Avalanche Forecast
Regions: Cascades - East.
Avoid slopes with shallow snow and without terrain anchors due to uncertainty about persistent slab layers. Possible wind slab formed over the weekend should linger along the east slopes mainly on northwest to southeast slopes near and above treeline.
Detailed Forecast
A weak upper trough should begin to linger over BC and the Northwest on Monday. West winds should generally decrease in the Olympics and Cascades by Monday. The trough will cause decreasing clouds in the Olympics and north Cascades but more clouds and some light rain or snow in the south Cascades mainly at Mt Hood Monday afternoon and night with further cooling.
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. Evaluate snow and terrain 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 to check for weak layers in areas with shallow snow and avoid steep slopes with shallow snow and especially slopes without terrain anchors.
Possible wind slab formed over the weekend should linger along the east slopes mainly on northwest to southeast slopes near and above treeline. But watch for firmer wind transported snow on other aspects.
Snowpack Discussion
Weather and Snowpack
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. The 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 Wenatchee 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.
A large upper ridge and warm air mass built over the Northwest Thursday to Saturday. By Saturday temperatures warmed into the 30s at higher elevations along the east slopes but stayed cold at lower elevations. Moderate west winds were seen at higher elevations.
A front is crossing the Northwest on Sunday afternoon. The front is causing strong southwest winds in most areas with cooling. Rain or snow amounts along the east slopes should be very light if any by Monday morning.
Recent Observations
North
Initial reports from the NCHG Barron Yurt Wednesday 1/25 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.
The NCMG were on north slopes in the Cutthroat drainage on Saturday. They found non-reactive storm layers in the top 20 cm of storm snow. New wind slab and loose wet snow on solar slopes were considered sensative to triggering.
The NCMG were in the Hairpin Valley on Sunday and noted strong W-SW winds along the ridge crests with colder temperatures at lower elevations. Compression tests gave no results on the 1/17 layer at 45-50 cm.
Central
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