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Avalanche Forecast

Archived

Dec 29th, 2017–Dec 30th, 2017

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 possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Lizard-Flathead.

With a buried weak layer in the snowpack, 30 cm of recent storm snow and more snow in the forecast, conservative terrain choices are the key for safe skiing and riding this weekend.

Confidence

Moderate - Due to the number of field observations

Weather Forecast

FRIDAY NIGHT: Snow. Accumulation 15-25 cm. Ridge wind light east, possibly strong at times. Temperature -22. Freezing level surface.SATURDAY: Mostly cloudy, flurries. Accumulation up to 5 cm. Ridge wind light west, possibly strong at times. Temperature -17. Freezing level surface.SUNDAY: Mostly sunny. Ridge wind light southwest. Temperature -14. Freezing level surface.MONDAY: Mix of sun and cloud. Ridge wind light southwest. Temperature -15. Freezing level surface.

Avalanche Summary

A natural avalanche cycle occurred overnight Thursday producing storm slab avalanches to size 2 on north to northeasterly, wind loaded, upper elevation start zones.  Avalanche control work on Friday produced explosive-triggered, storm slab avalanches from size 1.5-2. On Thursday natural and skier triggered storm slabs releases up to size 1.5 were reported including a remotely triggering a size 1.5 slab release on an open, rock slope on a north west aspect below treeline. Whumpfing and cracking in rocky areas was also reported on Thursday. See the report on the Mountain Information Network (MIN) here.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 and small avalanches failing on the mid-December surface hoar layer, which remains a concern with the on-going storm loading.

Snowpack Summary

Thursday night 10-15 cm of storm snow fell on top of 25 cm of very low density storm snow and has been modified by winds creating an "upside-down" storm slab layer with stiffer snow on top of softer snow.This upside-down storm snow layer overlies roughly 50 cm of old storm snow from last week that formed into stiff wind slabs in exposed terrain near ridge top, but remains low density in sheltered areas. The mid-December layer is 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.