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Avalanche Forecast

Archived

Mar 10th, 2021–Mar 12th, 2021

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

Regions

North Rockies.

Thursday is a classic late winter day with cool temps & a few clouds. A warm storm arrives Thursday night, & then warming continues through the weekend. Peak warming may initiate a natural avalanche cycle, especially in the east where there are deeper concerns in the snowpack.

Confidence

Low - Uncertainty is due to the timing, track, & intensity of the incoming weather system. Uncertainty is due to difficult to forecast freezing levels.

Weather Forecast

A shot of cold snow Thursday night and then a pretty significant warm up this weekend.

WEDNESDAY NIGHT: Freezing level at valley bottom, light west/northwest wind, no snow expected.

THURSDAY: A few clouds, freezing level rising to about 1350 m in the afternoon, strong southwest wind, no snow expected during the day, 5 to 10 cm possible Thursday night.

FRIDAY: Broken cloud cover, freezing level climbing to about 1800 m, strong southwest wind, 5 to 10 cm of snow at upper elevations with rain at valley bottom.

SATURDAY: Scattered cloud cover, freezing level potentially climbing as high as 2000 m, strong southwest wind, no precipitation expected. 

Avalanche Summary

Our field team found some large and disturbingly wide avalanches in the Hasler Tuesday, lots of compelling images in their MIN report here. It's suspected that these are running on facets that are about 80 cm below the surface.

This adds to the tally of recent large avalanches they started collecting in Pine Pass on Monday. All of this activity leaves me feeling uneasy about a big warm up this weekend. 

The warm temperatures last week triggered loose wet avalanches on solar (south through west) aspects. Strong winds triggered wind slabs naturally up to size 2 and explosive controlled avalanche (near Pine Pass) up to size 3. 

Snowpack Summary

Strong south to west winds in exposed areas have scoured some slopes and built wind slabs on others. Freezing levels reached treeline last Thursday and Friday. It has cooled since so you will likely find 5 to 10 cm of new new snow on the surface or a crust at treeline and below.

About a metre of snow covers a weak layer of facets buried mid February. Slightly deeper there is a widespread persistent weak layer from late January/early February that consists of surface hoar. It is most prevalent around treeline elevations, but likely reaches into the alpine and in openings below treeline too. These layers are both significantly shallower in the east of the region.

Terrain and Travel

  • Be careful as you transition into wind affected terrain.
  • Avoid freshly wind loaded terrain features.
  • Avoid thin areas like rock outcroppings where you're most likely to trigger avalanches failing on deep weak layers.

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.

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.