Avalog
Register
Get forecast notifications
Create an account to receive email notifications when forecasts are published.
Register
Login
Archived

Avalanche Forecast

Dec 29th, 2014–Dec 30th, 2014
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate
Alpine
2: Moderate
The avalanche danger rating in the alpine will be moderate
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
1: Low
The avalanche danger rating below treeline will be low

Regions: Cascades - South West.

Even as storm slab avalanche concerns diminish, old wind slab on easterly aspects and new wind slab on south and westerly aspects will require careful terrain management Tuesday. 

Detailed Forecast

Moderate east winds should ease Tuesday afternoon under mostly clear skies and cold temperatures. When traveling into the near and above treeline zones, be prepared for new wind slab formation on more southerly and westerly aspects. Older wind slab on more westerly aspects may still be sensitive in certain areas. Treat wind loaded slopes with caution, it is difficult to determine how sensitive they are with snowpack tests.  

 

Snowpack Discussion

A storm system moved through the Northwest on Saturday and Saturday night delivering some much needed snowfall. West winds changed to northwest, with moderate to heavy snow west of the crest and a cooling trend. A convergence zone contributed to snowfall in the central Cascades on Sunday. NWAC sites west of the crest picked up about 12-20 inches of storm snow over the weekend except lower amounts at Snoqualmie Pass due to initially very wet snowfall. 

Professional observers west of the crest reported storm snow instabilities quickly settling out and not exceptionally reactive over the weekend. The snow layering ended right side up with low density snowfall at the end of the storm. Mountain guide John Minier reported widespread loose dry avalanches in 40+ degree terrain in the Table Mt area of Mt. Baker Sunday.   

East winds both through the Cascade passes and along ridgetop began to increase Monday. This has begun to redistribute the recent snow on more S-W aspects. While the most recent storm snow avalanche problem should be decreasing, the new wind slab will be an issue with certain aspects generally near and above treeline.  

One notable exception for the west side zones is the presence of a spatially variable persistent weak layer in the Stevens Pass area. 2 out of 3 pro-observers have found PWLs over the last few days in this zone. Tom Curtis found a weak faceted layer 2-3mm thick in the mid-pack just east of Stevens Pass in the Nason Ridge area Saturday that propagated in snowpack tests. (See photo). Jeff Ward found a facet/crust combo at the bottom of the snowpack reactive in extended column tests on Cowboy Ridge near treeline almost 1 meter down on a SW aspect. 

With the amount of ski traffic in this area over the weekend and new snowfall making these layers harder to affect, we do not believe this problem is very sensitive to human triggering. However, the persistent slab problem will be added to the Stevens Pass zone with a low likelihood of triggering. 

Photo by T. Curtis 12-27-14 in the Stevens Pass area.  Location Nason Ridge/Rainy Pass 4900 ft

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: North East, East, South East, South, South West, West.

Elevations: Alpine, Treeline.

Likelihood: Likely

Expected Size: 1 - 1

Loose Dry

Release of dry unconsolidated snow. These avalanches typically occur within layers of soft snow near the surface of the snowpack. Loose Dry avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs. Loose Dry avalanches can trigger slab avalanches that break into deeper snow layers.

Loose Dry avalanches are usually relatively harmless to people. They can be hazardous if you are caught and carried into or over a terrain trap (e.g. gully, rocks, dense timber, cliff, crevasse) or down a long slope. Avoid traveling in or above terrain traps when Loose Dry avalanches are likely.

 

Loose Dry avalanche with the characteristic point initiation and fan shape.

Loose dry avalanches exist throughout the terrain, release at or below the trigger point, and can run in densely-treed areas. Avoid very steep slopes and terrain traps such as cliffs, gullies, or tree wells.

Elevations: Below Treeline.

Likelihood: Possible

Expected Size: 1 - 1