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
A break in the weather will allow recently formed wind and storm slabs time to heal. Ease into terrain Monday. Take time to identify and avoid slopes where winds have deposited snow and avalanches are more likely to be triggered.
Summary
Detailed Forecast
A break in the weather Monday will allow recently formed wind and storm slab layers time to heal.
Several days of snow and wind formed wind slabs on a variety of aspects near and above treeline. A backcountry traveler will be able to trigger a wind slab on Monday. Identify and avoid areas of wind deposited snow such as below cornices, snow drifts, and on cross-loaded terrain features.
Shallow small storm slabs will linger into Monday and are more likely to be triggered on steeper slopes, near convex rolls, or on unsupported slopes. Be aware of the consequences of even a small avalanche. Cliffs, rocks, gullies, and creeks can all magnify the consequences of a small avalanche.
Snowpack Discussion
Along the east side of the Cascades a stormy period from 1/16 through the present produced 12-20 inches of settled snow now sitting on the 1/16 melt-freeze crust. Observations show the bond between the new snow and the crust continuing to gain strength.
Winds observed during the stormy period formed wind slabs on a variety of aspects.
Observations above treeline have been very limited leading to a high level of uncertainty in this terrain.
Buried surface hoar was observed in the Cascade East - Central zone on Wednesday, January 17th. This layer has not been found in the north zone or in the area near Lake Wenatchee. The extent and distribution of this layer remains highly uncertain.
A widespread freezing rain event in the Cascade East-Central zone formed a supportive crust (1/5) which extends to the Stevens and Snoqualmie Pass areas. This layer was not present in the northeast from Washington Pass to Holden.
Snow depth still decreases substantially east of the Cascade crest. In many areas below treeline, there has not been enough snow to present an avalanche danger.
Observations
North
On Saturday guides from North Cascades Heli reported 10-14 inches of snow over the 1/16 crust. Observations indicate potentially touchy wind slabs above treeline.
On Thursday snow professionals in the Washington Pass area found good bonding of new snow to the old surfaces with favorable snow profiles and no avalanche activity, even traveling on steep slopes. No buried surface hoar was found in this zone.
Central
On Saturday an avalanche professional on Dirty Face Mountain near Lake Wenatchee observed a well bonded upper snowpack near and below treeline. No buried surface hoar was found and no avalanches were observed.
On Friday, NWAC observer Jeff Ward traveled near Mt Poe near Lake Wenatchee. No buried surface hoar was found over the Jan 16 crust layer in this terrain. Wind re-distribution was evident, especially in higher exposed terrain. Storm snow totals ranged from 16-20 inches in this area with no recent avalanches observed.
On Wednesday, 1/17, NWAC observers Jeremy Allyn and Jeff Ward traveled in the Icicle Creek drainage to 6800 ft. Buried surface hoar was found over a melt-freeze crust. The buried surface hoar was not limited to valley bottoms, but found to all elevations up to 6800 feet.
South
No recent observations
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
