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Avalanche Forecast

Archived

Mar 18th, 2021–Mar 19th, 2021

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

Regions

Cariboos.

New snow and strong wind may form isolated pockets of fresh wind slab.

The region will see mostly cloud cover on Friday, but if the sun shines it could heat up fast. Large loads like cornice fall may trigger a persistent slab from the slope below. 

Confidence

Moderate - Forecast snowfall amounts are uncertain.

Weather Forecast

A Pacific frontal system reaching the Coast today will bring cloudy skies and snowfall to the Interior regions through the weekend.

Thursday Night: Snow 5-10 cm. Ridgetop wind moderate from the southwest and alpine temperatures near -10. Freezing levels 1500 m.

Friday: Cloudy with sunny periods and possibly 5-10 cm. Ridgetop winds moderate from the South and freezing levels 1500 m.

Saturday: Mix of sun and cloud. Ridgetop wind strong from the West and alpine temperatures near -10. Freezing levels 1100 m.

Sunday: Snow 5-10 cm. Ridgetop wind light to moderate from the southwest. Freezing levels 1200 m. 

Avalanche Summary

Several reports of large size 2.5-3 natural slab avalanches occurred last weekend, likely during the first big warm-up. The suspect failing layer of these avalanches is the mid-February facet interface. This morning our North Rockies Field team reported a size 2.5 natural cornice failure in the McBride area. This cornice triggered an East facing alpine slope with the suspect interface being the deeper weak facets. 

These recent reports indicate that the buried persistent weak interface still lurks, however; it's likely that it would take a large load to trigger it. That being said the weight of a human and/or machine may be enough to trigger something deeper. It is a low probability - high consequence scenario with large N-E facing alpine slopes most suspect.

New snow and strong winds through the weekend could build fresh wind slabs on leeward slopes and behind terrain features. These would likely be reactive to rider triggers.

Snowpack Summary

Strong ridgetop wind accompanied by 5-10 cm of new snow is expected for Friday with additional snow through the weekend. This may form fresh wind slabs below alpine ridgetops. Dry snow can be found on north aspects and crusty snow surfaces exist on solar aspects. Large cornices loom over alpine ridges. They are very unpredictable and if they fail they could trigger a slab on the slope below.

A persistent weak layer made up of surface hoar at treeline elevations and a crust with facets in the alpine can be found down 50-150 cm in some parts of the region. Recent reports indicate that some very large avalanches have occurred on this layer in the past week. It seems to need a large trigger like a cornice fall or a rapid flux in weather like a big warm-up. 

Terrain and Travel

  • Watch for wind-loaded pockets especially around ridgecrest and in extreme terrain.
  • Back off if you encounter whumpfing, hollow sounds, or shooting cracks.
  • Pay attention to cornices and give them a wide berth when traveling on or below ridges.
  • Use caution on large alpine slopes, especially around thin areas that may propagate to deeper instabilities.

Problems

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.

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.

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.