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Avalanche Forecast

Archived

Dec 30th, 2019–Dec 31st, 2019

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely.

Regions

Northwest Coastal.

Avoid avalanche terrain. Heavy snowfall, rain, and warm temperatures have resulted in very dangerous avalanche conditions.

Confidence

Moderate - A small change in the upper snowpack could dramatically change avalanche conditions.

Weather Forecast

MONDAY NIGHT: Moderate rain up to 1200 m and 5-15 cm of snow above, moderate wind from the southwest, alpine temperatures around 0 C. 

TUESDAY: Moderate rain up to 1200 m and 5-15 cm of snow above, light wind from the west with 40 km/h gusts, freezing level around 1500 m with alpine high temperatures around +1 C.

WEDNESDAY: Scattered flurries with another 10-30 cm of snow, moderate wind from the southwest, freezing level drops to 300 m and alpine high temperatures drop to -5 C.

THURSDAY: Isolated flurries with 5-10 cm of snow, light wind from the south, alpine high temperatures around -5 C.

Avalanche Summary

Monday's storm likely caused natural avalanches and set Tuesday up for very dangerous avalanche conditions. The transition from snow to rain could cause a mix of avalanche problems including storm slabs, wind slabs, and wet loose avalanches. An additional concern is larger persistent slab avalanches on buried surface hoar layers. These weak layers produce large avalanches (size 3) on Saturday and will be under additional stress with the warm temperatures.

Snowpack Summary

As of 4 pm on Monday afternoon about 30 cm of snow has fallen across southern parts of the region. As the storm continues into Monday night it is expected to get warmer, with rain reaching up to 1200 m. By Tuesday afternoon there could be another 10-30 cm of snow in the alpine while lower elevations get soaked by rain. This will result in a mix of surface conditions with storm slabs, wind slabs, and wet loose avalanches occurring at different elevations. In addition to problems with the new snow and rain, the warm temperatures will cause significant stress on the two layers of surface hoar buried 50-120 cm deep. Reports from the Shames area suggest buried surface hoar can be found on all aspects, but is more prevalent on southeast to southwest aspects around 800-1400 m. Overall the snowpack will be under stress from the new snow, rain, and warming on Tuesday. Things should improve as temperatures cool the following days.

Terrain and Travel

  • Avoid all avalanche terrain during periods of heavy loading from new snow, wind, or rain.
  • Avoid the runout zones of avalanche paths. Avalanches could run full path.
  • Storm slabs in motion may step down to deeper layers resulting in large avalanches.
  • Extra caution for areas experiencing rapidly warming temperatures for the first time.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.