Avalanche Forecast

Issued: Jan 12th, 2015 10:00AM

The alpine rating is low, the treeline rating is low, and the below treeline rating is low. Known problems include Persistent Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

The avalanche danger is expected to be low. Watch for non-avalanche terrain hazards that may be present due to the thin snowpack. Be prepared to self-arrest if traversing steep and icy slopes.  

Summary

Detailed Forecast

Mostly sunny weather is expected on Tuesday. This should not change the overall low avalanche danger. Previous loose wet avalanches, consolidation and low solar input should make more loose wet avalanches very unlikely. Watch for soft wet snow on steep solar slopes or initial natural releases that would indicate conditions could be different than expected.

The only highlighted avalanche problem east of the crest will be in the Central Zone where the likelihood of triggering a persistent slab is very low.  

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 on 4 January was followed by about 1-3 inches of rain on 5 January with a warming trend. This led to a natural avalanche cycle east of the crest. In the northeast Cascades at higher elevations more of the precipitation fell as snow before precipitation ended than elsewhere.

Dry and unseasonably warm weather followed last week. This drained, consolidated and generally reset the upper or even entire  snowpack. There were also some loose wet avalanches. Melt-freeze crusts of varying thicknesses formed during the clear nights midweek with generally only southerly aspects softening.  On some windward or southerly aspects there is little snow cover.

Backcountry skier/pro patroller Dan Veenhuizen was on Mt Cashmere last Tuesday and noted debris from large avalanches from 4 and 5 January. At 6900 ft he found what should be the December facet layer at about 90 cm below the surface but it was not reactive in an ECT. Also from this cycle, the North Cascade Mountain Guides reported widespread large avalanches up to size D3 in the Washington Pass area, including reaching Highway 20 but not crossing the closed road.

Facet layer in a pit on Mt Cashmere on Tuesday by Dan Veenhuizen.

The Mission Ridge area saw less snow and rain but also experienced strong warming last week. On Friday the 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 the New Year 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 the central east zone.

Mild temperatures and a few light rain showers were received Saturday, followed by cloudy weather and a slight cooling trend Sunday, and decreasing clouds today. This should not change the overall stable  snow pack  and current low danger.

Problems

Persistent Slabs

An icon showing 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

Unlikely

Expected Size

1 - 2

Valid until: Jan 13th, 2015 10:00AM