Avalanche Forecast
Regions: Cascades - North East.
Watch for recent wind slabs formed over the past few days on exposed lee slopes, especially near and above treeline. Storm slabs should improve Monday, but may still be locally sensitive in the Washington Pass zone. Watch for wet snow avalanches on steep solar aspects if the sun comes out in the afternoon
Detailed Forecast
Mostly cloudy early Monday with a chance of a few light snow showers, light winds with cool temperatures. Clouds should decrease becoming partly cloudy late Monday.
These conditions should allow for a gradual decrease in avalanche danger as recent wind and storm slabs begin to settle and stabilize.Â
Allow existing storm and recent wind slabs time to continue stabilizing on Monday. Wind slabs are most likely on NE-W-SW aspects due to recent strong E-SE transport winds. Above treeline, look for wind slabs on a wide range of aspects due to more varied wind directions. Watch for recent wind effects near and below ridgelines and avoid steep slopes with obvious signs of recent wind loading.Â
Storm slabs may still be locally sensitive in the Washington Pass zone 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. 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 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.  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
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.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 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.Â
Jeff Ward was back in the Hairpin Valley of Washington Pass Saturday and in specific terrain found reactive tests (PST) on buried surface hoar just under the thin 1/17 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.Â
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. 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