Avalanche Forecast
Issued: Jan 19th, 2015 10:40AM
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
Watch for potentially dangerous storm and wind slabs sensitive to human triggering on lee slopes especially near and above treeline on Tuesday. The south-east Cascades and central-east Cascades away from the crest will likely have a lower avalanche danger.
Summary
Detailed Forecast
Tuesday should be partly to mostly sunny. Recent storm snow settlement will be slow with continued cold temperatures.
Expect a higher regional likelihood of finding sensitive wind slab on N-SE aspects near the Cascade crest Tuesday. Approach wind-loaded slopes with caution and practice conservative decision making especially when pushing into the near/above treeline band.
In the Washington Pass area, storm slabs releasing down a the Jan 5th crust/facet interface deserves your attention. Watch for cracking and collapsing while traveling as obvious instability clues and generally be conservative near avalanche terrain. There's plenty of good skiing to be found on lower angled slopes even in the alpine.
Sunshine may activate loose avalanches predominately on steeper southerly slopes with enough snow but this will not be listed as a top avalanche concern.
As a general backcountry travel safety note, tread carefully at lower elevations and on wind scoured aspects where terrain hazards (exposed rocks, trees, streams, etc.) are present.
Snowpack Discussion
Strong southwest flow carried a wet front across the Cascades Saturday night. This front tapped subtropical moisture but cold air trapped on the east side kept 48 hr water amounts through Mon AM of half an inch to 2 inches mainly in the form of snow. Snow was most significant in the northeast and central east Cascades with about 12-24 inches at Holden and in the Washington Pass area again through Mon morning. The warming trend seen late Sat night through Sunday led to an upside down layering.
Several reports have been received over the past couple days along the east slopes:
The North Cascade Mountain Guides in the Washington Pass Saturday reported about 10-20 cm of snow from the previous storm but no significant slabs. However, by Monday, the additional storm snow had formed a slab and collapsing was observed on a facet/crust combo down 70 cm on the Jan 5th crust. This layer was reactive in snowpit tests with propagation and one sizeable avalanche in the Cutthroat Drainage may have released down to this layer.
NWAC observer Tom Curtis was north of Blewett Pass on Thursday and found widespread 5mm+ surface hoar up to 5400 feet. This layer may not have survived through the weekend. The non-reactive persistent facet layer found on Jove Peak on Wednesday was also found in this area down 85 cm Thursday.
An avalanche class near Mission Ridge on Saturday reported via the NWAC Observations that facets between crust layers in the mid pack gave a Q1 shear but did not show a tendency to propagate via an ECT. The Mission Ridge ski patrol Sunday reported a surface rain crust with no natural or ski cut avalanches.
The south-east Cascades and central-east Cascades away from the crest will likely have a lower avalanche danger.
The overall snowpack remains well below normal for this time of year and some windward or southerly aspects have little if any snow cover.
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: North, North East, East, South East.
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 20th, 2015 10:40AM
