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East Slopes North - Canadian Border to Lake Chelan

Issued: 6:00 PM PST Sunday, March 26, 2017
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.

Fresh wind slabs and cornices will be the main problem Monday. Loose-wet avalanches will become likely on steeper sun exposed slopes during extended sun breaks Monday. Keep terrain selection simple and conservative. Cornices have recently proven dangerous and unpredictable and capable of triggering very large avalanches. 

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Avalanche Problems for Monday

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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Loose Weti

Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.

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Cornicesi

Cornices are easy to identify and are confined to lee and cross-loaded ridges, sub-ridges, and sharp convexities. They are easiest to trigger during periods of rapid growth (new snow and wind), rapid warming, and during rain-on-snow events. Cornices often catch people by surprise when they break farther back onto flatter areas than expected.

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Snowpack Analysis:

Special Note: For more information on the massive natural cornice triggered avalanche on the north side of Ruby Mountain on Sunday 3/19 and general thoughts about low-likelihood/high consequence avalanches, please see NWAC's blog post issued Sunday, March 26. 

Weather and Snowpack

The first week or so of March was very cool and snowy. NWAC and NRCS stations indicate about 2 feet of snow in the northeast Cascades with less elsewhere along the Cascade east slopes. 

The 2nd week of March was equally active with non-stop Pacific frontal systems pummeling the PNW. Unfortunately, these systems delivered more rain than snow to lower/mid elevations along the east slopes of the Cascades. At least two regional avalanche cycles occurred during the stretch. The most recent cycle from mid-March had many massive slides that covered Hwy 20 near Washington Pass up to 40' deep in places! Significant snowpack consolidation occurred over this period due to rainfall and warmer temperatures. Far less precipitation was seen further east of the crest during this period for areas like Blewett/Mission Ridge. 

A strong low pressure system brought 6-12 inches of snow in the northeast and central-east slopes of the Cascades Friday, 3/17 and mostly rain for the southeast Cascades. This was followed by snow levels rising to 6000-6500 feet in the northeast and central-east Cascades and likely 7000 ft in the southeast Cascades by Saturday morning 3/18. 

During this past week, weaker fronts crossed the Northwest from Tuesday through Saturday with light amounts of new snow along the east slopes of the Cascades, mainly near and above treeline and near the Cascade crest. The Barron Yurt near Hart's Pass reported about 20 cm (8 inches) of recent storm snow as of Saturday afternoon. About 6 inches of storm snow had accumulated in the Mission Ridge area.   

Recent Observations

North

Guides near Washington Pass Tuesday, 3/21 reported hearing many natural avalanches near midday following a prolonged sunbreak and subsequent brief warm up. Numerous avalanches were likely small, however, some avalanches sounded much larger, likely involving deeper layers below the relatively shallow recent storm snow.  

NCH reported no significant new avalanche activity observed outside of one small natural storm slab on a north aspect above treeline Thursday.

Reports from the Barron Yurt on Friday and again Saturday indicate the recent wind transport was mainly shallow but evident near ridges. Recent SW winds were building shallow wind slabs as of Saturday and were avoided. There were a few small natural avalanches that occurred on very steep shaded terrain, mainly above 40 degree's. These were D1-D1.5 in size and unknown whether they loose-dry or shallow wind slabs. No triggered slides were observed Saturday. 

Central

On Friday, Mission Ridge pro-patrol reported very sensitive storm and/or wind slab up to 6-8 inches deep on lee aspects near and above treeline. NW aspects were especially sensitive.

NWAC pro-observer Tom Curtis was on the lower slopes of Mt. Cashmere Friday below treeline. About 5 cm (2 in.) of new snow had accumulated in this area. On steep test slopes, Tom easily triggered loose wet avalanches entraining moist underlying snow with the potential to become large. Although not directly observed, Tom heard several large natural avalanches release up the Trout Cr drainage along ridge-crest in the morning. It is possible these avalanches were cornice triggered.  

South

No recent observations. 

Detailed Forecast for Monday:

Rain and snow late Sunday should change to showers with cooling overnight. Showers and moderate ridgetop winds should persist early Monday before tapering late Monday. This should maintain unstable wind slabs on lee slopes below ridges, in exposed terrain near and especially above treeline. 

Recent winds have been mostly SW-S-SE, so firmer wind slab should be found mainly on W-N-SE slopes near and above treeline. However, due to terrain effects, wind slabs may have formed on a variety of aspects 

Recent cornices are very large in many areas. Make sure to avoid areas on ridges where there may be an overhanging cornice as well as travel on steep slopes below cornices! Natural cornice releases and resulting slab avalanches are unpredictable. See a blog post regarding cornices here.