Avalanche Forecast
Issued: Feb 16th, 2018 11:10AM
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
Travel in avalanche terrain is not recommended. Heavy snowfall and strong wind will create very dangerous avalanche conditions. Stay far away from slopes steeper than 30 degrees or terrain where avalanches could come from above.
Summary
Detailed Forecast
Travel in avalanche terrain is not recommended Saturday. If you go into the backcountry today, you should be certain of your ability to avoid all avalanche terrain. Put a wide buffer of terrain between where you travel and any slopes over 30 degrees. Use extreme caution in areas where avalanches could come down from above.
Intense snowfall and strong to very strong winds will create large to very large wind slabs at all elevations with large storm slabs rapidly developing near and below treeline. You can easily trigger Storm Slabs on steep slopes. The upper snowpack will become very unstable as high snowfall rates of denser storm snow fall on lower density and faceted snow sitting above a slick sliding surface (a crust). Natural avalanches are likely and human-triggered avalanches are very likely on Saturday.
Snowpack Discussion
The east slopes of the Cascades saw a warming trend with light snow falling closer to the Cascade crest on Friday creating an inverted layer on top of the 2-8" of settled lower density snow which now on the 2/5 crust. Snow which fell on Wednesday bonded poorly to the older snow directly above the crust. On north through east aspects where sun and wind didn't affect this older snow, a shallow layer of sugary faceted snow exists.
Moderate winds created wind slabs near ridge crest on Friday. A large natural wind slab avalanche was observed Friday in the Washington Pass area (see observation, below). A previous round of NW winds transported snow on Wednesday (2/14) and created small, isolated wind slabs on easterly ridge crests in the Cascade East-North zone.
A relatively well consolidated snowpack exists below the 2/5 crust. While we are tracking several older crust layers there are currently no other layers of concern in the snowpack.
In the last week of January, observers reported isolated buried surface hoar on top of the 1/16 crust. This persistent weak layer was found or thought to be the cause of several avalanches. A recent observation (2/11) from the Mission Ridge area confirmed this layer is still present in some locations. This layer will be worth watching during Saturday's storm. Snow profiles and snowpack tests are the best means to confirm the presence of this layer.
Observations
North
North Cascades Heli was in the Washington Pass area on Friday. They witnessed a large natural wind slab on very steep northeast-facing terrain at 7500 ft., hanging above a roll-over. They reported light snow with periods of moderate snow and light southwest winds gusting to moderate throughout the day. The higher density snow was forming an inverted upper snowpack (storm slab).
NCMG reported moderate winds near Harts Pass on Friday transporting snow and forming wind slabs that were stubborn to touchy.
Avalanche professionals in the Cascade East-North zone Thursday triggered small loose dry avalanches. This was due to 2-4" of low density snow over the 2/5 crust. Wind slabs were small, isolated, and located close to ridges.One team was able to trigger a small pocket of wind slab 4 inches deep at 8700 ft on a steep north-facing aspect.
Central
NWAC Pro Observer Matt Promomo traveled in the Blewett Pass area where 1-3 inches of widely-distributed light faceting. The recent snow sits on a solid and stable rain-affected and now refrozen snowpack to ground below 5800 ft. Winds created variable snow surfaces above 5500 feet and snow depth was 80-90 cm below 5500 ft and greater than a meter above this elevation.
A recent observation from the Mission Ridge/Lake Clara 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: All elevations.
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
Valid until: Feb 17th, 2018 11:10AM
