Avalanche Forecast
Issued: Feb 28th, 2018 12:34PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Storm Slabs and Persistent Slabs.
NWAC,
Northwest Avalanche Center
Overlapping avalanche problems will combine Thursday to create complex and dangerous avalanche conditions. Use caution when traveling in the mountains and avoid all steep open slopes where an avalanche may start. Avalanches have the potential to become large enough to kill you.
Summary
Detailed Forecast
You will be able to trigger a variety of different avalanche problems Thursday. This complicated scenario deserves respect. Stay off all steep open slopes where avalanches may start.
New and reactive storm slabs and wind slabs will have formed Wednesday night. These will be larger and easier to trigger in areas receiving more snowfall and wind, particularly around Mt Baker and Mt Rainier. Identify and avoid slopes where winds have deposited new snow. In some location soft non-wind-effected snow may cover new wind slabs making them harder to identify. Stay off of any slope you expect may have received wind loading from the predominantly southerly winds during the storm. These areas may exist far below ridge-line and on mid-slope cross-loaded features.
In sheltered areas, you will be able to trigger soft storm slabs on slopes greater than 35 degrees. Subtle changes in temperature and snowfall rates formed weak layers in the new snow. These layers need time to heal.
Weak old snow layers found on slopes that have received direct sun were buried on 2/23. This persistent weak layer resulted in several avalanches 1.5-3 feet deep. Snow profiles and snowpack tests can show the presence of this layer but cannot prove its absence. This layer will still be able to be triggered by backcountry travelers on Thursday.
While it is not listed as a problem, there is still a potential for a Deep Persistent Slab avalanche to occur. These avalanches are becoming increasingly difficult to trigger, but may be caused by a smaller avalanche stepping down to older deeper weak layers or a cornice fall. The best way to avoid this low likelihood-high consequence problem is by avoiding triggering smaller avalanches in the surface snow.
Snowpack Discussion
New snow fell around the western regions of the Cascades beginning Wednesday. New snow combined with moderate to strong winds to transport snow in exposed terrain forming new wind slabs.
On E-S-W aspects, a thin breakable sun crust was formed early last week and buried on 2/23. Very small weak facets have been reported surrounding the crust. This was the weak layer found or suspected in several avalanches 1.5-3 feet deep. This layer has not yet had significant time to heal. It is found 2-3 feet below the surface on steeper slopes that have received direct sun during the past week.
Some observations suggest the presence of other persistent grains at this same interface on shaded slopes. Buried surface hoar and large preserved stellars have been reported in recent avalanches and snowpack tests at this interface.
Avalanche and snowpack observations continue to indicate that avalanches are possible on a layer of weak sugary facets buried on 2/13. This weak layer is generally 3 to 5 feet below the snow surface just above a very firm melt-freeze crust (2/8).
There are no significant layers of concern below the 2/8 crust.
Observations
On Wednesday NWAC professional observer Lee Lazzara traveled in the Mt Baker backcountry. Lee reported wind slabs forming on a variety of aspects near treeline. Snowpack observations showed a highly variable snowpack, but the facet/crust combination was found on steep sunny aspects 2 feet below the snow surface.
NWAC professional observers in the Crystal area Tuesday and Wednesday observed the 2/23 interface, involved in two previous avalanches, still reactive in snowpack tests. In this area the 2/23 layer was found 1-2 feet below the snow surface.
Avalanche and snowpack observations from around the western regions of the Cascades and Passes continue to indicate that propagation of an avalanche on the 2/13 facets is possible. This layer has been found 2.5 to 4 feet below the snow surface.
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
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.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
Valid until: Mar 1st, 2018 12:34PM
