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Avalanche Forecast

Archived

Feb 15th, 2019–Feb 16th, 2019

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 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.

The first half of the storm was a bit of a bust, but Friday night's wave could be strong. Snowfall amounts are uncertain and may be spatially variable in the region. The forecast herein assumes 30+ cm of snow accumulation.

Confidence

Moderate - Intensity of incoming weather systems is uncertain

Weather Forecast

FRIDAY NIGHT: Cloudy with snowfall, variable accumulation in the region between 5 and 20 cm, moderate west wind, alpine temperature -8 C.SATURDAY: Cloudy with snowfall, accumulation 5 to 15 cm, light to moderate west wind, alpine temperature -13 C.SUNDAY: Mostly cloudy with light snowfall, accumulation 5 cm, light north wind, alpine temperature -20 C.MONDAY: Mostly clear skies, light north wind, alpine temperature -18 C.

Avalanche Summary

On Thursday, there was further evidence of a natural avalanche cycle that occurred on Wednesday, with many large (size 2 to 3) storm slab avalanches observed. The avalanches were generally 20 to 40 cm deep, on northerly aspects, and within all elevation bands. There were also indications of instability within one of the buried weak layers described in the Snowpack Summary, as noted here.

Snowpack Summary

More snowfall is expected Friday night into Saturday, with total amounts uncertain but could be upwards of 20 cm or more. This new snow overlies around 30 to 50 cm of recent storm snow, which may have been redistributed by strong northeast wind at higher elevations.Two weak layers may be found within the snowpack. A weak layer of surface hoar crystals that was buried early-February may be found about 50 cm deep in shaded and sheltered areas above 1600 m. The mid-January layer of surface hoar is buried around 70 cm and is most prominent between 1600 m and 1900 m.The remainder of the snowpack is well-settled. Thin snowpack areas, such as in the far east of the region, may find weak and sugary faceted grains near the base of the snowpack.

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.

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.