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Avalanche Forecast

Archived

Dec 29th, 2014–Dec 30th, 2014

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

Snoqualmie Pass.

The avalanche danger should decrease with lingering wind and storm slab the main avalanche problems on Monday.

Detailed Forecast

Light northeast winds, clouds giving way to some sun and cool temperatures should be seen west of the crest on Monday.This should cause stabilizing and a decreasing avalanche danger on Monday.

Lingering wind slab will be rated as likely west of the crest on Monday. Watch for firmer wind transported snow on steep lee slopes.

Lingering storm slab may still be possible on Monday so watch for it in any areas that received heavier amounts of snow. Hand shears or shovel tilt tests can be fast ways to check for this type of layer.

Any hoar frost that survived the storm would increase the instability of wind or storm slab layers.

Instability should be limited to new snow and deeper instabilities in the snow pack are not currently expected west of the crest.

The sun should be out by later Monday but sun effects should be limited since we are barely past the solstice.

Note that northeast winds may increase Monday night and Tuesday so heads up for new wind slab potential on Tuesday.

Snowpack Discussion

A cooler period with little precipitation was seen about Thursday to Friday. NWAC observer Dallas Glass saw widespread 5 mm surface hoar which formed around Paradise Thursday night.

Sparkly surface hoar on a SW aspect at 6000 feet at Paradise Friday by Dallas Glass.

A cold front moved over the Northwest on Saturday and Saturday night. West winds changed to northwest, with moderate to heavy snow west of the crest and a cooling trend. Convergence contributed to snowfall in the central Cascades. NWAC sites west of the crest picked up about 6-14 inches of new snow by Sunday morning. 

Northwest winds are decreasing west of the crest on Sunday with little or no snow accumulation and cooler temperatures. NWAC observer Tom Curtis at Stevens Sunday morning reported natural storm slab avalanches due to heavy loading. By Sunday afternoon NWAC observers Jeff Hambelton at Baker and Jeremy Allyn at Crystal reported improved stability with mostly right side up uncohesive snow. So the cooling trend looks like it favored right side up snow and should help limit the extent and longevity of new slab layers.

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.

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.