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Avalanche Forecast

Archived

Feb 16th, 2018–Feb 17th, 2018

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Lizard-Flathead.

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.

Confidence

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

Weather Forecast

Snowfall is expected to begin Saturday afternoon, increasing to heavy snowfall during the evening, then easing overnight.SATURDAY: Snow. Accumulation 20-30 cm. Ridge wind light to moderate, east. Temperature -8 C. Freezing level valley bottom.SUNDAY: Mainly cloudy, light flurries. Accumulation trace. Ridge wind moderate to strong, east. Temperature -20. Freezing level valley bottom.MONDAY: Mix of sun and cloud. Ridge wind light, east. Temperature -18. Freezing level valley bottom.

Avalanche Summary

Reports of natural and skier triggered storm slab and loose snow avalanches up to size 1.5 were reported during the storm on Friday primarily in leeward areas in the alpine and at treeline.On Thursday, natural storm slab and loose storm snow avalanches up to size 1.5 were reported to have occurred overnight with active loading during the storm. Explosive control work on Thursday also produced storm slab results up to size 1.5 on northerly slopes between 1700-1800 m.Explosive control work and ski cutting on Wednesday produced widespread storm slab results up to size 2, between 1600-2100 m.On Tuesday skiers were able to cut size 1.5 wind slabs on north east features at treeline. Wind slabs to size 2 were also reported, starting at the base of steep head walls. Last weekend we received reports of impressive sizes of cornice chunks breaking off a tree line ridge feature near Harvey Pass (Flathead area), running far into the forest and meadow below. See here for photos. Avalanche activity on deeper layers has tapered off for now, but it's best to avoid likely trigger points such as thick-to-thin snow pack areas and also close to the base of rocky headwalls.

Snowpack Summary

About 30-50 cm of recent storm snow and wind have created wind slabs at upper elevations. This new snow overlies old wind slabs on leeward slopes at treeline and above, as well as sun crusts on solar aspects and a mix of surface hoar and facets in sheltered areas at near treeline. Below 1700m, this new snow covers a rain crust that was buried on February 8th.For the past month there has been regular avalanche activity on multiple weak layers in the mid and lower snowpack. A widespread weak layer from mid December composed of facets, crusts, and surface hoar is 120-160 cm deep. Also, a rain crust with sugary facets buried in late November is near the base of the snowpack.The take home message is that several weak layers are still lurking in the snowpack and they deserve a lot of respect. Conservative terrain use is recommended while avoiding overhead hazard.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.

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.