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Olympics

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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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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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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Snowpack Analysis:

Weather and Snowpack 

The first week of March saw a very active winter weather pattern with deep snow accumulations, followed by periods of heavy rain in the second week of March. This caused significant avalanche cycles in most areas March 9-10. Significant snowpack consolidation occurred over this period due to rainfall and warmer temperatures. 

Another strong low pressure system brought several inches of rain to the west slopes of the Olympics and Cascades on Friday, 3/17 through early Saturday morning 3/18. Rapid cooling later Saturday morning was followed by generally light snow showers with little in the way of new snow accumulation. The rain event 3/18-19 has formed a very strong crust layer, now buried by this past weeks storm snow, generally 20-25 inches in the Hurricane Ridge area.

Since Tuesday, daily weather systems crossed the Northwest. In the Hurricane Ridge area, about 2 feet of snow has fallen over the past five days as of Sunday afternoon.

Similar to most of the recent wind patterns, the latest front passing the area Sunday, has had moderate sustained S-SE winds. This has transported available snow to build fresh wind slabs in the Hurricane Ridge area.

Daily early spring warming temperatures has allowed surface snow melt and consolidation, at nearly the same rate as accumulations. With about 2 feet of snowfall received in the past five days, the total snowdepth has increased only about 10 inches. 

Recent Observations

NWAC pro-observer, Matt Schonwald was in the Hurricane Ridge area Thursday afternoon. Moderate S-SE winds were quickly building fresh 10-12" wind slab on lee aspects and scouring windward aspects to the most recent rain crust. Wind slabs near treeline were becoming increasingly sensitive by the end of the day. The new cornice formation was occurring along ridgelines. 

Matt was back in the Hurricane Ridge area on Friday. Hurricane Ridge continues to live up to its name because the wind was again the main story with fresh wind slab becoming deeper and more sensitive on lee slopes which included some westerly slopes near treeline. Fresh and large cornices were also building and deemed likely to fail. All wind loaded avalanche terrain was avoided. Generally shallow, loose wet avalanches occurred below treeline on solar aspects until the cloud cover increased late morning.  

Detailed Forecast for Monday:

Showers and moderate ridgetop winds overnight Sunday and early Monday should maintain unstable wind slabs on lee slopes below ridges, near and especially above treeline. 

Recent winds have been mostly S-SE, so firmer wind slab should be found mainly on W-N-SE slopes near and above treeline. 

Recent cornices are very large. 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.