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Avalanche Forecast

Archived

Feb 16th, 2019–Feb 17th, 2019

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

Lizard-Flathead.

Recent observations of persistent slab avalanches suggest buried weak layers may be touchy, particularly in openings like cutblocks. Also use caution at higher elevations, since the wind may have turned the recent snow into slabs.

Confidence

Moderate - Forecast snowfall amounts are uncertain

Weather Forecast

SATURDAY NIGHT: Cloudy with light snowfall, accumulation 1 to 5 cm, light to moderate northeast wind, alpine temperature -17 C.SUNDAY: Mix of sun and cloud with isolated flurries, trace accumulation, light to moderate northeast wind, alpine temperature -21 C.MONDAY: Mostly clear skies, light to moderate north wind, alpine temperature -18 C.TUESDAY: Partly cloudy skies, light west wind, alpine temperature -16 C.

Avalanche Summary

Widespread large (size 2) persistent slab avalanches were observed in the region on Saturday - check the report out here. The avalanches released on the surface hoar described in the Snowpack Summary. The slabs were 40 to 100 cm deep. They released between 1550 m and 1700 m on north aspects. There was also further evidence of instability within the buried weak layers, as noted here and here.Otherwise, small to large (size 1 to 2) wind, storm, and persistent slabs were triggered by explosives and skiers, all on northerly aspects and around 2000 m. The slabs varied from 5 to 40 cm deep.

Snowpack Summary

Around 10 cm of recent snow overlies a weak layer of surface hoar crystals found between 1600 m and 1800 m. Strong wind may have redistributed this recent snow and formed wind slabs in lee terrain features at higher elevations. The wind was first westerly and switched to northeast, so wind slabs may be found on all aspects.Two weak layers may be found within the snowpack. Surface hoar that was buried early-February may be found about 40 to 50 cm deep in shaded and sheltered areas above 1600 m. The mid-January layer of surface hoar is buried around 70 cm and is most prominent between 1500 m and 1900 m.The remainder of the snowpack is well-settled. Thin snowpack areas, such as in the far east of the region, may have weak and sugary faceted grains near the base of the snowpack.

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.