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East Slopes North - Canadian Border to Lake Chelan

Issued: 6:00 PM PST Saturday, January 21, 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.

Watch for new wind slabs forming early Sunday on exposed lee slopes at higher elevations. Allow recent storm and wind slabs further time to stabilize Sunday. Wind slabs are most likely on N-W-SW aspects. Storm slabs should continue to slowly stabilize Sunday, but may still be locally sensitive in the Washington Pass zone where new snow has bonded poorly to an underlying freezing rain crust. 

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Avalanche Problems for Sunday

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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Storm Slabsi

Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.

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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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Snowpack Analysis:

Weather and Snowpack

An arctic air mass was over the Northwest with fair, cold weather last week. Surface hoar and near surface faceted crystals formed in wind and sun-sheltered areas during this period. Observations over the next few days should help to determine what extent any of these weak persistent grain types might have survived the atmospheric river event and remain relevant to the forecast moving forward.

An atmospheric river moved over the Northwest Tuesday and Wednesday, causing heavy snow in the northeast zone (Washington Pass storm totals were observed to be 24"+). A mix of snow, sleet and freezing rain was likely seen in the central-east zone and also in some lower elevation areas east of the crest. Lyman Lake Snotel, Holden Village and Berne Camp east of Stevens saw about a foot or more of new snow while Mission Ridge and Lake Wentachee saw about 6 inches. Rain was likely seen up to about 5-6000 feet in the southeast zone.   

With the arctic air mass finally displaced and cool easterly flow abating Thursday, many lower elevation sites east of the crest warmed to near freezing. Light upslope snow showers may have produced another inch or two along the east slopes Friday. Winds at Mission Ridge summit were light to moderate out of the E-NE. 

A very pleasant day Saturday allowed temperatures to moderate to near the freezing mark in many areas. Light snow showers began to spread from south to north late Saturday afternoon.

Recent Observations

An observation via the NWAC observation page from last Saturday 1/14 continued to identify weak persistent grains near the base of the relatively shallow snowpack in the Mission Ridge area. While these layers were reactive in column tests, no recent avalanche activity had been observed involving these layers until Thursday, 1/19. Mission Ridge ski patrol produced 8-10 foot(!) hard wind slab avalanches (see photo below) that released down to the basal facets formed earlier this winter with hand charges. These very large avalanches were on very specific wind loaded slopes, NE aspects, just below ridgelines. Elsewhere, 3" storm slabs could be ski triggered on steeper slopes, but were only capable of producing small avalanches. 

NWAC observer Jeff Ward was out at Sandy Butte in the NE zone (outside of Mazama) Wednesday traveling up to 6000'.  The main avalanche problem was small loose dry sluffs with about 20 cm of new snow through Wednesday afternoon.  

Recent observations on Friday from the NCMG found some faceting below a melt-freeze crust around on Delancey Ridge that gave a few sudden collapses during trail breaking at 4000 ft. The melt-freeze crust became thicker, higher in elevation. No direct avalanche activity was observed down to this interface. Storm snow totaled 45-55 cm in this area. 

Jeff Ward was back in the Hairpin areas of Washington Pass Saturday and in specific terrain found reactive tests (PST) on buried surface hoar over the recent crust, buried 65 cm (2 ft) in that area. It is unsure how widespread the buried surface layer is in this zone, so proceed with caution, especially lower elevation and near valley bottom slopes. 

NWAC observer Tom Curtis was in the Blewett Pass/Diamond Head area Saturday to assess post storm conditions. The major finding relates to the well documented 10-15 cm of basal facets in that region. Extensive observations in this region determined that significant rounding has occurred with these grains and no facets above the ground remain. The rounding of the basal facets may be limited to this specific terrain, so tests in other areas are encouraged, especially when travelling in terrain of consequence.  

Mission Ridge Ski Patrol 1-19-17

Detailed Forecast for Sunday:

A period of light snow showers is expected Sunday with a period of moderate to strong S-SE crest level winds early Sunday. The winds early Sunday may move available surface snow and build some new shallow wind slab layers, mainly below exposed ridges.  

Allow existing storm and wind slab time to continue stabilizing on Sunday. Wind slabs are most likely on N-W-SW aspects due to recent and expected E-SE transport winds, but will be listed on all aspects due to variable loading patterns over the past several days. Look for recent wind effects near and below ridgelines and treat wind loaded slopes with caution.  The considerable avalanche danger is forecast above treeline to highlight the potential for increased sensitivity and size of wind slabs.

Storm slabs may still be locally sensitive in the Washington Pass zone Sunday and in areas further south where new snow has bonded poorly to an underlying freezing rain crust. Give these layers time to settle and approach steeper slopes with caution.

The 12/17 PWL and other persistent weak layers closer to the surface have been unreactive lately in the northeast zone and persistent slabs have been removed from that zone. However, the basal facets in the Mission Ridge area continue to occasionally produce alarming results either in snowpack tests, or more recently with very large hard slab avalanches released during control work at Mission Ridge on specific wind loaded slopes. Continue to think about the possibility of low likelihood/high consequence persistent slab avalanches in this area when choosing terrain.  

Observations