Avalanche Forecast
Issued: Feb 13th, 2018 10:25AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Storm Slabs.
NWAC,
Northwest Avalanche Center
Avalanche conditions will change Wednesday due to a quick moving frontal system bringing fresh snowfall to the Cascades. Heightened avalanche conditions will exist on any slope greater than 35 degrees where significant new snowfall bonds poorly to the underlying surface including older weaker snow or a smooth crust.
Summary
Detailed Forecast
Avalanche conditions will change on Wednesday as a quick moving frontal system brings fresh snowfall to the Cascades Tuesday night and Wednesday along with much cooler temperatures.
On average, 6 to 12 inches of new snow may bond poorly to a medley of snow surfaces including weak surface snow in the form of persistent grain types, wind sculpted snow surfaces or various surface crusts. Heightened avalanche conditions will exist on any slope greater than 35 degrees where significant new snowfall bonds poorly to the underlying surface.
Shallow storm slabs will be more sensitive to human triggering in specific areas if they develop over weaker snow (near surface facets) found in wind sheltered, non solar aspects. Also watch for developing storm slab instabilities in areas that experience extended periods of intense snowfall.
While winds are not forecast to be especially strong with this system, westerly winds should build fresh wind slabs on lee slopes near and above treeline. Avoid steep slopes with wind deposited snow such as below cornices, on wind drifts, and near uneven snow surfaces. Wind slabs will be more likely to trigger in areas where wind deposited snow bonds poorly to an underlying crust or weak surface snow.
Loose dry avalanches will not be listed as a problem but will be possible on very steep slopes. Be aware of the consequences of a loose dry avalanche near terrain traps such as cliffs, gullies, or open creeks. In areas where shallow loose new snow bonds poorly to the underlying crust, avoid steep icy slopes where it will be difficult to stop a fall.
Continue to avoid other non-avalanche-related hazards such as glide cracks, creeks, and openings within the snowpack formed during warm wet weather in early February. Falling into these holes poses a danger to backcountry travelers.
Snowpack Discussion
Heading into Tuesday night's storm, a strong and stable snowpack exists around the region. Up to 6 inches of older weak snow sits on the supportable 2/8 crust in most wind and sun sheltered locations along the west slopes of the Cascades. On slopes that have received direct sun over the past few days, spring-like conditions have developed. Above treeline and to a lesser extent near treeline, a medley of wind sculpted snow surfaces or surface crusts exist.
Warm wet weather from the beginning of February created a well consolidated snowpack. While we are tracking some deeper buried crust layers, there are no significant layers of concern.
There have not been any reports of avalanches in several days.
Observations from the West Slopes of the Cascades
NWAC field staff over the last several days have reported a strong snowpack and no recent avalanche activity throughout the west slopes of the Cascades. Older debris from the 2/4 avalanche cycle is still visible in many areas.
In non-wind or sun affected terrain, several inches of weak surface snow sits upon the 2/8 crust mainly in the form of near surface facets although some observations of surface hoar were also noted. NWAC staff in the Mt. Baker and the Paradise backcountry near and above treeline on Tuesday reported landscapes reshaped by recent winds and sun. Highly variable snow surfaces consisted of melt-freeze crusts, sastrugi, old wind deposits and scoured surfaces exposing slick crusts to name a few.
Over the weekend, numerous glide cracks and openings in the snowpack were present and created a non-avalanche travel hazard.
Problems
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: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
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
Expected Size
Valid until: Feb 14th, 2018 10:25AM
