Avalanche Forecast

Issued: Jan 14th, 2015 10:00AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

New shallow storm and wind slab mainly on lee aspects may bond poorly to the existing snow surface Thursday afternoon in the above treeline elevation band. 

Summary

Detailed Forecast

On Thursday, an approaching weather system should bring light snow during the daylight hours. Precipitation will become more intense Thursday evening. This should minimally increase the avalanche danger Thursday. Shallow new storm and wind slab may develop by the end of the day above treeline. 

East of the crest in the Central Zone the likelihood of triggering a persistent slab is considered very low.

On 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 January 4th was followed by about 1-3 inches of rain on January 5th 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.

Unseasonably high freezing levels with strong temperature inversions and without any significant precipitation has characterized the weather since the last significant storm. No avalanches have been reported recently east of the crest.

The Mission Ridge pro-patrol last Friday reported that the wet and warm weather had stabilized the upper snowpack and formed a supportable crust in most areas. Depth hoar involved in large skier triggered avalanches in this area around the New Year remained intact but unreactive to explosives on previously uncontrolled slopes. Pro-observer Tom C. near Jove Peak on Wednesday found a faceted layer 85 cm down on a southerly aspect generally unreactive in snowpit tests. 

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.

Problems

Storm Slabs

An icon showing Storm Slabs

Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.

 

You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.

 

Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.

Aspects: All aspects.

Elevations: Alpine.

Likelihood

Possible

Expected Size

1 - 1

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 15th, 2015 10:00AM