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Avalanche Forecast

Archived

Feb 24th, 2021–Feb 25th, 2021

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

Regions

Lizard-Flathead.

Danger will increase as the day progresses with new snowfall and strong wind. Continue to travel diligently, as the buried weak layer is still surprising riders.

Confidence

Moderate - Uncertainty is due to the track & intensity of the incoming weather system.

Weather Forecast

WEDNESDAY NIGHT: Increasing clouds, 30 to 50 km/h southwest wind, alpine temperature -10 C.

THURSDAY: Cloudy with snowfall, accumulation 10 to 20 cm, 30 to 50 km/h southwest wind, alpine temperature -10 C.

FRIDAY: Cloudy with snowfall, accumulation 5 to 10 cm, 30 km/h west wind, alpine temperature -9 C.

SATURDAY: Clear skies, 30 km/h northwest wind, alpine temperature -10 C.

Avalanche Summary

Many large (size 2 to 2.5) storm and wind slab avalanches were triggered naturally, by skiers, and using explosives on Monday and Tuesday. The slabs were generally 30 to 50 cm thick, most often at treeline and alpine elevations, and occurred on all aspects but many of them on north to east aspects.

Looking towards Thursday, riders are still capable of triggering storm slabs, wind slabs in wind-loaded terrain, and persistent slabs. Conditions are not easily managed right now, warranting a conservative mindset!

Snowpack Summary

Stormy conditions enter the region on Thursday, bringing new snow and strong wind. Storm slabs are expected to build in sheltered areas at all elevations and on all aspects. Wind slabs are expected to build over the day in exposed terrain at treeline and alpine elevations in lee terrain features. These slabs will build on top of storm slabs formed early this week that still may be triggerable by riders, particularly where they overly sugary faceted snow from mid-February's cold drought.

A weak layer found around 50 to 80 cm deep that was buried late-January has been prevalent in the region during February. The layer may consist of weak and feathery surface hoar, sugary faceted grains, or faceted grains associated with a hard melt-freeze crust. Many avalanches in the past few weeks have been triggered on this layer by riders and naturally.

There are presently no deeper concerns.

Terrain and Travel

  • Watch for changing conditions today, storm slabs may become increasingly reactive.
  • The trees are not the safe-haven they normally are at this time. Terrain at treeline is primed for human triggered avalanches.
  • Choose conservative terrain and watch for clues of instability.
  • 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.

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.