Avalanche Forecast
Regions: Cascades - North East.
Allow existing storm and wind slab to continue stabilizing on Saturday. Wind slabs are most likely on N-W-SW aspects due to recent E-SE transport winds but will be listed on all aspects due to variable loading patterns during the last few days. Storm slabs may still be locally sensitive in the Washington Pass zone Saturday and in areas further south where new snow has bonded poorly to an underlying freezing rain crust.
Detailed Forecast
Light upslope snow showers should taper off Saturday morning before increasing once again for the south and central-east zone in the afternoon. Cool snow levels and generally light SE winds should not dramatically alter the avalanche danger on Saturday.Â
Allow existing storm and wind slab time to continue stabilizing on Saturday. Wind slabs are most likely on N-W-SW aspects due to recent E-SE transport winds but will be listed on all aspects due to variable loading patterns during the last few days. Look for recent wind effects near and below ridgelines and treat wind loaded slopes with caution.
Storm slabs may still be locally sensitive in the Washington Pass zone Saturday 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/Blewett 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. Â
Snowpack Discussion
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 determine to 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 probably 18"+). 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 L. Wentachee saw about 6 inches. Rain was likely seen up to about 5-6000 feet in the southeast zone.  Â
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.Â
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.Â
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. Â
Recent observations on Friday from the NCMG found some faceting below a melt-freeze crust around on Delaney 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.Â
Mission Ridge Ski Patrol 1-19-17
Avalanche Problems
Wind Slabs
Release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
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.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 1
Storm Slabs
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
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.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 1 - 1