Avalanche Forecast
Issued: Feb 23rd, 2018 11:21AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Persistent Slabs.
NWAC,
Northwest Avalanche Center
New reactive wind slabs will continue to form Saturday. Identify and avoid steep slopes were the wind is depositing snow near and above treeline. It will still be possible for you to trigger a persistent slab avalanche Saturday. Stay away from steep large open slopes where this difficult to manage avalanche problem may be found.
Summary
Detailed Forecast
New winds slabs will continue to form on lee aspects Saturday. Look for signs of wind transporting snow onto nearby slopes such as blowing snow, fresh cornices, and snow drifts. Wind loading may occur near ridgeline and by cross-loading terrain features mid-slope. Avoid steep slopes where the winds deposited snow.
The snowpack structure continues to support potential persistent slab avalanches on some slopes. Weak sugary facets may be found 1 to 2 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 this past week in the Washington Pass area. Continue to stay off of large open slopes where you may trigger a persistent slab avalanche.
Loose surface snow will be found in sheltered terrain. Be careful in terrain where even small avalanches may have large consequences.
Snowpack Discussion
Small isolated wind slab avalanches were reported in the Washington Pass area Thursday and Friday. Winds continued to transport snow in some locations as winds increase Friday afternoon.
In sheltered areas loose surface snow exists. 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 and Friday reported small wind slabs isolated to ridgelines, with several small skier triggered wind slabs observed.
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
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
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.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Persistent Slabs
Release of a cohesive layer of soft to hard 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 Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Feb 24th, 2018 11:21AM
