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Avalanche Forecast

Archived

Mar 14th, 2017–Mar 15th, 2017

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Olympics.

We have already experienced a region-wide avalanche cycle such that the overall likelihood of wet snow avalanches has diminished. However, destructive wet slab avalanches are still being reported in isolated areas. Realize that wet snow avalanches, specifically wet slab avalanches, are very hard to predict and demand an extra amount of caution when engaging with avalanche terrain.

Detailed Forecast

The relatively high snow levels will continue with rain forecast below 5500-6500 feet through early Wednesday morning. Steady precipitation will taper off during the day on Wednesday in the Olympics with the snow levels gradually lowering. Check the mountain weather forecast for details on the gradual cool down. Periods of moderate S-SW winds above treeline will continue to transport new and recent snowfall in the upper portion of the above treeline band Tuesday night through Wednesday. 

We have already experienced a region-wide avalanche cycle such that the overall likelihood of wet snow avalanches has diminished. However, destructive wet slab avalanches are still being reported in isolated areas in the Cascades. Realize that wet snow avalanches, specifically wet slab avalanches, are very hard to predict and demand an extra amount of caution when engaging with avalanche terrain. The cooling trend on Wednesday is not occurring fast enough to lower the regional avalanche danger although the avalanche danger will likely peak earlier in the day for the Olympics.  

Wet slab avalanches are highly unpredictable and not necessarily tied to peak warming or rainfall. Besides their huge destructive potential, wet slab avalanches often surprise backcountry travelers when they run further than expected. 

Loose wet avalanches are possible on steeper slopes with the additional rainfall. While most loose wet avalanches will be shallow, a few may gouge down to deeper layers and become more powerful than you expect. 

Cornices have been weakened and prone to failure due to rain and mild temperatures. Very large cornices have been reported in many areas and have been involved in recent accidents and close calls. A cornice failure could trigger a large and destructive wet slab avalanche.

If you manage to venture into the above treeline zone Wednesday, expect new or previous wind slab mainly on NW-SE aspects due to recent SW-W winds. Continue to watch for firmer wind transported snow on all aspects especially in areas of complex terrain.

Snowpack Discussion

Weather and Snowpack

The first week or so of March was very cool and snowy. It looks like Hurricane had about 40-50 inches of snowfall during this period.

A strong frontal system brought increasing precipitation and winds along with a warming trend to the Olympics and Cascades on Thursday 3/9. On Thursday night rain probably pushed up to about 5000-5500 feet in the Olympics. Natural and skier triggered avalanches were reported on Friday 3/10. 

Moist W-SW flow and a stalled frontal boundary have produced periods of moderate to occasionally heavy rainfall at most locations throughout the Olympics and the west slopes of the Cascades on Monday and Tuesday. Rainfall totals through 4 PM PST (or 5 PM PDT)

Recent Observations

The Hurricane NPS rangers reported that a snowboarder triggered a large cornice release on Friday 3/10 which in turn triggered a very large slab avalanche on the locally named Pit Bull path (NE aspect at about 5000 ft). NWAC is working on a blog post featuring photos of this avalanche incident. We hope to post online in the next few days. 

A loose wet avalanche near the summit of Mt. Angeles in turn triggered a large slab avalanche on S-SE slopes at about 6000 ft. Other loose wet avalanches from S-SE slopes crossed the main road below the level of Hurricane Ridge on Thursday 3/9 or Friday 3/10. 

Goat Creek avalanche where it crossed the road at about 3500 ft. Avalanche occurred 3/9 and the photo was taken 3/10. Photo by Dave Turner.

Problems

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.

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.

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.