Avalanche Forecast
Issued: Feb 11th, 2016 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Loose Wet, Wind Slabs and Glide Cracks.
NWAC,
Northwest Avalanche Center
Rainfall will renew the loose wet avalanche potential on all aspects of steeper slopes. New snowfall should bond well to a moist snow surface in most locations, but wind-driven snow may build unstable wind slab on lee aspects of higher terrain by late Friday afternoon.
Summary
Detailed Forecast
Mild snow levels should keep precipitation in liquid form until a cold front passes through the Cascades Friday mid-afternoon with snow levels lowering to 4000-4500 feet by late afternoon. Precipitation should be light to moderate, except occasionally heavy in the Mt. Baker area Friday afternoon through evening.
Rainfall will renew the loose wet potential on all aspects of steeper slopes. New snowfall should bond well to a moist snow surface in most locations, but wind-driven snow may build unstable wind slab on lee aspects of higher terrain by late Friday afternoon.
Continue to watch for loose wet avalanche potential above terrain traps such as above cliffs, near gullies or where avalanche debris would deeply accumulate. These are the types of places that even small wet avalanches can have serious unintended consequences.
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 dropping snow levels Friday, powerful glide avalanches are possible in the right terrain features until we have an extended period of cold weather.
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. Freezing levels came down a few notches Thursday, but mild conditions continued with spotty light rain increasing Thursday afternoon.
The very warm temperatures and solar effects caused loose wet avalanches, snowpack consolidation, and melt-freeze surface crusts. This will have further stabilized the lower and mid snow pack and turned the most recent storm snow into moist to wet snow in most areas. The upper snowpack should be well bonded to the 1/28 rain crust in all areas.
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. The Alpental pro-patrol gave a similar report for 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.
Problems
Loose Wet
Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
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, North West.
Elevations: Alpine.
Likelihood
Expected Size
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
Expected Size
Valid until: Feb 12th, 2016 10:00AM
