Avalanche Forecast
Issued: Jan 20th, 2016 11:57PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Persistent Slabs, Wind Slabs and Storm Slabs.
NWAC,
Northwest Avalanche Center
Shallow new or lingering storm or wind slabs or small loose wet avalanches could step down to areas where persistent weak layers are reactive resulting in large avalanches in specific areas. If you venture out, use conservative decision making and slope selection to stay safe Thursday.
Summary
Detailed Forecast
A slow warming trend will begin late Wednesday night and continue on Thursday as a frontal system stalls offshore. Moderate snow and strong alpine winds should target the northeast Cascades Thursday. Strong alpine winds but much lighter precipitation amounts are forecast for the central and southeast Washington Cascades Thursday. A period of freezing rain is possible for the lower east slopes and valleys Thursday morning through mid-day with a delayed warm-up versus the west slopes.
In the northeast Cascades, moderate snowfall should gradually stress and load buried weak layers, potentially resulting in large and destructive avalanches. Travel in backcountry terrain is not recommended Thursday, especially near and above treeline where wind loaded slopes should tip the balance for natural avalanches from possible to likely. The danger from storm and wind slab avalanches will increase on Thursday as well, with warming throughout the day building an upside down snowpack and winds transporting snow to lee NW through SE aspects.
The avalanche forecast will be trickier for the south and central-east Cascades where precipitation should be lighter. However, shallow new or lingering storm or wind slabs or small loose wet avalanches could step down to areas where persistent weak layers are reactive resulting in large avalanches in specific areas.
Loose wet avalanches for the south and central east Cascades are likely to be small and confined to steeper slopes. Be especially wary near terrain traps, where even a small avalanche could have unintended consequences.
Snowpack Discussion
Weather
Two fair weather periods earlier this month allowed surface hoar and near surface faceting to occur. These persistent weak layers were buried intact on Jan 3rd and 11th and have been reported through Monday from the Methow, the Icicle Creek area, the Blewett Pass area, the Nason Ridge area and the Chiwaukum range east of Stevens Pass. It's safe to say this layer is extensive throughout the east slopes.
A parade of weather systems this week added about .5-1 feet of snowfall to the east slopes from about Friday to Sunday.
Only a few inches of additional snowfall accumulated with a weather system that passed through Oregon Tuesday and Tuesday night.
Snow and Avalanche Observations
NWAC pro observer Jeff Ward on Sunday found the persistent weak layer on shaded slopes up to about 5000 feet in the Early Winters drainage. This layer was reactive prompting very cautious terrain selection of lower angled slopes. A video by Jeff shows a similar persistent weak layer and a remotely triggered avalanche in the Icicle Creek Drainage near Leavenworth on Monday.
On Sunday the observer at the Scottish Lake High Camp in the Chiwaukums reported a natural 12 inch slab release on a southerly aspect around 5000 feet and easily identified two buried surface hoar layers in snowpits on a north-northwest slope at 5500 feet.
On Monday NWAC pro observer Tom Curtis was on Diamond Head near Blewett Pass in the 4-6000 foot range and found a persistent weak layer at 30-35 cm on north to east aspects with propagation indicated via PST and ECT tests. A 50 foot crack also released upon approach to a north facing starting zone indicating this layer remains reactive. Tom's video is here.
A report via the NWAC Observations page from Wedge Mountain for Monday reports a skier was caught and carried but not injured by a 18-24 inch storm slab avalanche.
NWAC pro observer Tom Curtis visited the Rainy Creek drainage of the central east zone today and traveled from 2-5800 feet and found limited lingering wind and storm slab. He did not find any signs of lingering persistent weak layers.
NCMG on Monday found several persistent weak layers near Goat Peak at 6400' on a north aspect, reactive to column test and sensitive to human triggering.
Remotely triggered avalanches on persistent weak layers were reported in TAY in the Mission Ridge area Monday.
Problems
Persistent Slabs
Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Aspects: North, North East, East, West, North West.
Elevations: All elevations.
Likelihood
Expected Size
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, North West.
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, 2016 11:57PM
