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Avalanche Forecast

Archived

May 1st, 2014–May 2nd, 2014

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural and human triggered avalanches likely.
Alpine
Below Threshold.
Treeline
Below Threshold.
Below Treeline
Below Threshold.

Regions

Mt Hood.

Very dangerous avalanche conditions are expected with widespread wet snow avalanches likely. Some of these slides may become destructive in force and as a result, travel in avalanche terrain is not recommended. 

Detailed Forecast

Friday will be the last very warm day of this hot stretch. Cooling should begin in the Olympics as a weak front and onshore flow (cool westerlies) develop in the morning and spread inland to the west slopes of the Cascades/Mt. Hood later in the day. Gradual cooling will be delayed until Friday night for the east slopes. Cloud cover should increase in the afternoon over the mountains. Upper mountain winds should increase, especially at Mt. Hood.  However, a warm Thursday night and part of Friday coupled with morning to potentially early afternoon sunshine will prolong the high avalanche danger Friday.  

Natural or triggered avalanches should be likely to very likely Friday and travel in avalanche terrain is not recommended.  Some of these slides may become large and potentially destructive in force.  

Initially small loose avalanches could still involve large amounts of wet or moist snow and lead to very large and destructive avalanches. Even smaller wet snow avalanches are heavier and more powerful than you think and can easily lead to injury or pushing a caught person into unintended terrain. For more info on wet slab avalanches please read this document.  

Glide avalanches can release at any time as water lubricates a smooth bed surface resulting in a full depth release. Watch for glide cracks and limit exposure to potential and hard to predict glide avalanches.

Think about the terrain you are connected to and the consequences of a larger wet slab avalanche as you travel through the backcountry. 

The avalanche danger is expected to be relatively lower over the weekend due to cooler weather. Please see the weekend outlook below.

Weekend Avalanche and Mountain Weather Outlook:  An upper level low pressure system will allow a series of fronts to swing through the Pacific Northwest this weekend. The snow level should fall Friday night and settle to between 4500-5500 feet with periods of light to occasionally moderate showers with mostly cloudy skies. The front arriving on Sunday looks to be the wetter front this weekend. Alpine winds out of the SW look to the be the strongest over Mt. Hood this weekend.

Over the weekend wet snow avalanche concerns should eventually shift to new storm and wind slab concerns above treeline, however expect lingering effects from our very warm week.  It will take time for the snowpack to transition to a more typical springtime profile. Natural wet snow avalanches may become less likely and less widespread over the weekend, but plenty of free water will keep the upper snowpack in a weakened state and maintain large amounts of heavy wet snow available for entrainment.  Melt-freeze crusts may be weak, or may not develop at lower elevations. 

The weekend outlook will be updated Friday evening when the warning expires. 

Snowpack Discussion

After a stormy and cool pattern came to end late last weekend the PNW has been baking in sunny, hot and dry weather the last few days with freezing levels around 12000-13000 feet. NWAC stations have pushed into the 50s and 60s during the day with overnight lows mostly above freezing which has generally led to a poor re-freeze of the snow surface.    

On Tuesday, Apr. 29th the WSDOT crew reported a very large avalanche released from a large explosive that triggered an initial slab as much as 5 feet deep (see photos below).  The slide propagated and triggered additional slab releases, entraining impressive amounts of deep wet snow. Besides the recently fallen snow from late April, there is a deep layer of weak and wet snow available to slide. The old weakly bonded melt forms extended down to over 5 feet where tested. Wednesday saw equally impressive avalanches from more control work and an increase in natural wet loose avalanches as well.  The photos below are from the WSDOT Chinook Pass operation.  Note that large and potentially deadly avalanches were triggered from small ski cuts, not just big explosives!

On Wednesday, NWAC forecasters found generally small wet loose avalanches both natural and skier triggered at Mt. Pilchuck by late morning below and near treeline. They also observed small but sudden and powerful glide avalanches that were releasing naturally from rock bed surfaces, even on non-solar aspects.  Alpental pro-patrol reported a large glide avalanche that occurred Wednesday night as well as numerous wet loose releases in the area over the last few days. 

 Chinook Pass, J Stimberis, 4-29-14    

Problems

Loose Wet

Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.

Wet Slabs

Wet Slab avalanches are the 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 Slabs can be very unpredictable and destructive.

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.