Avalanche Forecast
Regions: Cascades - West.
As we enter a storm cycle Thursday, human triggered avalanches will become increasingly likely as denser storm snow builds over weaker snow during the day. Watch for changing conditions, travel conservatively and rein in your plans if conditions deteriorate faster than expected. The avalanche danger is expected to further increase Thursday evening so plan your exit accordingly.
Detailed Forecast
As we enter a storm cycle Thursday, human triggered avalanches will become increasingly likely as denser storm snow builds over weaker snow during the day. Storm slabs will likely become touchy by the end of the day. Watch for changing conditions, travel conservatively and rein in your plans if conditions deteriorate faster than expected. The avalanche danger is expected to further increase Thursday evening so plan your exit accordingly. See the mountain weather tab for weather forecast details.Â
Watch for building wind slab on a variety of aspects Thursday, especially near and above treeline where there is plenty of low density snow available to transport onto lee slopes. Â
There remains significant deep, soft snow out there from the past week. Unconsolidated snow, particularly around small trees, can present a non-avalanche, snow immersion hazard. Keep visual and verbal communication with your travel partners when traveling in treed terrain.
Also 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
Our last major storm cycle ended 12/19-12/20. Since then a series of weak weather disturbances have traversed the area over the last week, depositing about 6-12 inches of low density snow.Â
Periods of moderate and shifting winds have caused both wind stiffened surfaces and built wind slabs in exposed terrain near and above treeline. The few recent avalanches reported over the last week have involved wind slabs located in higher terrain and on steep features such as terrain convexities.Â
The upper snowpack has been widely reported as well bonded to the December 15th interface. Where found, the December 15th layer varies from about 2-3Â feet in depth throughout the west slopes of the Cascades.Â
Observations
North
On Tuesday 12/26, NWAC Observer Lee Lazzara traveled between 2000-6000 feet in the Swamp Creek drainage. The recent cold low density storm snow from this past week has been preserved with a favorable and stable density profile near and below treeline. Snowpack tests failed to produce results within the upper snowpack and the 12/15 layer was not identifiable in this area. The wind slab problem in higher terrain was difficult to accurately assess due to  shallow new snow/low visibility. Wind slabs appeared to be the main current problem in exposed terrain and specific terrain features, such as slope convexities. Wind slabs were featured on a variety of aspects due to shifting wind patterns over the past week.
CentralÂ
NWAC pro observer Jeremy Allyn was in the Source Lake area of the Alpental Valley Monday 12/26. Visibility was poor and no wind slab was directly observed in the terrain he traveled.  A thin crust formed by freezing fog was noted from valley floor up to 4900 ft. Â
A skier on Humpback Mountain via the NWAC Observations page for Saturday 12/24 reported getting caught by a 2 ft wind slab on a west facing slope near the summit. No serious injuries were reported. Note that areas west of Snoqualmie typically have stronger winds during periods of offshore/easterly flow.
The Stevens pro patrol on Saturday found the 12/15 layer down 3 ft (85-90 cm) and unreactive in large column tests performed on north aspects. No wind slab was observed.
South
A professional in the Crystal backcountry Wednesday observed a settled snowpack well bonded to the 12/15 interface and no obvious layers of concern.Â
A nice report via the NWAC Observations page from the Tatoosh on Saturday reported several triggered D1-1.5 loose dry avalanches on various steep slopes.
Avalanche Problems
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
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