Ministry of FLNRO

Snow Survey and Water Supply Bulletin - April 1st, 2017

The April 1st snow survey is now complete. Data from 143 snow courses and 68 automated snow weather stations around the province, collected by the Ministry of Environment Snow Survey Program and partners, and climate data from Environment and Climate Change Canada have been used to form the basis of the following report.

Click here for the Snow Survey and Water Supply Bulletin as a PDF file.
Click here for the complete April 1, 2017 Snow Survey Data.
Click here for the Basin Snow Water Index Map.

Weather
March weather was dominated by prolonged periods of cool and moist unstable conditions particularly in southern British Columbia. These conditions were interspersed with periods of strong influence from cold arctic air, particularly over northern BC, and a number of Pacific frontal systems which delivered warmer and wetter weather over southern BC.

For the month of March, temperatures were 0.5 to 2˚C below normal through most of BC, and up to 4˚C below normal in northern British Columbia. Temperature anomalies were more pronounced for daily maximum temperatures than daily minimum temperatures. March was extremely wet for most areas of southern BC, with many areas in the south-west recording 150-200% of normal March precipitation, and areas in the south-east receiving 200-300% of normal precipitation. In the northern half of the province, March precipitation tended to be closer to normal, or slightly below-normal.

Snowpack
Cool and wet weather through March has led to a significant increase in snowpack conditions across the province, with the biggest increases being observed in south and south-east BC. Provincially, snow basin indices trended closer to normal, with the provincial average for all April 1st snow measurements at 98% of normal. This has increased from the average of 85% for March 1st.

Snow basin indices for April 1st 2017 range from a low of 62% of normal in the Liard to a high of 123% of normal in the Lower Fraser (Table 1 and Figure 1). Below-normal snowpack (65-80%) is present in the Upper Fraser and Peace, and well below-normal (<65%) in the Stikine and Liard. Slightly elevated snowpacks (110-120%) are present in the West Kootenay, East Kootenay, Skagit and South Coast, and elevated (>120%) in the Lower Fraser. Other areas of the province have near-normal April 1st snow basin indices. The April 1st basin index for the entire Fraser Basin is 100% of normal.

 


Table 1 - BC Snow Basin Indices – April 1, 2017


Basin

% of Normal

Basin

% of Normal

Upper Fraser West

71

Boundary

86

Upper Fraser East

75

Similkameen

95

Nechako

106

South Coast

115

Middle Fraser

97

Vancouver Island

102

Lower Fraser

123

Central Coast

96

North Thompson

103

Skagit

111

South Thompson

106

Peace

76

Upper Columbia

100

Skeena-Nass

85

West Kootenay

119

Stikine

64

East Kootenay

116

Liard

62

Okanagan

105

Northwest

80

Streamflow
With extremely wet weather through March, many rivers of the province are flowing well above normal for early April. Snowmelt at lower elevations has led to increased runoff in smaller stream systems and the start of freshet flows in some large river systems. 

Outlook
Neutral El Niño (ENSO) conditions are now present in the Pacific Ocean, and the Climate Prediction Centre (CPC) at the U.S. National Weather Service/NOAA is forecasting that neutral conditions are favoured to persist through the spring of 2017, with increasing chances for El Niño development into the fall period. Negative sea surface temperature anomalies are present in the North Pacific Ocean west of British Columbia. Seasonal forecasts from Environment and Climate Change Canada indicate an increased likelihood of above-normal temperatures over the April-June period, particularly for northern BC.

Seasonal volume runoff forecasts (see below) are near normal for most basins across the province. Higher than normal seasonal runoff is expected in the Nicola and Okanagan basins.

By early April, 96% of the annual BC snowpack has typically accumulated, with maximum accumulation generally occurring in the middle of the month. Significant snow accumulation over the past month has led to an increase in seasonal flood risk across most of the province. Most areas of the province are entering into the spring freshet with normal levels of seasonal flood risk. This includes the Nechako, Middle Fraser, South Thompson, North Thompson, Upper Columbia, Okanagan and Similkameen. The West Kootenay and East Kootenay have moderately high snow packs, and subsequent moderately increased seasonal flood risk. In the Lower Fraser, tributary watersheds typically experience flooding due to fall-winter storms; however they can experience high freshet flows that can cause flood issues. Due to high snowpacks in the Lower Fraser (>120%), areas such as the Lillooet River and tributaries have an increased likelihood of higher than normal freshet flows this season. 

Normal seasonal flood risk is present for the Fraser River through the Fraser Valley and Lower Mainland. The expected peak flow for the Fraser River at Hope is 8000-9000 m3/s, or mean annual flood level; higher or lower flows are possible depending on weather conditions.

While snowpack is an important factor in seasonal flooding, weather during the melt season, particularly extreme rainfall events can also create flood risk even in years with low or average snowpack.

Well-below normal snowpack in the Stikine and Liard, and below normal snowpack in the Upper Fraser indicate the potential for increased risk of low flows this summer. Summer weather is also an important factor in summer low flows, particularly on Vancouver Island and northeast BC, and wet or dry weather through the summer period can affect the risk of low streamflow regardless of snowpack.

With two to four weeks remaining in the snow accumulation season, changes to the seasonal runoff outlook are possible, but at this stage would require extremely wet or cool conditions to make a significant impact on seasonal flood risk by either accumulating more snow, or delaying the melt season.  

The River Forecast Centre will continue to monitor snowpack conditions and will provide an updated seasonal flood risk forecast in the May 1st 2017 bulletin, which is scheduled for release on May 8th.

BC River Forecast Centre
April 6, 2017