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Archived

Avalanche Forecast

Feb 9th, 2017–Feb 10th, 2017
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
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate

Regions: Olympics.

After a period of wild weather Wednesday night and Thursday that closed major highways and limited observations, there is a higher than normal amount of uncertainty about the snowpack heading into Friday. Account for the uncertainty with more conservative travel plans than you might otherwise given the same danger rating.

Detailed Forecast

What a difference a day makes as we quickly transition from mild and wet to cool and showery weather on Friday. Snow levels will slowly lower Thursday night and lower further during the day on Friday. Showers should decrease overnight but then increase again late Friday morning as a low pressure system passes to our north. SW winds will be moderate to occasionally strong Thursday night and Friday at Hurricane Ridge. 

After a period of wild weather Wednesday night and Thursday that closed major highways (in the Cascades) and limited observations, there is a higher than normal amount of uncertainty about the snowpack heading into Friday. Account for the uncertainty by choosing more conservative travel plans than you might otherwise given the same danger rating.

Moderate to occasionally strong SW winds will transport new and recent snow onto lee slopes near and above treeline. Wind slab may build down into the below treeline band on Friday. NW-SE aspects will be identified in the elevation/aspect diagram but watch for firmer wind transported snow on a variety of aspects.   

Despite the cooling trend, new storm slab instabilities may develop during periods of intense showers.   

For areas west of the Cascade crest including the Olympics, wet slabs will be listed as unlikely due to the cooling trend. However, avoid travel below unsupported slopes, especially near and below treeline, with the low likelihood/high consequence of large wet slab avalanches in mind. Wet slab releases are hard to predict and can happen a day or two after peak warming and rainfall.   

Snowpack Discussion

Weather and Snowpack

A strong storm cycle was seen Friday through Monday with about 3 feet of snow accumulating at Hurricane Ridge. 

A strong plume of moisture brought heavy rain to the west slopes of the Cascades and Olympics. About 2 inches of water fell mainly as rain at Hurricane Ridge from Wednesday afternoon through Thursday afternoon with significant snowpack settlement. A strong cold front swept through mid-day. Bands of light to moderate showers in SW flow followed the front with a slow cooling trend beginning Thursday afternoon.  

Recent Observations

No observations have been received since last Saturday. 

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

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: Possible

Expected Size: 1 - 1

Wet Slabs

Release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slab avalanches can be very destructive.

 

Avoid terrain where and when you suspect Wet Slab avalanche activity. Give yourself a wide safety buffer to handle the uncertainty

 

A Wet Slab avalanche. In this avalanche, the meltwater pooled above a dusty layer of snow. Note all the smaller wet loose avalanches to either side.

Wet slabs occur when there is liquid water in the snowpack, and can release during the first few days of a warming period. Travel early in the day and avoiding avalanche paths when you see pinwheels, roller balls, loose wet avalanches, and during rain-on-snow events.

Aspects: All aspects.

Elevations: All elevations.

Likelihood: Unlikely

Expected Size: 1 - 1