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Avalanche Forecast

Archived

Feb 27th, 2019–Feb 28th, 2019

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.
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.

Recently formed wind slabs and wind-affected snow may quickly be hidden under a thin cover of flurries falling through Thursday. Expect a more reactive snowpack in areas where winds pick up and 10 cm snowfall accumulates.

Confidence

Moderate -

Weather Forecast

WEDNESDAY NIGHT: Clear with cloudy periods. Alpine temperatures near -10C. Ridgetop winds 10-20 km/h from the east.THURSDAY: Cloudy with scattered flurries, 5-10 cm snow. Alpine temperatures near -9C. Ridgetop winds 15-30 km/h south-southeast.FRIDAY: Mainly cloudy with scattered flurries, 5 cm snow. Alpine temperatures near -18C. Ridgetop winds 25-30 km/h from the east.SATURDAY: Sunny. Alpine temperatures near -20C. Ridgetop winds 15-20 km/h from the east-northeast.

Avalanche Summary

Cracking and other signs of instabilities have been reported around ridgetops, check out this MIN report from Tuesday here.On Monday, small (size 1) natural wind slab avalanches were reported on west to south-south east aspects and reactive to skiers on north to east-southeast aspects at treeline and above. Additionally, machine operator on a road at 1700 m triggered a small (size 1) slab avalanche with a 40 cm crown which failed on on a layer of well preserved surface hoar between crusts.Last Saturday, numerous natural and explosives triggered storm slab avalanches to size 2-2.5 were reported. These were 50-100 cm deep running on east through north aspects in the alpine and treeline. Loose, dry snow is sloughing in steeper terrain. On Sunday, a skier triggered a size 1 storm slab avalanche on a north aspect around 2100 m.Last Thursday, before the weekend snowfall, an explosive triggered a large (size 2.5) persistent slab avalanche. The slab was about 40 cm deep and failed on a southerly aspect between 1650 and 2000 m.

Snowpack Summary

Winds are redistributing recent snowfall forming scouring ridgetops and producing wind slabs in open areas at treeline and above. The recent snow covered wind-affected surfaces in the alpine and treeline. In sheltered areas and lower elevations, the new snow is settling over facets and a patchy layer of surface hoar and a crust on solar aspects.Between 1600 and 1800 m, another weak layer of surface hoar crystals is now 30-50 cm below the snow surface. Two more weak layers of surface hoar may be found within the snowpack. One buried early-February, is 40-80 cm deep in shaded and sheltered areas above 1600 m. The other buried mid-January, is 80-110 cm down and is most prominent between 1600 m and 1900 m.The remainder of the snowpack is well-settled.

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.