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East Slopes South - South of I-90 to Columbia River

Issued: 7:37 PM PST Wednesday, February 21, 2018
by Kenny Kramer

NWAC avalanche forecasts apply to backcountry avalanche terrain in the Olympics, Washington Cascades and Mt Hood area. These forecasts do not apply to developed ski areas, avalanche terrain affecting highways and higher terrain on the volcanic peaks above the Cascade crest level. 

While it is still possible to trigger a large avalanche in larger terrain, there are still excellent travel conditions in more protected lower angle terrain of lower consequence. The ongoing Persistent Slabs problem is uncommon and with a high degree of variability still requires terrain choices with a wide margin of safety. 

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Avalanche Problems for Thursday

Persistent Slabi

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.

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Wind Slabi

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.

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Forecast for Thursday:

Cold temperatures with light snow showers at times and generally light winds are expected Thursday. This weather should do little to change the current avalanche danger.

The very cold temperatures are helping preserve the weak interface, where present, at the buried crust layer from 2/5 or 2/13, maintaining the possibility of triggering a large avalanche in open big terrain steeper than about 35 degrees.  This layer varies throughout the terrain but ranges in general about  1.5-2.5 feet below the surface. This uncommon avalanche problem in our snow climate requires a step back in terrain choices. Under these conditions, be prepared not to see avalanches but this doesn't mean there is no inherent danger. 

The good news is that great snow conditions can be accessed in smaller more protected, lower angled terrain with lower exposure risk. 

Shallow fresh wind slabs may be found near and above treeline so watch for wind stiffened or wind sculpted surface snow.

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.

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

Avalanche Summary:

Cold temperatures and light snow showers since Sunday have not significantly changed the overall danger since Sunday. Since Sunday most areas have received 2-4 inches of cold low density snow. In most sheltered terrain this surface snow is low cohesion and lacks the ability to propagate a slab avalanche. 

The low cohesion snow is very susceptible to wind transport and there have been some reports of fresh but shallow wind slabs forming in exposed terrain near or above treeline.   

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. From many recent observations, the distribution of the Persistent slab problem along the east slopes of the Cascades is inconsistent with some areas lacking the weak grain type and others lacking an overlying slab. 

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.

Observations

North

In terrain near Harts Pass Wednesday, a ski cut produced a size 1.5 slab avalanche of 2 ft on a NE aspect at about 6000 ft. The avalanche released on a persistent weak layer above a crust that has been seen through the region since the latest storm cycle.

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 field staff traveled in the Icicle Creek area Wednesday up to about 6000 feet on mainly NE exposures. No avalanches were triggered or seen. The persistent layer we are tracking was found consistently buried about 2 ft from the surface, though the distribution of this layer was intermittent. In many areas observed the snow above the facet/crust layer lacked slab properties and was low cohesion powder, lacking the ability to propagate a slab avalanche. Shallow fresh wind slabs were noted near and above treeline. 

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. 

No Corresponding Mountain Weather Forecast Available

USE AT YOUR OWN RISK

This Backcountry Avalanche Forecast is provided in conjunction with the US Forest Service, and is intended for personal and recreational purposes only. Safe backcountry travel requires preparation and planning, and this information may be used for planning purposes but does not provide all the information necessary for backcountry travel. Advanced avalanche education is strongly encouraged.

The user acknowledges that it is impossible to accurately predict natural events such as avalanches in every instance, and the accuracy or reliability of the data provided here is not guaranteed in any way. This forecast describes general avalanche conditions and local variations will always occur. This forecast expires 24 hours after the posted time unless noted otherwise.