rising, but not forecast to reach high levels through the following week. They will likely reach peak flows in early to mid-June, or possibly later for higher elevation watersheds or with wetter weather.
The next measurements will be taken June 1 and released on June 8, 2020.