Avalanche Forecast
Issued: Feb 18th, 2018 1:00PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Persistent Slabs.
NWAC,
Northwest Avalanche Center
Avalanche conditions will not significantly improve Monday. Very cold temperatures will slow new and recent wind slabs from stabilizing. N-NW winds above treeline will transport recent snowfall onto lee slopes, forming fresh wind slabs. Expect wind slabs to be touchy on all aspects. Limited observations suggest you may be able to trigger dangerous Persistent Slab avalanches that are big enough to injure or kill you. Make conservative terrain choices until we know more about these avalanches.
Summary
Detailed Forecast
Avalanche conditions will not significantly improve Monday. Very cold temperatures will slow new and recent wind slabs from stabilizing. N-NW winds above treeline will transport recent snowfall onto lee slopes Monday, forming fresh wind slabs. Due to shifting winds, avoid steep wind loaded terrain especially near and above treeline and on all aspects and expect wind slabs to be touchy. Wind sculpted features, cracks in the snow, pillows, cornices, and variable height of recent snow are all indicators that you could trigger a new or old Wind Slab avalanche.
Limited observations suggest you may be able to trigger dangerous Persistent Slab avalanches that are big enough to injure or kill you. Make conservative terrain choices until we know more about these avalanches.Persistent Slabs can surprise even very experienced travelers and can break over terrain features. Put a wide buffer of terrain between where you travel and any steep slopes. Avalanches may be surprisingly large and run farther than you expect. Make conservative terrain choices until we know more about these avalanches.
Small loose dry avalanches are likely on very steep slopes and could be problematic around terrain traps. This problem will be listed due to larger and more dangerous avalanche problems forecast for Monday.
Snowpack Discussion
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 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
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
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
Valid until: Feb 19th, 2018 1:00PM
