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Avalanche Forecast

Archived

Mar 4th, 2020–Mar 5th, 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 possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Northwest Inland.

The snowpack is complex with numerous concerning avalanche problems. As we leave an active stormy period, conservative terrain choices are recommended to handle these tricky conditions.

Confidence

Moderate - Uncertainties in both the snowpack structure and the weather forecast limit our confidence.

Weather Forecast

Stormy and windy on Thursday then clearing weather for the weekend...

WEDNESDAY NIGHT: Frontal system crosses the region overnight delivering 5-15 cm of new snow, strong south wind, alpine temperatures drop to -12 C.

THURSDAY: Scattered flurries with another 5 cm of snow, strong south wind, alpine temperatures around -10 C.

FRIDAY: Sunny with a few clouds, light south wind, alpine high temperatures around -8 C.

SATURDAY: Sunny, light wind, alpine high temperatures around -8 C.

Avalanche Summary

Over the past few days there have been reports of wind slab, persistent slab (failing on the February 19 surface hoar layer), and deep persistent slab (failing on basal facets) avalanches. This pattern highlights how current stormy weather is aggravating multiple weak layers in the snowpack.

Reports from Tuesday suggest wind slab avalanches occurred naturally in alpine terrain, while at treeline elevations several size 2-2.5 persistent slab avalanches released naturally and with artificial triggers. These slabs failed on the Feb 19 surface hoar layer on north and east aspects.

Size 1 human triggered avalanches on the Feb 19 surface hoar layer have been reported almost daily since last Thursday. These have typically been 30 cm thick on a range of aspects. 

Cornice falls have been a common trigger for wind slabs on the slopes below, and last Friday a cornice fall triggered a size 3.5 deep persistent slab on a northeast facing slope at 1800 m.

Snowpack Summary

Extreme wind from the southwest, west, and northwest has hammered open terrain, scouring some slopes and forming wind slabs on other slopes. Another 5-15 cm of snow is expected to form fresh wind slabs on Thursday.

Loading from new snow and wind has made several persistent weak layers problematic over the past week. A combination of surface hoar and sun crust layers were buried on February 19 and currently sit 30-60 cm below the surface that seem to be most sensitive to human triggering at treeline elevations. A couple of weak layers that formed in January are buried in close proximity to one another 80-140 cm below the surface. Below that, an early season crust/facet layer lurks at the base of the snowpack.

Terrain and Travel

  • Avoid freshly wind loaded features, especially near ridge crests, roll-overs and in steep terrain.
  • Use careful route-finding and stick to moderate slope angles with low consequences.
  • The trees are not the safe-haven they normally are at this time. Terrain at treeline is primed for human triggered 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.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.