Avalanche Forecast
Issued: Jan 20th, 2018 10:59AM
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
The avalanche danger will rise throughout the day Sunday as another round of precipitation and winds effect the area. Watch for changes in the weather and be prepared to use terrain conservatively as new wind and storm slabs form.
Summary
Detailed Forecast
Another round of precipitation and winds will reach the Cascades during the day Sunday. Avalanche conditions should deteriorate as the storm moves through the area. Slightly higher precipitation amounts and winds should lead to a slightly elevated avalanche danger in the Crystal and Paradise areas.
Expect winds to transport snow and form new sensitive wind slabs on exposed terrain features near and above treeline. Watch for an increase in wind speed and visual signs of winds actively transporting snow such as plumes and newly forming cornices. As wind transportation increases through the day, identify and avoid areas where winds are depositing snow.
Small shallow storm slabs will develop on Sunday during period of higher precipitation intensity. Lingering storm slabs formed throughout the storm period are gaining strength but may be found in in isolated locations. These older storm slabs are most likely to trigger in steep terrain, on convex rolls, and on unsupported slopes. As storm slabs grow consider the consequences of the terrain where being caught in even a small avalanche may have more drastic consequences such as above cliffs, gullies, and open creeks.
Snowpack Discussion
A stormy period beginning Wednesday has brought 12-18 inches of storm snow to the Passes and 6-12 inches in the Paradise/Crystal areas. A general cooling trend throughout the storm period created a generally strong bond to the most recent crust layer.
Moderate to strong southwest winds redistributed the new snow creating a variety of snow surfaces near and above treeline. Wind slabs have been observed on several aspects in exposed terrain especially along ridgelines.
Observations indicate recent snow is bonding well to the 1/16 crust.
Observations
Central
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.
Several avalanche professionals throughout the Snoqualmie Pass area Saturday reported 12-15 inches of generally right-side-up storm snow well bonded to the 1/16 crust. Only small loose dry 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 1/18 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 21st, 2018 10:59AM
