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Avalanche Forecast

Archived

Feb 25th, 2018–Feb 26th, 2018

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
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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New snow and wind have created dangerous avalanche conditions. Avoid steep slopes were the wind is depositing snow near and above treeline. It may now be easier to trigger a dangerous Persistent Slab avalanche. Stay away from large open slopes steeper than 35 degrees that are suspect for these avalanches.

Detailed Forecast

Fresh Wind Slabs and new snow brought dangerous avalanche conditions to the East Slopes. Expect Wind Slabs on leeward sides of ridges. You may also find them in unique areas like cross-loaded terrain features mid-slope and in open areas below treeline. Avoid steep slopes where the wind deposited thick pillows of snow or obvious wind features. 

The new snow has added to the Persistent Slab avalanche problem. Slopes that previously lacked cohesive snow over the facets, may now have developed slabs. Weak sugary facets can be found 2-3 feet below the snow surface and above a firm crust. This is the perfect depth for human triggered avalanches. Two skier triggered Persistent Slabs have occurred last week in the Washington Pass area. It may now be easier for you to trigger these avalanches than prior to this storm. Continue to stay off of large open slopes where you could trigger a Persistent Slab avalanche. Collapsing and cracking are obvious signs that you could trigger a Persistent Slab. Use extra caution if you observe these signs of instability. 

 

Snowpack Discussion

One person was killed in an avalanche south of Snoqualmie Pass on Sunday. Our condolences go to the friends and family of the victim. If conditions allow, NWAC staff will visit the accident site. A very large natural avalanche crossed Highway 20 east of Newhalem on Sunday.

Up to 1.5 feet of snow fell with strong wind across the East Slopes on Sunday. Winds Slabs capped the existing snowpack on leeward slopes. Small isolated wind slab also formed Thursday and Friday. Prior to this storm about 1 to 2 feet of generally soft snow has been reported over an old (2/5) crust from early February. Weak sugary facets can be found above this crust in some locations. This persistent weak layer has failed in two reported skier triggered avalanches over the last week.

While several layers exist in the snowpack, there are no significant layers of concern below the 2/5 crust.

Observations

North

Avalanche professionals in the Washington Pass area Thursday through Saturday reported several small skier triggered wind slabs. Collapsing was reported on Saturday.

The most recent persistent slab avalanche occurred on Wednesday near Harts Pass. The avalanche was about two feet deep and 90 feet wide. It occurred on a NE aspect at 6000’.

Central

Mission Ridge Ski Patrol Reported many small natural avalanches in wind drifted areas on Sunday.

Observers reported triggering small avalanches on Blewett Pass and in the Wenatchee Mountains over the weekend.

NWAC field staff traveled in the Icicle Creek area Wednesday on NE facing slopes up to about 6000 feet. The persistent layer we are tracking was found consistently buried about 2 ft from the surface, though the distribution of this layer was intermittent. In many areas the snow above the facet/crust layer was low cohesion powder, lacking the ability to propagate a slab avalanche. Shallow fresh wind slabs were noted near and above treeline.

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.

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.