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Avalanche Forecast

Archived

Feb 13th, 2018–Feb 14th, 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 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 unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Lizard-Flathead.

Wind slabs, cornices and persistent slabs make for a tricky situation out there. Ride in conservative terrain and avoid all overhead hazard to manage your risk.

Confidence

Moderate - Intensity of incoming weather systems is uncertain on Wednesday

Weather Forecast

Wednesday will see an upslope event bringing small amounts of low density snow, followed by clearing and unsettled weather. TUESDAY NIGHT: Flurries (5-10cm of low density snow). Winds moderate gusting strong from the north west. WEDNESDAY: Flurries (5 -10cm accumulation). Ridge wind moderate gusting strong from the north / west. Temperature -7. Freezing level valley bottom.THURSDAY: Sunny with cloudy periods. Ridge wind gusting moderate from the west. Temperature -9. Freezing level valley bottom.FRIDAY: Isolated flurries. Ridge wind becoming strong from the southwest. Temperature -5. Freezing level 1000m in the afternoon.

Avalanche Summary

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. On Saturday 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. On Saturday, explosive control produced wind slab results up to size 2 on north facing slopes near 2000m. 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

Since the last snowfall almost a week ago, we've seen gusty winds (strong at times) from virtually all directions. The end result is wind slabs up to 20cm thick have formed on down wind (lee) features higher up, on a wide range of aspects. Below 1700m, rain last Wednesday formed a thick crust and capped the snow pack. 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 all overhead hazard.

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.

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.

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.