Avalanche Forecast
Regions: Cascades - North East.
Natural or triggered wind and storm slab avalanches should be very likely in the above treeline along most of the east slopes on Saturday. Travel in back country avalanche terrain above tree line is not recommended along the east slopes on Saturday. Careful snowpack evaluation, cautious routefinding and conservative decision making will be essential in other areas.
Detailed Forecast
A very important pattern change is taking place over the Northwest Friday and Saturday. The dry weather and strong cold E-NE winds seen Wednesday and Thursday are being replaced by increasing strong southwest flow aloft, increasing snow and warmer temperatures Friday and Saturday.
Saturday over the east slopes should be a day of strong southwest alpine winds with light to moderate snow along the east slopes and warming temperatures. Along the east slopes at higher elevations from Friday to the end of the day on Saturday there should be about 6-12 inches of warmer, denser new snow.
This will generally build new upside down wind and storm slab layers. This new snow may build over weak or faceted snow from the cold weather mid-week on some or many slopes.
New sensitive wind slab is very likely on NW to SE slopes.
Wind slab formed by strong E-NE winds on Wednesday and Thursday may linger on Saturday. The primary aspects for these wind slabs should be W facing slopes.
New sensitive storm slab will continue to build on sheltered slopes on Saturday in areas that see more than a few inches of rapidly accumulating snow. The warming trend will help build upside down layers and promote storm slab formation.
It looks like there may be a bit less snow in the northeast zone on Saturday and the avalanche danger may be a little less there compared to the central east or southeast zones. But this is uncertain and for now one forecast will be used for the east slopes for Saturday.
Natural or triggered wind and storm slab avalanches are likely mainly in the above treeline along the east slopes on Saturday. Travel in back country avalanche terrain is not recommended above treeline along the east slopes on Saturday.Â
Snowpack Discussion
Weather and Snowpack
An atmospheric river arrived over the Northwest 1/17 causing heavy snow in the northeast zone, a mix of snow, sleet and freezing rain in the central-east zone and also in some lower elevation areas east of the crest. Rain was likely seen up to about 5-6000 feet in the southeast zone during this stretch.
A large upper ridge and warm air mass built over the Northwest last Thursday to Saturday 1/26-1/28. By Saturday temperatures warmed into the 30s at higher elevations along the east slopes, but stayed cold at lower elevations.
Very strong NE-E winds have been seen Wednesday and Thursday with very cold temperatures. Fresh, touchy wind slabs formed with large plumes of snow visibly transporting loose snow from exposed terrain.Â
Recent Observations
North
Reports from the NCH Barron Yurt the week of 1/23 indicated no direct signs of instability, but avalanche professionals did find unstable results in snowpack tests around faceted grains just below the 1/17 interface in isolated areas on cold northerly aspects. Persistent slab will be re-listed in the northeast zone to deal with this uncertainty.Â
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.
NWAC pro-observer Jeff Ward was in the Washington Pass area on Friday and found 10 cm of new snow in the near and below treeline and no signs of instability though observations were limited.
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 column tests. No avalanches were seen on this layer.
Stevens DOT personnel were on Tumwater Mountain on Friday 1/27 and found a continental-like profile with 77 cm total snow, 20 cm of basal facets, foot penetration to the ground.Â
Jeff Ward was at Cannon Mountain off of Icicle Creek Monday. He found a variable and shallow snowpack that ranged from 1 to 2 m, depending on elevation and wind affect. Many windward slopes had been scoured to the 1/17 crust with thin wind slab present on lee slopes. The 1/17 crust was down 20-40 cm but no avalanche activity was observed on this layer. Large surface hoar was found at all elevations in non-wind affected terrain.
Both Tom Curtis and Jeff Ward travelled independently in terrain east of Stevens Pass Wednesday 2/1, covering the areas of Rock Mountain, Jove and Union Peaks. Both reported rapidly forming, very touchy wind slabs by mid-morning Wednesday. Plumes of wind transported snow were seen along all ridges and exposed terrain Wednesday. Wind slabs were building much further downslope due to the strength of the winds. Wind slabs up to 12 inches were seen and noted forming well below treeline. Any small feature or convexity would produce shooting cracks or release small wind slabs, even on relatively shallow angled terrain. Both witnessed natural wind slab releases Wednesday and both avoided travel on any steep terrain capable of avalanches.Â
Active wind transport and fresh sensitive wind slabs on south slopes of Rock Mountain, east of Stevens Pass, Wednesday 2/1. Photo: Jeff Ward
South -Â No observationsÂ
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: Very 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: Likely
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