Avalanche Forecast
Regions: Cascades - South East.
A mixed bag of conditions is possible in the Olympics and Cascades on Sunday. Along the east slopes there is a high amount of uncertainty regarding the extent and sensitivity of persistent slabs, please read the discussion for details. Note that old wind slab may linger mainly on specific, steep northwest to southeast terrain features near and above treeline.
Detailed Forecast
A weakening front will move over the upper ridge and cause increasing alpine winds on Sunday. It may still be mild or rather warm at higher elevations east of the crest through the morning hours. Then light to moderate rain or snow should spread to the Olympics and northwest Cascades with a cooling trend beginning in all areas by Sunday afternoon.Significant precipitation is not expected to reach the east slopes until the end of the daylight or after the daylight hours.
More information is needed about several potential persistent slab threats, 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. 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. Dig a snow pit in areas with shallow snow and avoid steep slopes with shallow snow and especially those without terrain anchors.
Note that old wind slab may linger Sunday mainly on specific, steep northwest to southeast terrain features near and above treeline.
Snowpack Discussion
Weather and Snowpack
An arctic air mass settled over the Northwest with fair, cold weather from the New Year 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.
An atmospheric river arrived over the Northwest Tuesday 1/17 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 were seen since Monday to Wednesday.
A large upper ridge and warm air mass built over the Northwest Thursday to Saturday. By Saturday temperatures have reached the 40's at Hurricane and at higher elevations especially along the west slopes. Cooler temperatures are seen Saturday at Stevens and Snoqualmie and at lower elevations along the east slopes.
Recent Observations
North
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.
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 faceted grains just below the 1/17 interface. Persistent slab will be re-listed in the northeast zone to deal with this uncertainty.Â
On Thursday 1/26 and again on Friday, 1/27, Jeff Ward reports very good stability while ski testing extensive terrain in the Washington Pass area. Older wind slabs had bonded and stabilized and no triggered avalanches occurred. Small loose-wet avalanches were seen on some very steep solar aspects. Extensive surface hoar was seen in this zone from large (12mm) in the valleys, with small surface hoar extending above treeline as well. If not destroyed by wind or sun, and buried intact by future light snowfall, this may become a layer to watch for.Â
Central
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.
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 feet 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 feet in areas further east of the crest with a shallower snowpack.
Tom Curtis was at Dirty Face above Lake Wenatchee Thursday, 1/26 and found a persistent weak layer of buried surface hoar, consistently buried about 2 feet (60cm). Test results were inconsistent upon multiple tests, some showing it to be reactive and others not. No avalanches were seen on this layer.
Stevens DOT personnel were on Tumwater Mountain on Friday and found a continental-like profile with 77 cm total snow, 20 cm of basal facets, foot penetration to the ground. You will need to avoid steep slopes with shallow snow and especially those without terrain anchors.
South
No recent observations.Â
Avalanche Problems
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
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