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West Slopes North - Canadian Border to Skagit River

Issued: 6:08 PM PST Monday, December 11, 2017
by Robert Hahn

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.

Large, but isolated avalanches are possible in the Mt Baker area on steep, rock faces. These avalanches are difficult to predict and best managed by avoiding terrain below glide cracksWatch for shallow wet surface snow conditions on some steep sun exposed slopes, especially below rocks or cliffs and above terrain traps. This weather provides the perfect opportunity to check out surface snow conditions.

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Avalanche Problems for Tuesday

Glidei

Glide avalanches occur when water lubricates the interface between the snowpack and the ground. These avalanches are difficult to predict and best managed by avoiding terrain below glide cracks.

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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:

High pressure has now been over the region for over a full week. Sunshine and very mild temperatures have been the story with cooler temperatures at lower elevations and in the passes. This weather has allowed for overall strengthening and consolidation of an already strong snowpack.

No human-triggered avalanches have been reported since Tuesday.

Some small loose wet slides have occurred on mainly steep sun exposed slopes through the week, but have become much less frequent over the past several days.

The total snowpack height in these zones varies mostly from 3-5 feet, with the most over the volcanic peaks.

The major story has been the persistent and anomalously warm temperatures. At Mt. Baker, we've had temperatures near or above 50F for the last five days, both day and night! Normally we don't have to talk about glide/climax releases during high pressures in December, but this high pressure is so persistent and warm that the snow is now draining water to the ground, particularly on solar aspects. Here is a plot of temperature over the past week at Mt. Baker.

Strong crest level southerly wind Friday night to midday Saturday may have built some wind slab layers in isolated terrain. However, moist surface snow on windward slopes likely limited available snow for transport. Watch for isolated recent wind slabs on shaded slopes in the north WA Cascades where the winds were strongest.

The surface snow consists mainly of surface melt freeze crusts forming overnight on solar aspects, On shaded aspects, settled storm snow is still providing some nice skiing and riding conditions.  

The snowpack is well consolidated and strong with settled old storm snow of about 1.5-2.5 feet over the strong Thanksgiving rain crust. 

During this high pressure pattern, surface hoar and or surface facets have been developed in some areas and not in others. At some lower elevations under the strong inversion, very large surface hoar has been seen. Less surface hoar growth has been reported in typical start zone terrain however. These potential weak surface snow conditions will be important when the snowfall returns, as they may have an impact on future avalanche conditions. 

Observations

Don't forget to check the numerous recent high quality observations posted to NWAC.

North

On Monday, Pro patrol at Mt Baker Ski area reported two climax/glide avalanches off rocks during the morning hours. The slides were in typical locations on an east and a south face, both were about 30 feet across and 3 feet deep, with debris piles 4-5 feet deep. No other avalanche activity was observed.

NWAC observer, Lee Lazzara travelled extensively Sunday, December 10th in the Swamp Creek / Winchester area from 3000-6100 feet. While the temperatures were quite warm and melting evident, surprisingly little current or previous loose wet avalanche activity was noted, mainly tree bombs. No avalanche activity was noted with variable but good ski quality in well settled storm snow, well bonded to the T-Day crust. Very little to no surface hoar or near surface facets were seen, other than in non-avalanche terrain near valley bottoms and creeks. 

Central

Professional Observer Dallas Glass skied non-solar slopes in the Alpental valley on Monday. He observed a very strong temperature inversion up to 4800' with cold, dry snow below the inversion. He found moderate easterly winds near treeline at 5200’. No snow was being transported. He found no avalanche problems that affected his travel. He noted the biggest hazard was low snow cover. Surface snow conditions were extremely variable.

Numerous recent reports over the past few days indicate creamy settled storm snow still providing nice skiing and riding conditions on sun sheltered aspects.

South

Friday the Crystal pro-patrol reported only a few roller balls on steep solar slopes that were not entraining snow, with some previously scoured areas getting melted to the ground.

NWAC staff avalanche training at Mt Rainier Thursday reported an overall strong snowpack on westerly aspects of Mazama Ridge near Paradise. At 5400 feet the height of snow was about 4 feet with the Thanksgiving crust well bonded about 2 feet below the surface. Settled storm snow with few remaining storm layers provided nice conditions on non-solar aspects. Previous wind effected snow was visually evident on exposed slopes near and above treeline but not visited.

Detailed Forecast for Tuesday:

Recent days have featured mostly sunny and warm with light winds at higher elevations and cold and cloudy conditions in the lower passes and some low elevations. More of the same conditions are expected Tuesday, with slightly lower temperatures aloft weakening the inversion.

At the higher terrain Monday, temperatures reached the upper 50's and are expected to drop into the 40's on Tuesday.

Large, but isolated avalanches are releasing to ground in the Mt Baker area on steep, rock faces. These avalanches are difficult to predict and best managed by avoiding terrain below glide cracks. Isolated slabs above lubricated rock faces may continue to release in spite of slightly cooler temperatures Tuesday.

With the warm temperatures, isolated wind slabs formed Friday and Saturday morning should have bonded and settled to the point that we are removing this as an avalanche problem, Tuesday.

Surface melt-freeze crusts are forming overnight on some solar exposed slopes and softening through the day. Loose wet avalanches are unlikely, but might be encountered in isolated steep solar exposed slopes below rocks or trees absorbing more radiation.  

The nice riding and skiing conditions on non-solar aspects found Sunday should remain on Tuesday as little change in the snowpack structure should have occurred.

Early season terrain hazards still exist, such as poorly covered rocks, vegetation and creeks, particularly at lower elevations.

No Corresponding Mountain Weather Forecast Available