Avalanche Forecast
Issued: Feb 20th, 2018 11:27AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Persistent Slabs and Wind Slabs.
NWAC,
Northwest Avalanche Center
You can still trigger dangerous Persistent Slab avalanches that are big enough to kill you on slopes steeper than about 35 degrees. A combination of Wind Slabs, uncommon Persistent Slabs and a high degree of variability over short distances require terrain choices with a wide margin of safety. Avoid big terrain, which could produce large avalanches.
Summary
Detailed Forecast
Continued cold temperatures with light winds Wednesday will not significantly improve the current avalanche danger, allowing recent slabs and the known weak persistent layer to persist longer than usual.
Even a small triggered Loose-Dry avalanche or a small Wind Slab may be enough to trigger a Persistent Slab. Snowpack observations and one serious skier triggered avalanche from along the west slopes of the Cascades Sunday show you can still trigger dangerous Persistent Slab avalanches that are big enough to kill you on slopes steeper than about 35 degrees. Persistent Slabs can surprise even very experienced travelers and can break over terrain features. Avoid large terrain, especially areas exposed to avalanches that could come from above. Avalanches may be surprisingly large and run farther than you expect. Make conservative terrain choices until we know more about these avalanches. A small triggered Loose-Dry avalanche or a small Wind Slab may be enough to trigger a Persistent Slab avalanche.
The persistent slab problem is likely more sensitive on shaded, non-solar aspects, but can not be ruled out on any aspect.
The combination of Wind Slabs and the more uncommon Persistent Slabs require terrain choices with additional safety margins.
Snowpack Discussion
Winds abated Monday but cold temperatures have persisted through Tuesday, slowing the stabilization process of recent slabs.
Sunday saw light snow showers and much colder air move in along the east slopes of the Cascades. Moderate N-NE winds transported recent snowfall in the Washington Pass area and at Mission Ridge. Persistent slab avalanches were skier triggered near Harts Pass on small test slopes and identified in snowpack tests as likely to propagate. The distribution of the Persistent slab problem along the east slopes of the Cascades is unknown at this time.
On Saturday, from the Hwy 2 corridor to the northeast Cascades up to 8 to 15 inches of snow fell. The snow level rose up to at least 4000' for the central-east zone Saturday afternoon; Blewett Pass mixed with rain while Mazama stayed all snow. Dirty Face and Mission Ridge stations reported very strong winds and although the Washington Pass station winds were not very strong, strong gusts were observed by professionals near Hart's Pass Saturday afternoon.
In the Washington Pass area, approximately 14-25 inches of snow sit on the 2/5 crust. The crust likely extends up to around 6800’. In some areas, the 2/5 crust sits over weaker snow (facets).
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 tracking after Saturday's storm. Snow profiles and snowpack tests are the best means to confirm the presence of this layer.
Observations
North
NCH professionals in the Cutthroat Creek drainage Tuesday found the persistent slab in snowpack tests on ESE aspects mainly near and below treeline. The shallow weak faceted snow over the 2/5 crust was found consistently about 2 ft below the surface.
NCH professionals in the Hart's Pass area Sunday found nearly 2 feet of snow failing on facets buried above the 2/5 crust. Snowpack tests and skier triggered avalanches on small test slopes indicated the potential for Persistent Slab avalanches in this zone. Wind slabs were sensitive to human triggering below ridges.
A professional in the Hart's Pass area found deteriorating avalanche conditions Saturday as human and skier triggered slabs within the storm snow were reported by the afternoon. Winds became very strong and gusty in the afternoon along with the heavier snowfall.
Central
NWAC Pro Observer Matt Promomo traveled in the Blewett Pass area Thursday. Matt found 1-3 inches of weak snow above the early February crust that had become faceted. No deeper persistent weak layers were identified in the lower snowpack. Winds had created variable snow surfaces above 5500 feet and snow depths averaged about 3 feet.
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: All aspects.
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: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Feb 21st, 2018 11:27AM
