Avalanche Forecast
Regions: Cascades - North East.
There is a high amount of uncertainty regarding the extent and sensitivity of persistent slabs in the northeast and central-east zones, please read the discussion for details. Other avalanche problems will consist of a mix of lingering wind slab near and above treeline and loose wet avalanches on solar slopes during extended periods of afternoon sunshine.
Detailed Forecast
Generally cloudy weather with light winds should be seen Thursday morning with at least partial clearing expected in the afternoon. A slow warming trend begins Thursday. Â
These conditions will continue to help recent wind slabs settle and further stabilize. Lingering wind slabs are most likely on SW-NW-NE aspects due to recent strong E-SE transport winds. Watch for recent wind effects near and below ridgelines and avoid steep slopes with obvious signs of recent wind loading.Â
Watch for loose wet snow conditions on steep solar slopes if the sun pops out for an extended period of time on Thursday. Loose wet avalanches may become large in isolated areas if they entrain most of the recent storm snow.Â
More information is needed about the persistent slab threat particularly in the northeast and central-east zones. Due to the lack of direct avalanche activity on any PWL, the avalanche danger will be maintained at Moderate. Please take the travel advice to heart, evaluating snow and terrain your traveling through carefully, erring on the side of caution if you are unsure about the snowpack structure in your area and realizing large avalanches are possible in isolated areas under Moderate danger. Â
Snowpack Discussion
Weather and Snowpack
An arctic air mass settled over the Northwest with fair, cold weather from the New Year through the second week of January. Surface hoar and near surface faceted crystals formed in wind and sun-sheltered areas during this period. Observations are beginning to paint a picture that these weak persistent grain types were buried intact in portions of the central-east and northeast zones.Â
An atmospheric river moved over the Northwest 1/17-1/18 causing heavy snow in the northeast zone with about 2 feet of snow accumulating at Washington Pass. 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 during this stretch.  Â
With the arctic air mass finally displaced and cool easterly flow abating Thursday 1/19, many lower elevation sites east of the crest warmed to near freezing. Showers Saturday and Sunday deposited about 2-5 inches of new snow with local strong S-SE winds observed during this period.Â
Fair weather with light winds and cool temperatures have been seen Monday through Wednesday.
Recent Observations
On Thursday, 1/19 the Mission Ridge pro-patrol produced 8-10 foot hard slab avalanches with hand charges that released down to the basal facets formed earlier this winter. These very large avalanches were on very specific wind loaded NE slopes just below ridgelines.
Observations on Friday 1/20 from the NCMG found some faceting below a melt-freeze crust around Delancey Ridge that gave a few sudden collapses during trail breaking at 4000 feet. No direct avalanche activity was observed down to this interface. Storm snow totaled 45-55 cm in this area.Â
NWAC pro-observer Jeff Ward was in the Hairpin Valley of Washington Pass Saturday 1/21 and in specific terrain found reactive tests (PST) on buried surface hoar just under the thin 1/17 crust, buried 65 cm (2 feet) 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. Jeff provided this video.
NWAC observer Tom Curtis was in the Blewett Pass/Diamond Head area Saturday 1/21 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.
In Icicle Creek at 4500 ft on Saturday 1/21, a Stevens pro-patroller reported 20 cm of storm snow over buried surface hoar/faceted snow producing collapsing and shooting cracks. This corresponds with local Methow reports of instability related to a similar snowpack structure roughly between 4-5000 ft in areas further east of the crest with a shallower snowpack.  Â
The NCMG were out on Delancey Ridge and Vasiliki Ridge on Sunday 1/22 and reported wind transport at 6000 feet. Numerous ski tests gave no results on Delancey. A storm shear was seen at about 25 cm and no results were seen at a 1/17 interface. On Vasiliki ski tests gave very local results on the 1/17 interface. The NCMG were out again on Monday on Delancey Ridge and saw no avalanche results while skiing. Compression tests on a SE slope at 6000 feet indicated a hard resistant planar shear at 45 cm below the 1/17 interface so some of the surface hoar or faceted snow from the 2nd week of January may still be present.
Initial reports from the NCH Barron Yurt Wednesday indicate no direct signs of instability but did indicate unstable results in snowpack tests, presumably around the buried surface hoar layer just under the 1/17 interface. Persistent slab will be listed in all of the east slope zones to deal with this uncertainty.Â
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: Possible
Expected Size: 1 - 1
Loose Wet
Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
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.
Aspects: South East, South, South West, West.
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 - 1