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Avalanche Forecast

Archived

Feb 7th, 2015–Feb 8th, 2015

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

Regions

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Expect a decreasing avalanche danger Sunday with a break between systems. Watch for areas where more significant new snow has accumulated, mainly above treeline where new storm and wind slab are more likely. 

Detailed Forecast

A break between systems is expected during the daylight hours Sunday. This should give time for any recent shallow storm slab or wet new snow to settle and possibly refreeze to form new surface crust layers. This should allow for a slightly decreasing danger at higher elevations where more recent snow may have accumulated. Watch for wind transported new snow on some higher elevation lee slopes, mainly NW-SE facing   

It is unknown how well storm snow received Thursday and Thursday night has bonded to the underlying crust or old snow surface in the northeast and central east Cascades near the crest... and a potential storm slab triggered in this zone could step down to this interface.

Remember, due to the unseasonably low snowpack especially at lower elevations, numerous terrain hazards such as open creeks, partially covered rocks and vegetation are widespread.  

Snowpack Discussion

Periods of light snowfall were this week east of the crest with more snowfall at higher elevations in the northeast and central east zones. Steady but generally light precipitation was seen during the day Thursday with a gradual warming trend that continued into Friday. The snow level hung around 5500 feet in the northeast Cascades for most of Thursday night with snowfall accumulation likely in the 6-10 inch range in the Washington Pass area before likely mixing with light rain Friday. Much less new snow was reported in the Stuart range and Mission Ridge area above 6500 feet Friday.  Slight cooling with a frontal system Saturday has deposited an additional about 2-4 inches of snow above about 4-5000 feet 

NWAC pro-obs and North Cascade Guides, report last week in the northeast and central east zones have the January 15th facet/crust layers at 40 cm-1 m below the surface with the facets starting to turn to rounded grains and not reactive or not releasing in pit tests. It thus appears the January 15th facet/crust layers are strengthening and stabilizing. This problem has been removed from the central east zone forecast, but retained in the northeast zone until more information becomes available following this storm cycle.

Other private groups in the Washington Pass area on February 2 and 3 report some upside down densities in the recent snow from earlier in the week and natural and ski cut loose dry avalanches. 

Jeff Ward was in the Stuart Range Friday and found small but sensitive, loose wet avalanches naturally and skier triggered above about 6500 ft. Below this elevation, there was not enough new snow for wet loose danger and the most recent crust had not significantly softened. In the area traveled, the rest of the pack was consolidated and the Jan 15th layer was non-reactive in snowpit tests.

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.

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.