Avalanche Forecast
Issued: Jan 9th, 2015 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Persistent Slabs.
NWAC,
Northwest Avalanche Center
The avalanche danger is low, however be aware of the persistent slab problem in this zone despite the very low likelihood for human triggering. Non-avalanche terrain hazards are present due to the thin snowpack. Also, be prepared to self-arrest if traversing steep and icy slopes.
Summary
Detailed Forecast
Very light rain and snow showers along with cool and cloudy conditions are expected on Saturday. This will not change the overall low avalanche danger. The only specific avalanche problem along the east slopes of the Cascades exists in the Central Zone where the likelihood of triggering a persistent slab is very low but the consequences are high and worth considering when entering avalanche terrain.
As a general backcountry travel safety note, watch for terrain hazards (exposed rocks, trees, streams, etc) at lower elevations and on wind scoured aspects. On non-solar aspects with a slick and supportable crust, take extra caution when traversing steep slopes and be prepared to self-arrest if necessary.
Snowpack Discussion
East of the crest about 6-12 inches of snowfall last Sunday was followed by about 1 to 3 inches of rain on Monday with a warming trend especially on Monday. This event led to a natural avalanche cycle east of the crest. At higher elevations in the northeast Cascades more of the precipitation fell as snow before precipitation ended on Monday. A period of freezing rain likely occurred at lower elevations Sunday night.
Backcountry skier/pro patroller Dan Veenhuizen was on Mt Cashmere on Tuesday and noted debris from large avalanches from Sun/Mon. At 6900 ft he found what should be the December facet layer at about 90 cm below the surface, however it was not reactive to an ECT. Also from this cycle the North Cascade Mountain Guides reported large avalanches up to D3 in the Washington Pass area, including slides reaching Hwy 20 but not crossing the closed highway.
Mostly sunny and unseasonably warm weather followed mid-week in the alpine while the valleys stayed colder and cloudier. In most areas this weather has led to draining, consolidation and a general resetting of the upper or entire snowpack. While the weather has been pleasant in the mountains, ski conditions have generally been reported as poor over the last few days
The Mission Ridge area saw less precipitation but strong warming as well earlier this week. On Friday, Mission Ridge pro-patrol reported the warm and wet regime had stabilized the upper snowpack and formed a supportable crust in most areas. However, depth hoar involved in large skier triggered avalanches in this area around NYE remained intact post-storm. While it is highly unlikely for a human to trigger a persistent slab down to this layer, we will continue to call this avalanche problem out in this zone.
Facet layer in a pit on Mt Cashmere on Tuesday by Dan Veenhuizen.
Problems
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: Jan 10th, 2015 10:00AM
