Avalanche Forecast
Regions: Snoqualmie Pass.
Significant new snow and strong winds over the last several days continue to warrant patience and conservative terrain use. The snowpack is slowing gaining strength, but caution should be taken before stepping into larger terrain.
Detailed Forecast
Easterly flow will continue to bring cooler temperatures and low elevation clouds to the Passes Sunday.
Newly formed wind slabs will continue to be a problem into Sunday. Look for areas of recently wind transported snow such as fresh cornices, snow drifts, and uneven snow surfaces. Identify and avoid locations where recent wind loading has occurred. Strong ridge top winds may have loaded slopes well below ridgeline.
While we expect wind slabs to primarily exist near and above treeline, keep a watchful eye out for exposed terrain features below treeline where wind slabs may have formed.
Wind slabs can be deceptively difficult to manage in the terrain. Take a moment and read our recent blog post by NWAC Pro Observer Jeremy Allyn on wind slabs.
With the lack of significant warming in the Passes, Storm Slab instabilities will continue to linger into Sunday. While these layers are settling and gaining strength, the shear amount of new snow warrants caution. Avalanches originating within the new snow may entrain additional snow from lower layers. Storm slabs will have the potential to be their largest at higher elevations where significant new snow loads and colder temperatures exist.
The snowpack is quite deep in many locations. Small trees can present a non-avalanche, snow immersion hazard. Keep visual and verbal communication with your travel partners when traveling in treed terrain.
Despite all this new snow, early season hazards still exist at some lower elevation locales and especially around creek beds that are not filled in.
Snowpack Discussion
Saturday afternoon brought a close to a wet and wild storm pattern which began on Thursday. East flow in the passes maintained cooler temperatures, allowing for mostly snow throughout the storm systems. Snoqualmie pass did receive short rain, and freezing rain events Friday during the day, but the passes did not receive the same warm temperatures experienced across much of the west slopes of the Cascades.
One to two feet of snow fell from Thursday to Saturday afternoon. Temperature fluctuations during the storm created several layers within the new snow. Settlement and strengthening of the upper snowpack can be seen in weather station data and has been confirmed by recent field observations.
Moderate winds associated with the end of the storm cycle have redistributed snow in exposed terrain, creating newly formed wind slabs on a variety of aspects.
The upper snowpack is generally right-side-up and gaining strength. Freezing rain and freezing fog crust buried 12/29 and 12/28 respectively exist in the Snoqualmie pass area. The extent of these crusts is not well known.
Observations
Stevens Pass
On Friday, a professional guide in the Stevens backcountry reported a resistance increase within the 15" of storm snow and that storm snow was bonding well with the old snow interface. The layers were not producing significant test results.
Snoqualmie Pass
On Friday, the WSDOT crew at Snoqualmie Pass reported numerous small loose wet avalanches on steep NW-facing terrain and at that elevation there was not much snow to entrain.
On Friday, Alpental Pro Patrol reported evidence of a natural cycle from 1' of storm slab in the early morning hours on Friday. The snow switched to rain and rain was reported to the top of Alpental by late morning.
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
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: Possible
Expected Size: 1 - 1