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Avalanche Forecast

Archived

Dec 28th, 2017–Dec 29th, 2017

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

South Rockies.

With snow and wind in expected Thursday night and Friday, watch for fresh wind slabs to develop in open areas. If triggered these slabs may step down to a deeper layer.

Confidence

Moderate - Due to the number of field observations

Weather Forecast

THURSDAY NIGHT: Snow. Accumulation 10-15 cm. Ridge wind light southeast possibly strong at times. Temperature -22. Freezing level surface.FRIDAY: Cloudy, snow developing and overnight. Accumulation 10-20 cm. Ridge wind light northeast possibly strong at times. Temperature -22. Freezing level surface.SATURDAY: Mix of sun and cloud, flurries. Accumulation 5-10 cm. Ridge wind light southwest. Temperature -20. Freezing level surface.SUNDAY: Mostly sunny. Ridge wind light southwest. Temperature -14. Freezing level surface.

Avalanche Summary

Limited reports suggest triggering slab avalanches remains possible. Two skier triggered avalanches have been reported east of the divide the past few days. One was a small wind slab triggered in a steep gully and the other was a larger persistent slab (size 2) triggered low on a slope that propagated up and across the slope on a 50 cm deep crust. The later highlights the potential for persistent slab avalanches to have wide propagations.

Snowpack Summary

Up to 10 cm of new snow fell overnight Wednesday and during the day on Thursday and overlies up to 60 cm of old storm snow in southern and eastern parts of the region and 30 cm in the Elk Valley. Wind has formed harder wind slabs in exposed terrain, but low density storm snow remains in sheltered areas.The main question in the snowpack surrounds the mid-December layer sitting beneath the recent snow. This layer includes crusts on solar aspects and feathery surface hoar in sheltered terrain at and below treeline, and has produced whumpfing as well as moderate, sudden results in recent snowpack tests. The lower snowpack is composed of mostly soft sugary snow and a few early season crusts that have not produced significant test results recently.For an update on recent conditions check out the Mountain Information Network (MIN) report here. And please post your own reports!

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.