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Avalanche Forecast

Archived

Dec 31st, 2017–Jan 1st, 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 unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Lizard-Flathead.

We're into a tricky period as a critical amount of storm snow settles (into a slab) and sits on a buried weak layer. Keep your terrain choices on the conservative side until conditions stabilize.

Confidence

Moderate - Due to the number of field observations

Weather Forecast

Dry with gradually warming alpine temperatures during the week. Cold in the valley bottoms. MONDAY: Mix of sun and cloud. Ridge wind light southwest. Temperature -12. Freezing level surface.TUESDAY: Mix of sun and cloud. Ridge wind light west. Temperature -10. Freezing level surface.WEDNESDAY: Mostly sunny. Ridge wind light southwest. Temperature -8. Freezing level surface.

Avalanche Summary

On Sunday, an explosives-triggered Size 2.5 persistent slab avalanche was reported on a south east aspect at 1700m near Fernie, running on the mid-December surface hoar layer (down 65cm in that location). A widespread natural avalanche cycle occurred on Friday into Saturday producing storm slab avalanches to Size 2 on north through south easterly aspects between 1700 m and 2100 m.A surprising amount of natural avalanche activity was also reported below treeline on Saturday - see here for the MIN report and details. On Thursday, natural and skier triggered storm slab releases up to size 1.5 were reported including a remote-triggered Size 1.5 slab 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.

Snowpack Summary

On Friday into Saturday, 20-35 cm of storm snow fell on top of 25 cm of previous very low density storm snow. Winds (at times strong from the south west) have modified the snow pack, 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 60-80 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.