Avalanche Forecast
Regions: Cascades - South East.
A pattern change will begin to take place over the Northwest on Friday. You will need to pay lots of attention to conditions and forecasts during the transition Friday and Saturday if you decide to venture into the back country.
Detailed Forecast
A pattern change will begin to take place over the Northwest on Friday. Dry weather and strong cold E-NE winds seen Wednesday and Thursday will gradually get replaced by increasing southwest flow aloft, increasing rain or snow and milder temperatures Friday and Saturday. You will need to pay lots of attention to conditions and forecasts during the transition Friday and Saturday if you decide to venture into the back country.
Wind slab formed by strong E-NE winds on Wednesday and Thursday will linger on Friday. The primary aspects for these wind slabs should be any W facing slopes but could be found on other aspects.
New sensitive wind slab is likely to begin forming as shower bands of snow begin to move across the Olympics and Cascades on Friday with a slight warming trend. The primary aspects for these wind slabs should be NW to SE facing slopes.
Wind slab may be more touchy in areas where poorly bonded poorly to an underlying crust or firm old layer. Watch for firmer wind transported snow in all elevation bands especially in open terrain in the upper elevations of the below treeline band. All aspects are listed for wind slabs to account for the strength of recent winds and for any lingering older wind slabs formed over the weekend. Â
New storm slab is also likely to begin forming on Friday on more sheltered slopes 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.
More information is needed about several deeper potential persistent slab threats, particularly in the northeast and central-east zones. 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.
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. The 1/17 crust is very thin in the northeast zone and most notable in areas further south.  Â
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. Moderate west winds were seen at higher elevations.
A front crossed the Northwest Sunday afternoon, causing continued west-southwest winds at higher elevations along with slight cooling and little to no measurable precipitation. Light snow fell Monday night through Tuesday morning with generally 2-6 inches recorded along the east slopes, with the most at the Berne Camp east of Stevens. An observation from Mt. Cashmere Tuesday reported locally up to 10 inches of low density snow between 4000-5700 feet.
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 last week 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.
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: 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