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Avalanche Forecast

Archived

Dec 1st, 2020–Dec 2nd, 2020

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

We're going from cold and stormy to unseasonably warm and sunny in quite a hurry which is expected to de-stabilize our snowpack. Cornice failures and very large natural avalanches are possible as we head into the warm spell.  

Confidence

Moderate - We are confident the snowpack will rapidly lose strength with the arrival of the forecast weather.

Weather Forecast

Wednesday looks to be warm right up into the alpine. Temperatures return to more seasonal norms Thursday and Friday before an Atmospheric River delivers what is expected to be heavy precipitation on Saturday.

TUESDAY NIGHT: Freezing level around 2500 m through the night, strong to extreme southwest wind, 1 to 5 cm of snow possible at ridge top with rain falling at most elevations.  

WEDNESDAY: Mostly clear skies through the bulk of the day with cloud cover increasing after sunset, freezing level holding at 2500 m, strong to extreme southwest wind, no precipitation expected.

THURSDAY: Overcast, freezing level between 500 and 1200 m, moderate to strong southwest wind, 5 to 10 cm of snow.

FRIDAY: Overcast, freezing level between 500 and 1500 m, strong south/southwest wind, up to 5 cm of snow possible. 

Avalanche Summary

Warm temperatures and direct sunshine are coming our way. Natural avalanches are possible.

We've got some more details from the natural avalanche cycle that occurred during the storm Nov 28 through 30th.

In the North, around Bear Pass wind slabs from size 2 to 3 released naturally in steep alpine features. Several persistent slabs up to size 3.5 were observed which likely failed on the November 3rd crust.

Around Terrace, the Skeena and Bulkley valleys, moist avalanche debris from size 2 avalanches was observed. This makes sense as there was much less storm snow around Terrace when compared with the north.

Snowpack Summary

The storm over the last few days produced 30 to 70 cm of snow above 500 m. Areas to the north and west of the region got the bulk of the snowfall.  

The new snow may be resting on the late November surface hoar and facets which could set up a scenario where storm slabs are surprisingly touchy.

Strong to extreme winds generally out of the south/southwest have formed wind slabs in open features at upper elevations.

We're learning that the early November crust is widespread, even in the alpine and treeline elevations. You should be able to find it 25 to 50 cm above the ground. 

The total snowpack depth is roughly 150 to 200 cm at upper treeline and alpine elevations. The bottom of the snowpack consists of several thick crusts. There could be weak snow developing around these crusts in colder inland parts of the region.

Terrain and Travel

  • Avoid all avalanche terrain during periods of intense solar radiation.
  • Minimize overhead exposure; avalanches triggered by warming or cornice fall may be large and destructive.
  • Avoid exposure to slopes that have cornices overhead.
  • Avoid freshly wind loaded terrain features.
  • Be mindful that deep instabilities are still present and have produced recent large avalanches.

Problems

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.

Wind Slabs

Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.

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.