Avalanche Forecast
Issued: Jan 21st, 2018 11:31AM
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 danger will generally decrease during a break from the active weather Monday. While avalanches will become increasingly difficult to trigger, resulting avalanches may entrain significant snow, causing them to grow larger than expected. Ease into terrain Monday. Identify and avoid slopes where avalanches are more likely to trigger.
Summary
Detailed Forecast
A relative break in the weather Monday will allow avalanche hazard to decrease slightly in most areas. Isolated locations may experience higher precipitation rates in the morning hours and in convergence north of Stevens Pass.
Wind slabs formed throughout the storm period will continue to linger along ridge lines and crossloaded features near and above treeline. Stronger winds in the southern part of the region may have produced wind loading father down slope than expected. Identify and avoid areas of wind deposited snow such as below cornices, wind drifts, and crossloaded slopes.
Storm slabs layers will continue to gain strength. Identify and avoid areas were storm slabs are more likely to trigger such as steeper slopes, convex rolls, and unsupported slopes.
Avalanches may entrain significant new snow allowing them to grow larger than expected. Select terrain wisely and minimize exposure of your group to avalanche paths above you.
Snowpack Discussion
An active weather pattern Wednesday through Sunday deposited 12-18 inches of settled snow over the most recent crust layer. Observations show the recent snow is generally well bonded to the old snow surface.
Fluctuations in temperature and precipitation intensity created several layers within the upper snowpack during the day Sunday. These firmer layers lie above very soft snow from Saturday.
Winds, particularly at Paradise and Crystal, transported snow forming wind slabs in exposed terrain near and above treeline.
Observations
Central
An avalanche professional in the Stevens Pass area Sunday reported 18 inches of generally right-side-up snow well bonded to the 1/16 crust. No avalanches were observed.
An avalanche professional in the Snoqualmie Pass area Sunday reported 12 inches of snow well bonded to the 1/16 crust. They observed wind transportation of snow above 4000 feet. Several natural, small and isolated wind slab avalanches were seen in very steep northerly terrain.
A public observation from Jim Hill on Saturday reported an avalanche triggered by cornice fall on a near treeline N-facing slope.
NWAC Pro Observer Jeremy Allyn was in the Snoqualmie Pass area Friday and Saturday. On Saturday Jeremy observed 18 inches of recent snow over the 1/16 crust layer. Wind transportation was limited to exposed ridgeline terrain features. No new avalanches were observed.
South
On Friday, NWAC forecaster Dallas Glass traveled in the Crystal Mountain backcountry. The recent winds have caused highly variable storm snow conditions with exposed areas striped of recent snow. New snow was observed to be generally well bonded to the most recent crust.
Pro Patrol at Crystal Mountain Thursday morning reported shallow 2-4" very soft slabs. The underlying rain crusts have become very firm by Thursday morning above 6000 ft and gradually softer at lower elevations.
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: Jan 22nd, 2018 11:31AM
