Following is the April snowpack report, available online at http://bcrfc.env.gov.bc.ca/bulletins/watersupply/current.htm, published by the British Columbia provincial government and valuable for predicting local summer water supplies as well as providing spring flooding indicators.
Current Snow Survey and Water Supply Bulletin
The April 1 snow survey is now complete. Data from 139 snow courses and 55 snow pillows around the province and out-of-province sampling locations, and climate data from Environment Canada, have been used to form the basis for the following reports.
The entire document including text, data, graphs and basin index map can be viewed as an Adobe pdf file.
Weather patterns in March have been dominated by Pacific frontal systems delivering moisture across British Columbia. Precipitation in March was above normal, and temperatures were below normal for most areas of British Columbia. Weather patterns are consistent with the La Niña cycle currently being observed.
By this date, about 96% of the annual peak BC snowpack has accumulated. This year, as of the first week of April, snow packs across BC are normal or slightly above normal, with the exception of well above normal snow packs in the Lower Fraser, South Coast and Vancouver Island.
Basin snow water indices for BC at April 1 vary from a low of 95% of normal in the North Thompson to a high of 153% of normal on Vancouver Island. Indices have increased for most basins in the province since March, most notably in the Lower Fraser, South Thompson, Nicola, Columbia, Kootenay, Okanagan, Similkameen, South Coast and Vancouver Island. Indices have remained about the same since last month in the Upper Fraser, Middle Fraser, North Thompson, Peace and Skeena-Nass.
In most basins, low to mid elevation snow packs are well above normal, consistent with the cooler and wetter than normal accumulation season. Snow sampling in BC is biased towards high elevation sites. Therefore, some caution is warranted in interpreting the snow basin index values reported here, as they do not fully capture the above normal low to mid elevation snow pack that has been observed.
BC Snow Basin Indices – April 1, 2011
% of Normal
% of Normal
Individual snow courses range from 88% of normal (McBride-Upper) to 155% (Prince George Airport), with an overall basin index of 105%.
Snowpack levels are fairly consistent in the Nechako basin, with a range from 94% of normal (Mount Pondosy) to 113% (Mount Swannell). The overall basin index is 105%
Snow conditions in the Middle Fraser are quite variable, and range from near normal to well above normal. Low and mid elevation snowpacks are much higher than normal, and at sites below 1200 meters elevation, individual sites vary from 125% to 206% of normal. The overall basin index is 105%.
Snow pack levels in the Lower Fraser are consistently above normal, with values ranging from 86% of normal (Wahleach Lake) to 144%. Overall basin index is 131%.
Snow course values are fairly consistent in the North Thompson and range from 76% to 103% of normal. The one low elevation snow survey site in the North Thompson is at above normal levels (133% of normal at Blue River). The overall basin index is 95%.
Snow pack levels in the South Thompson range from 75% of normal at Bouleau Lake, to 147% of normal at Enderby. The overall basin index is 110%.
Snow pack in the Upper Columbia ranges from 90% of normal (Bush River) to 154% of normal (Field), with well above normal levels observed in the low to mid elevations. The overall basin index is 101%
Snow pack levels are variable in the Kootenay region. Individual snow course measurements range from 95% of normal (Nelson) to 238% (Duncan Lake). The overall basin index is 110%.
The Kettle is at near normal snowpack, with a range from 93% at Carmi, to 108% at Farron. The overall basin index for Okanagan-Kettle is 107%.
Snow pack levels are variable in the Okanagan basin, and range from 84% of normal (Islaht Lake) to 135% of normal (Summerland Reservoir). Overall basin index for the Okanagan-Kettle is 107%.
Snow pack levels in the Similkameen basin are variable, ranging from 83% of normal at Hamilton Hill, to 120% at Lost Horse Mountain. Overall basin index is 101%.
Snow pack levels in the South Coast are consistently well above normal. Values range from 117% of normal (Orchid Lake) to 169% (Powell River – Lower). Overall basin index is 133%.
Snow pack levels on Vancouver Island are consistently well above normal, with values ranging from 147% of normal (Upper Thelwood Lake) to 222% (Wolf River-Lower). The overall basin index is 153%
Snow pack levels are near normal for most sampling locations in the Peace. Snow pack levels vary from 80% of normal (Pine Pass) to 126% (Mount Sheba). Overall basin index is 99%.
Snow pack levels are variable in the Skeena-Nass basins. Values range from 57% of normal at Ningunsaw Pass to 201% of normal at Terrace. Overall basin index is 102%.
Water Supply Outlook
In general, the snowpack as of April 1st is at near normal levels throughout the province. At this point in time, snow pack levels do not indicate extraordinary conditions for potential flood or drought risk through most of the province.
Above normal snow packs exist in the Lower Fraser, South Coast and Vancouver Island. Peak flows in these regions are not typically associated with extreme spring peak flows, rather they experience major flood events during rainfall storm events in the fall and winter. An exception to this is in basins with a significant portion of the watershed at high elevation. In these watersheds in the Coast and Lower Fraser, above normal spring freshet runoff can be expected. These basins include the Lillooet, Squamish, Birkenhead and Harrison Rivers.
Normal to above normal water supply conditions are forecast for the spring runoff season. This should be particularly beneficial for replenishing groundwater, lake and reservoir levels in regions affected by dry conditions in the 2010 season (north-east BC, Skeena-Bulkley and Middle Fraser).
Snow conditions at the end of the winter snow accumulation period comprise only one part of the peak flow and water supply picture. Spring weather has a large influence on the timing and rate at which the snow pack melts.
The La Niña cycle that has been present in the equatorial Pacific Ocean throughout the 2010-2011 winter is beginning to weaken. The National Oceanic and Atmospheric Administration (NOAA) are commenting that La Niña conditions will continue to weaken, and neutral conditions are expected by June 2011. The persistence of La Niña conditions into the spring period may affect the continued accumulation and melt of the snow pack through the freshet period. Environment Canada are forecasting cooler than normal conditions through the spring, which is indicative of normal La Niña effects. This may lead to delayed timing of the onset of freshet.
Weather during the April to June period will affect the specific flood risk in snow melt driven rivers of the province. A delayed melt of the snow pack will mean that more snow may be present when we enter into the late spring-early summer period (where there is an increased likelihood of hot weather), and therefore flood risk may increase. The River Forecast Centre will continue to monitor the snowpack and weather conditions throughout the snow melt period, and will revise our assessment of runoff and flood potential accordingly.
Produced by: River Forecast Centre
April 8th, 2011