Avalanche Forecast
Issued: Mar 1st, 2018 10:03PM
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
You can trigger Persistent Slab avalanches that could break widely over terrain features. Put a wide buffer of terrain between where you travel and open slopes over 35 degrees as well as large avalanche paths. Reduce your risk of triggering a Wind Slab avalanche by avoiding fresh wind drifts and cross loaded features on steep slopes at upper elevations.
Summary
Detailed Forecast
You can trigger Wind Slab avalanches near and above treeline, today. You can avoid these avalanches by staying off of recent snow drifts, deeply pillowed features, and fresh cornices on slopes 35 degrees and steeper. These areas may exist far below ridge-lines and on mid-slope cross-loaded features. In some location soft non-wind-effected snow may cover new wind slabs making them harder to identify.
It's currently much easier to trigger dangerous Persistent Slab avalanches on the East side of the Cascades compared to the West Slopes. Several old weak layers exist in the snowpack. Put a wide margin of terrain between you and any slopes 35 degrees and steeper where you suspect the Persistent Slab problem. Continue to be cautious and stay away from large open slopes large avalanche paths. A resulting avalanche will likely be large and could kill you. If you experience collapsing or audible whumphs, stay away from any nearby avalanche terrain. Snow profiles and snowpack tests can help confirm the presence of a weak layer but cannot prove its absence.
Snowpack Discussion
Up to a foot of snow has fallen with variable snow totals around the East Slopes since Wednesday. This fresh snow has fallen on a variety of snow surfaces including thin sun crust, uneven wind surfaces, and soft unconsolidated snow. Moist snow surfaces and roller balls may be found at lower elevations.
A number of potential persistent weak layers exist in the snowpack around the eastern Cascades. Two common layers that have been reported in many locations are a facet/crust combination buried on 2/23 and a facet/crust layer buried on 2/13. The exact layer and depth depend on aspect, elevation, and proximity to the Cascade crest. A high level of uncertainty remains surrounding these layers.
The upper (shallower 2/23) layer can be found 1-2 feet below the snow surface on steeper slopes that have received direct sun. Small weak facets have been found in other regions surrounding a thin sun crust formed early last week and buried during last weekend’s storms.
The deeper (2/13) facet/crust combination is typically found 2-3 feet below the snow surface. This layer has been around for two weeks with two confirmed skier triggered avalanches and more recent collapsing and whumphing. With significant new snow added to the snowpack last weekend, this layer may be reactive in areas where we have not seen previous avalanche activity nor snowpack test results. These weak sugary facets are located above a firm wide spread crust buried on Feb 5th.
While several layers exist in the snowpack, there are no significant layers of concern below the 2/5 crust.
Observations
North
On Tuesday and Wednesday, avalanche professionals near Washington Pass observed small wind slabs near ridgeline and localized cracking. Moderate, gusty winds were transporting snow.
On Monday, the crown of a large avalanche was visible on Scaffold Ridge (Twisp River) in the North Cascades. Interestingly the red line in the photo marked the initial crown width that released Sunday and the remainder of the slab released sometime later Monday morning.
A persistent slab avalanche occurred on February 21st near Harts Pass. The avalanche was about two feet deep and 90 feet wide. It occurred on a NE aspect at 6000’.
Large natural slab avalanche visible on Scaffold Ridge in the North Cascades, starting zone about 7300'. Image, Matt Firth
Central
On Wednesday, An avalanche professional in the Chiwakum Mountains reported collapses and whumps on the 2/13 buried facet layer. Depth to the layer was highly variable (1-3 feet). Another observer triggered an avalanche almost 3 feet deep on a small steep slope near McCue Ridge.
Another avalanche professional in the Chiwaukum mountains traveled in a low-elevation terrain on a N-NW aspect and found the 2" thick 2/13 buried facet layer well preserved and 16" down. He also observed moist surface snow conditions up to 3400 feet with lots of roller balls.
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: Mar 2nd, 2018 10:03PM
