Avalanche Forecast

Issued: Feb 12th, 2016 10:15AM

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is moderate. Known problems include Wind Slabs, Storm Slabs and Glide Cracks.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Moderate W-SW winds will transport new snow onto lee slopes near and above treeline Saturday. New snow is generally expected to bond well to moist snow surfaces and various crusts, but wind-driven snow may build unstable wind slab on lee aspects. The avalanche danger will increase rapidly Saturday night as snowfall increases along with a slow warming trend. 

Summary

Detailed Forecast

Light snowfall accumulations are expected Friday night through Saturday above 3500 feet along the west slopes.

Moderate W-SW winds will transport new snow onto lee slopes near and above treeline. New snow is generally expected to bond well to moist snow surfaces and various crusts, but wind-driven snow may build unstable wind slab on lee aspects. 

Wet loose avalanches may still be locally a concern on steeper slopes at lower elevations, especially in areas that received heavier rainfall Thursday night and Friday like in the vicinity of Mt. Baker and Mt. Rainier but loose wet will not be listed as a primary avalanche problem. 

Avoid cornices along ridges and slopes below cornices since cornices may still be weaker due to the warm weather.

Glide avalanches occur when a smooth bed surface (ex.rock face) becomes lubricated by water and the wet slab above releases. They are not tied to specific air temperature or rainfall trends other than they occur during or after prolonged mild conditions, similar to what we have experienced over the last several days. Be aware that even with lower snow levels Saturday, isolated but powerful glide avalanches are possible in the right terrain features until we have an extended period of cold weather.   

The avalanche danger will increase rapidly Saturday night as snowfall increases along with a slow warming trend. This unstable combination of moderate to heavy precipitation and warming may continue on Sunday leading to high avalanche danger in many areas.

Snowpack Discussion

Weather and Snowpack

A large upper ridge and warm air mass aloft over the West Coast from this past Sunday through Wednesday led to the warmest weather of the winter. During this stretch temperatures were generally well above freezing. A warm front brought periods of rain to the west slopes Thursday night through mid-day Friday, with snow levels falling to 5000 ft after the frontal passage Friday afternoon. 

The warm temperatures and solar effects earlier in the week caused loose wet avalanches and overall snowpack consolidation while recent rainfall has kept the upper snowpack moist in many areas. The upper snowpack should be well bonded to the 1/28 rain crust in all areas along the west slopes. 

Recent Observations

NWAC pro-observer Ian Nicholson was in the Snoqualmie Pass backcountry on Tuesday and reported that an extensive loose wet avalanche cycle with some large avalanches had occurred on Monday. In the same area, only a few small natural loose wet avalanches were seen on Tuesday. 

Another sign of the ongoing warmth, both the Alpental and Mt. Baker pro-patrol reported glide cracks with small glide avalanches releasing off of steeper rock faces within their respective areas Thursday. Mt. Baker pro-patrol reported additional small glide avalanches Friday. 

Problems

Wind Slabs

An icon showing 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: North, North East, East, South East.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

1 - 1

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: All elevations.

Likelihood

Possible

Expected Size

1 - 1

Glide Cracks

An icon showing Glide Cracks

A release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. They are often preceded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.

 

Predicting the release of Glide avalanches is very challenging. Because Glide avalanches only occur on very specific slopes, safe travel relies on identifying and avoiding those slopes. Glide cracks are a significant indicator, as are recent Glide avalanches.

 

This Glide avalanche broke to the ground on a smooth, grassy slope. From all the mud on the bed surface, water pooling at the base of the snowpack likely caused the failure.

Glide avalanches occur when water lubricates the interface between the snowpack and the ground. These avalanches are difficult to predict and best managed by avoiding terrain below glide cracks.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Unlikely

Expected Size

1 - 1

Valid until: Feb 13th, 2016 10:15AM