As much as 60 cm of low density storm snow could arrive by Friday morning. Expect both natural and human triggered avalanches in steep terrain and where wind loading has occurred.
Confidence
Moderate - Intensity of incoming weather systems is uncertain
Weather Forecast
THURSDAY NIGHT: Snow. Accumulation 15-25 cm. Ridge wind light northeast, possibly strong at times. Temperature -18. Freezing level surface.FRIDAY: Mostly cloudy, snow developing and overnight. Accumulation 5-25 cm. Ridge wind light northeast possibly strong at times. Temperature -17. Freezing level surface.SATURDAY: Mostly cloudy, flurries. Accumulation 5-10 cm. Ridge wind light west. Temperature -16. Freezing level surface.SUNDAY: Mostly sunny. Ridge wind light southwest. Temperature -13. Freezing level surface.
Avalanche Summary
Thursday's storm snow has produced numerous natural, storm slab and loose dry avalanches from Size 1-1.5 in steep terrain on all aspects. Avalanche control work produced similar results with loose, dry and storm slab avalanches up to Size 1.Earlier in the week on Monday, one large naturally-triggered avalanche (size 2) was reported on a north-facing alpine feature that likely failed on the mid-December surface hoar layer. There have been some isolated reports of whumpfs at small avalanches failing on the mid-December surface hoar layer, but in most areas the snow above this layer has not settled into a slab yet, this will change if winds are stronger than forecasted.
Snowpack Summary
Wednesday night and through the day on Thursday approximately 30 cm of low density snow fell with generally light winds.This overlies roughly 50 cm of old storm snow from last week that formed harder wind slabs in exposed terrain near ridge top, but remains low density in sheltered areas. The main question in the snowpack surrounds the mid-December layer now buried 80 cm below the surface. The layer includes crusts on south and west facing slopes and feathery surface hoar in sheltered areas at and below treeline, and has been producing moderate, sudden results in recent snowpack tests.The lower snowpack is generally well settled with a thick prominent crust that was buried near the end of November.
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.