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Archived

Avalanche Forecast

Feb 20th, 2017–Feb 21st, 2017
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
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 - East.

Lingering storm and wind slab should be less sensitive on Tuesday but evaluate recent snow carefully before committing to steeper terrain. Wind slab will mostly likely be found on NW-E-SE aspects near and above treeline. 

Detailed Forecast

Another low pressure system will track across Oregon Tuesday but this one is forecast to bring less snow to the Washington Cascades. Periods of light snow should mainly affect the southeast Washington Cascades with light showers possible further north. A slight warming trend may occur Tuesday afternoon with sunbreaks possible in the northeast Cascades. 

Lingering storm and wind slab should be less sensitive Tuesday but evaluate recent snow carefully before committing to steeper terrain. Wind slab will mostly likely be found on NW-E-SE aspects near and above treeline. 

Loose wet avalanches will not be indicated as an avalanche problem but watch for loose wet surface snow if you find yourself on solar slopes during sunbreaks.

Snowpack Discussion

Weather and Snowpack

Hopefully the last atmospheric river of the season arrived Valentine's Day 2/14 through Thursday 2/16 along the Cascade east slopes. An avalanche cycle occurred Wednesday night in the northeast zone and Thursday for lower slopes and areas further east of the crest in the central-east zone. 

In the northeast zone the NWAC station at Washington Pass received about 10-14 inches of moist snow over a thin freezing-rain crust formed early in the storm cycle.  In the central-east and southeast zones, light rain likely reached up about 5-6000 feet in the central-east and up to about 7000 feet in the southeast zone. Fair and sunny weather on Friday formed a sun crust on solar aspects.

Light amounts of new snow accumulated over the weekend. A low pressure system tracking across the south Washington Cascades during the day on Monday brought a few inches of new snow to the central-east and southeast Cascades. A slight warming trend was observed during the storm Monday.   

Recent Observations

North

The NCMG were out near Washington Pass on Sunday and reported that ski tests on steep features gave no results and that wind slab was generally unreactive. A size 2.5 slab, 1 m x 200 m crown was seen on a north aspect at 7200 feet that probably released after cornice fall on 2/15.

On Monday, the NCMG observed a fresh and large storm slab that was triggered by cornice fall mid-day above treeline in the Hairpin Valley on a NNE aspect near 7000 feet. No other avalanche activity was observed. The slab averaged around 40 cm to the most recent crust.  

Central

Reports from the Mission Ridge pro-patrol Friday 2/17 indicated that the previous wet snowpack had locked up and formed a strong surface crust in most terrain with little to no new snow above.

NWAC observer Tom Curtis was at Jove Peak on Monday and found the recent snow generally bonding well to the Valentine's Day crust. Some previous wind slab was apparent below ridgelines near treeline. The possibility for skier triggered loose wet avalanches increased around mid-day during a slight warming trend.  

South

No recent observations. 

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: 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