Avalanche Forecast
Regions: Cascades - South East.
A period of moderate E-SE winds Friday may build new wind slab near and above treeline on westerly aspects, especially in places with lower density snow available for transport. Storm slabs may still be locally sensitive in the Washington Pass zone Friday and in areas further south where new snow has bonded poorly to an underlying freezing rain crust. We are still getting a handle on the snowpack post storm, so be prepared to evaluate the snowpack carefully in your local area.
Detailed Forecast
Light snow showers on Thursday should taper down overnight. A frontal band rotating up from the south Friday should bring generally light but persistent upslope snow showers mainly for the south and central-east Cascades with light accumulations possible. Out ahead of this frontal band, there should be a period of moderate E-SE winds.Â
A period of moderate E-SE winds Friday may build new wind slab near and above treeline on westerly aspects, especially in places with lower density snow available for transport. All aspects are indicated to cover older wind slab on more easterly aspects. Look for recent wind effects near and below ridgelines.
Storm slabs may still be locally sensitive in the Washington Pass zone Friday 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.
We are still getting a handle on the snowpack post storm, so be prepared to evaluate the snowpack carefully in your local area.Â
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 possibility of low likelihood/high consequence persistent slab avalanches in this area. 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 around 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 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.Â
Recent Observations
An observation via the NWAC observation page from Saturday 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 has been observed involving these layers until today, Thursday 1/19. Mission Ridge ski patrol produced 8-10 foot(!) hard wind slab avalanches 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" of 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
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
Persistent Slabs
Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
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.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Unlikely
Expected Size: 1 - 2