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

Avalanche Forecast

Dec 5th, 2015–Dec 6th, 2015
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
4: High
The avalanche danger rating in the alpine will be high
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

Strong frontal systems will be moving across the Northwest this weekend and next week. Be prepared to change your plans especially if conditions are worse than expected.

Detailed Forecast

Strong alpine winds and moderate to heavy rain and snow will accompany a cold front across the Northwest Saturday afternoon to Sunday morning. This should bring 1-2 feet of new snow near and above treeline with the most expected in the Mt Baker area. Temperatures may not change much over the west slopes Sunday following the front.

New potentially large storm and wind slab should be the focus on Sunday along the entire west slopes. Human triggered storm slab should be likely on varied aspects. Human triggered wind slab should be likely mainly on lee north to east slopes especially above treeline.

A short minor break between weather systems should be seen Sunday afternoon. This may slightly decrease the avalanche danger.

The next strong system should begin to move into the Olympics and Cascades Sunday night. But be prepared to curtail your plans later Sunday if conditions deteriorate sooner than expected.

Early season hazards exist for much of the below treeline band throughout the Northwest, so ski and ride with caution. 

Snowpack Discussion

Note: The snowpack discussion is for the entire west slopes of the Cascades but the avalanche danger forecast varies by zone.  

We had a wild and wet month of November, with 12- 25 inches of water at west side NWAC stations only amounting to about a 1 meter or 3 foot snowpack in the north Cascades above 4000 feet and above 6000 feet elsewhere. Then strong high pressure led to strong temperature inversions and brought very cold air through the Cascade Passes over Thanksgiving weekend. The weather pattern has become more active for the first few days of December with a few inches of new snow over the west slopes paired with moderating temperatures in the Passes. 

Snowpack and Avalanche Observations: Recent activity along the west slopes away from the Cascade Passes has been confined to new storm snow. New NWAC pro-observer Lee Lazzara noted a natural cycle of loose wet avalanches on Wednesday as temperatures warmed. This natural cycle likely extended to higher elevations in the north Cascades on Thursday as rain pushed above 5000'. The Mt. Baker and Crystal patrols reported minor avalanches involving new storm snow during control work Friday morning.   

The west slopes do not have a uniform maritime snowpack! Due to the cold temperatures and clear skies near the end of November, buried surface hoar can be found in the Stevens Pass area. NWAC pro-observer Dallas Glass found this layer at 4-5000 feet at Skyline on E-NE aspects around 45 cm down. Faceting may also be found at a similar depth on solar aspects around a buried sun crust.  Stevens pass pro patrol reported a 12-14 inch (30-35 cm) natural persistent slab avalanche in closed Corona Bowl of the ski area Friday on a N-NW aspect.  Professionals also reported shooting cracks and whumpfing in the area as temperatures warmed and the slab above the PWL became cohesive.

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: North, North East, East, South East.

Elevations: Alpine, Treeline.

Likelihood: Likely

Expected Size: 1 - 1