Avalanche Forecast
Issued: Feb 23rd, 2018 11:21AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Storm Slabs and Deep Persistent Slabs.
NWAC,
Northwest Avalanche Center
A variety of new and old avalanche problems will create dangerous avalanche conditions Saturday. With incoming storms, increasing avalanche danger, and a buried persistent weak layer this is a time to dial back terrain travel. Avoid all steep open slopes where avalanches may start. Use caution if traveling in areas where large or very large avalanches may run or stop.
Summary
Detailed Forecast
With several overlapping avalanche problems on Saturday this will be a challenging time to travel in the mountains. Dangerous avalanche conditions will develop Friday night into Saturday morning with newly forming wind and storm slabs. These new problems will develop on top of our buried persistent weak layer. Stay off of steep slopes where avalanches may start. Do not linger in the tracks and runouts of larger avalanche paths. Select travel routes and techniques to minimize your exposure to overhead hazards.
Winds will transport snow near and above treeline forming reactive wind slabs on lee aspects. Visual clues such as blowing snow and fresh cornices indicate wind slabs are nearby.
Storm slabs will develop with additional snowfall Saturday. These will be most reactive during periods of peak snowfall intensity. If you find strong new snow over softer weak snow expect storm slabs. Hand pits and small slope tests can help identify storm slab instabilities as you travel.
Our persistent slab problem has not gone away. Conditions over the past week have not allowed it to significantly heal. As additional snow buries this layer it will become more difficult to trigger. However, if you are involved in an avalanche on this persistent weak layer it could be very large and destructive. You are most likely to trigger a deep persistent slab from shallow areas in the snowpack or when a smaller avalanche steps down into this deeper layer. Continue to avoid large terrain where large avalanche may occur.
If you find loose unconsolidated surface snow, you may be able to trigger a loose dry avalanche. Even small loose dry avalanche can have dangerous consequences if they push you off a cliff, into a creek, or bury you in a gully.
Excellent travel conditions can still be found on slopes less than 30 degrees that are not connected to steep overhead slopes.
Snowpack Discussion
In general 2-4 feet of soft settled surface snow exists along much of the region. In many areas this snow sits on top of weak sugary facets buried on 2/13. These facets are located just above the most recent robust crust (2/8). Several avalanches occurred on this weak layer Saturday through Monday. Snowpack tests from around the area have generally shown this layer can still fail and produce avalanches. Observations from Stevens Pass, Snoqualmie Pass, and Mt Rainier confirm this persistent slab problem.
Steep SE-SW slopes formed a thin unsupportable surface crust Wednesday and Thursday. On shaded aspects loose surface snow exists in most locations.
Below the 2/8 crust no significant layers of concern exist.
Observations
An avalanche professional at Stevens Pass reported snowpack tests Wednesday indicating the weak facet layer could fail and produce avalanches. These were the most recent of numerous snowpack tests conducted over the last several days. All observations supported the same conclusion.
Several large persistent slab avalanches were triggered last Saturday 2/17 through Monday 2/19 in both the Snoqualmie and Stevens Pass backcountry (Stevens Pass-Hollywood Bowl-2-19). These avalanches failed on facets just above the 2/5 crust.
South
On Friday NPS rangers in the Paradise area reported 3-4 feet of snow over weak facets. No wind transported snow was observed.
NWAC Forecaster Dallas Glass observed a large audible whumph caused by a collapsing weak layer Tuesday near Paradise. Snowpack tests in the area indicated the persistent weak layer could fail and produce avalanches up to 4 feet deep.
No recent observations have been received from the Crystal backcountry. A higher level of uncertainty exists in this area.
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.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
Deep Persistent Slabs
Release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer, deep in the snowpack or near the ground. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage. They commonly develop when Persistent Slabs become more deeply buried over time.
Deep Persistent Slabs avalanches can be destructive and deadly events that can take months to stabilize. You can trigger them from well down in the avalanche path, and after dozens of tracks have crossed the slope.
A snowboarder triggered this Deep Persistent Slab near treeline, well down in the path.
Deep, persistent slabs are destructive and deadly events that can take months to stabilize. You can triggered them from well down in the avalanche path, and after dozens of tracks have crossed the slope. Give yourself a wide safety buffer to handle the uncertainty, potentially for the remainder of the season.
Aspects: North, North East, East, South East, West, North West.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Feb 24th, 2018 11:21AM
