Avalanche Forecast

Issued: Feb 16th, 2018 5:01PM

The alpine rating is considerable, the treeline rating is considerable, and the below treeline rating is moderate. Known problems include Wind Slabs, Cornices and Deep Persistent Slabs.

jmcbride, Avalanche Canada

South Rockies.

Periods of intense loading from snow and wind are expected to begin late Saturday afternoon into the evening. If the storm arrives earlier than forecasted, avalanche danger will increase sooner.

Summary

Confidence

Moderate - Timing, track, or intensity of incoming weather system is uncertain on Saturday

Weather Forecast

Snowfall is expected to begin Saturday afternoon, increase during the evening then ease overnight.TONIGHT: Light flurries. Accumulation up to 5cm. Ridge wind light, variable. Temperature -14. Freezing level valley bottom.SATURDAY: Snow. Accumulation 20-30 cm. Ridge wind moderate to strong, northeast. Temperature -14. Freezing level valley bottom.SUNDAY: Mainly cloudy, light flurries. Accumulation trace. Ridge wind moderate to strong, northeast. Temperature -22. Freezing level valley bottom.MONDAY: Mix of sun and cloud. Ridge wind light, northwest. Temperature -20. Freezing level valley bottom.

Avalanche Summary

Friday, ski cuts and explosive control produced loose storm snow avalanches up to size 1.On Thursday both natural and skier triggered wind slabs up to size 1.5 were reported as well as loose, storm snow releases from steep rocky terrain.Tuesday we received reports of size 3 avalanches that were likely triggered by cornice fall over the weekend and ran full path to valley bottom. See here for the MIN report. A widespread natural avalanche cycle occurred late last week with several storm slabs and deep persistent slabs to size 4 reported. See this MIN post for more information. These avalanches were failing on weak layers deep in the snowpack and running to valley bottoms. See this video for more details.

Snowpack Summary

About 20-40 cm on new snow and winds have led to the creation of wind slabs on leeward features at upper elevations on a wide range of aspects. In sheltered areas near treeline and below, these slabs overly a layer of surface hoar buried mid-February that has produced very easy, sudden results in recent snowpack tests.The lower snowpack in this region is weak with two main concerns; a widespread weak layer from mid-December composed of facets, crusts, and surface hoar that is 100-150 cm deep, and second, a rain crust with sugary facets buried in late-November near the bottom of the snowpack. Although the snowpack structure is variable across the region, these persistent weak layers are generally widespread.The take home message is that weak layers are still active and deserve a lot of respect. The solution is to stick to conservative terrain and avoid overhead hazard.

Problems

Wind Slabs

New snow and winds will form touchy slabs, especially at higher elevations and in wind-exposed terrain. A slab release near the surface may step down and trigger deep persistent weak layers, resulting in large destructive avalanches.

Watch for whumpfing, hollow sounds, shooting cracks or recent avalanches.Use extra caution as you transition into wind affected terrain.Avoid avalanche terrain during periods of heavy loading from new snow and wind.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

1 - 2.5

Cornices

Cornices have triggered very large avalanches recently. Be especially wary of long runout distances in avalanche paths, and the possibility of mature timber being taken out by a surprisingly large avalanche.

Give cornices a wide berth when traveling on or below ridges.Avoid steep slopes below cornices.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible - Likely

Expected Size

1 - 3.5

Deep Persistent Slabs

Several deeply buried weak layers have the potential to produce large avalanches reaching run out zones. Cornices have been a recent trigger for very large avalanches. Avoid shallow rocky snowpack areas where human triggering may also be possible.

Minimize exposure to overhead avalanche terrain, large avalanches may reach run out zones.Avoid steep convexities or areas with a thin or variable snowpack.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

2 - 4

Valid until: Feb 17th, 2018 2:00PM