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Avalanche Forecast

Archived

Feb 11th, 2015–Feb 12th, 2015

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

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The primary avalanche problems Thursday will continue to be loose wet avalanches in all elevation bands and lingering wind slab on lee aspects near and above treeline.

Detailed Forecast

Mild snow levels will prevail again on Thursday with light rain and snow developing for the central east and northeast Cascades. There may be a few sunbreaks for the southeast Washington Cascades Thursday. Thursday should be a few degrees warmer than Wednesday along the east slopes. 

Loose wet avalanches are possible Thursday on steeper slopes below and near treeline that either receive enough rainfall, or in areas further south, solar aspects that receive substantial sun breaks. If neither happen, the loose wet avalanche potential would be minimal.   

Lingering wind slab will be listed as a problem for one more day east of the crest above treeline. Watch for firmer wind transported snow on previous lee slopes mainly on N-SE slopes near ridges.

Expect a lower regional danger further from the crest in the central and southeast Cascades due to patchy snow cover. 

As a result of the overall low snowpack, especially below treeline, watch for terrain hazards such as open creeks, partially covered rocks and vegetation.

Snowpack Discussion

A series of fronts in an atmospheric river crossed the Northwest late last week. The east slopes have seen quite a variety of weather and snow conditions. The Harts Pass Snotel and the NWAC Washington Pass weather station's total snow gages indicate about a foot of snow during this period. Other areas east of the crest have had less snow or rain.  

NWAC pro-observer Tom Curtis was at Dirty Face Peak on Saturday and found small roller balls in recent snow with no recent avalanches and no signs of instability. The North Cascade Mountain Guides were at Washington Pass on Sunday and found 30-40 cm of storm snow with touchy 6 inch storm slab above 7000 ft. They also reported lots of small loose wet avalanches at lower elevations.

The North Cascade Mountain Guides near Washington Pass on Monday also reported a size 1.5 20-30 cm deep x 25 m wide wind slab on a north slope that was remotely triggered  from a shallow spot with facets near rocks. So there is definitely still some variability out there.

Regarding the potential January 15th persistent slab combo in the northeast Cascades zone; NWAC pro-observers and North Cascade Guide reports as recent as Feb. 10th have found the January 15th facet/crust layers at 1 m below the surface on a north aspect of Silverstar Mt 6600 ft with the facets starting to turn to rounded grains and not reactive or not releasing in pit tests. The January 15th facet/crust layers are stabilizing and becoming very unlikely for a human to trigger. A very large natrual avalanche that may have run on this layer east of Washington Pass was likely tied to the heaviest precipitation/warming from last week's event. 

Problems

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

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.

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.