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Snowpack above normal in many area, seasonal flooding likely this spring

Despite cooler than normal temperatures, dry weather contributed to modest snow pack accumulation over the month of March.

The provincial average of snow measurements is 112 per cent of normal. Seasonal flood risk is elevated in many regions, including the Upper Fraser West, Upper Fraser East, North Thompson, South Thompson, West Kootenay, Boundary, Cariboo Mountains, Central Coast and Skagit.

The snow basin index for the Fraser River is 116 per cent, with high snow pack levels in its major tributaries. The likely peak flow forecast for the Fraser River at Hope this freshet is 8,000-11,500 m3/s, though higher flows are possible with extreme weather.

There were additional challenges accessing manual snow courses this year due to the COVID-19 situation, and 28 (19 per cent) scheduled snow courses were not sampled, primarily within the South Coast and Bridge region of the Middle Fraser. This reduces the accuracy of the snow basin indices for certain regions; however, this is not expected to materially affect overall results.

Typically, the provincial snow pack reaches its maximum level in mid-April. Significant changes to the current snow pack are not expected. However, continued cool weather can lead to a delay in the snowmelt season and lead to increased seasonal flood risks. Snow pack is one element of seasonal flood risk in B.C. and alone does not predict whether flooding will occur. Spring weather is a critical factor determining the rate that snow melts, and extreme rainfall can also cause spring flooding. Spring freshet poses a seasonal risk across the B.C. interior, irrespective of snow pack levels.

Snow basin indices for April 1 range from a low of 88 per cent of normal on Vancouver Island to a high of 148 per cent in the Skagit. Snow basin indices in many regions have remained steady or dropped slightly relative to March values. Despite cooler than normal temperatures, dry weather contributed to modest snow pack accumulation in March, where most regions remained similar or declined slightly relative to their March snow basin index values.

The Upper Fraser East, Lower Fraser, East Kootenay, South Coast, Skagit, Peace, and Liard experienced modest increases in snow basin indices. Most notably, the Upper Fraser East region experienced significant accumulation in the first and last week of March, increasing its snow basin index from 135 per cent of normal on March 1 up to 147 per cent of normal for April 1.

As a result of snowpack growth in Interior mountainous areas, the overall average of province-wide measurements increased slightly from 111 per cent of normal last month to 112 per cent of normal for April 1. High snow packs are generally present in the mountainous interior ranges of the province and in the Central Coast region. Extremely high snowpack (>135 per cent) is present in the Upper Fraser East, Upper Fraser West and the Cariboo Mountains (Quesnel River).

Several snow measurement locations recorded an all-time record high Snow Water Equivalent (SWE) for April 1. These include:

  • 1A05 Longworth (Upper) 1378 mm with 61 years of record (Upper Fraser – East)
  • 1C13A Horsefly Mountain 735 mm with 50 years of record (Middle Fraser/Quesnel)
  • 1C41P Yanks Peak East 1215 mm with 23 years of record (Middle Fraser/Quesnel)
  • 1E14P Cook Creek 789 mm with 11 years of record (North Thompson) High snow packs (>120%) are present in the South Thompson, Boundary, Central Coast (Bella Coola), Skagit and Northwest basins.

Moderately high snowpacks (110-120 per cent) are also present in the North Thompson, Upper Columbia, West Kootenay, East Kootenay, Okanagan, Similkameen, and Peace. The overall snow basin index for the entire Fraser River basin (e.g. upstream of the Lower Mainland) is 116 per cent.

During freshet, most of the flow (roughly two-thirds) in the Fraser River originates from the Upper Fraser East, North Thompson, South Thompson and Cariboo Mountains. Snowpack in these areas is very high (131 per cent of normal). Snowpack in unregulated watersheds of the Fraser River upstream of Hope (e.g., excluding the Nechako and Bridge systems regulated by dams) are currently among the highest on record. For perspective, recent years with high snow basin indices for unregulated watersheds of the Fraser River upstream of Hope include 2018 (119 per cent), 2012 (127 per cent), 2007 (120 per cent), and 1999 (121 per cent).

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