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Avalanche Forecast

Archived

Apr 15th, 2015–Apr 16th, 2015

Alpine
Below Threshold.
Treeline
Below Threshold.
Below Treeline
Below Threshold.
Alpine
Below Threshold.
Treeline
Below Threshold.
Below Treeline
Below Threshold.

Regions

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Expect a sharp increase in the avalanche danger as you travel above treeline Thursday:  A warm and sunny day will make natural and skier triggered loose wet avalanches likely and bring an increased risk of cornice failure. Wind slab may linger on lee slopes above treeline, with shifting winds earlier in the week redistributing recent snow to many aspects.   

Detailed Forecast

This special avalanche advisory is meant to highlight the increased danger particularly in the above treeline band as we are jolted from winter-like conditions to spring Thursday. While recent storm snow instabilities are expected to have settled out, rapid warming and light winds Thursday will shift the avalanche problems to loose wet avalanches and cornices concerns. The one exception may be the Washington Pass area, where recent snow settlement may allow soft slabs to fail on a previously identified weak layer (see discussion).  

Freezing levels on a sunny Thursday should peak between 9000-10000 feet and make natural and skier triggered loose wet avalanches likely as well as bring an increased risk of cornice failure. Even small loose wet avalanches can be problematic in the wrong terrain; see the Mt. Shuskan accident report from last May that occurred in the late morning as an example. 

Wind slab may be linger on lee slopes above treeline, with shifting winds earlier in the week redistributing recent snow.  A large cornice failure could be the right trigger to find those pockets of wind slab.   

Expect slightly decreasing avalanche danger Friday as a weak weather system approaches. 

Snowpack Discussion

Our cold and snowy late March through mid April run of wintry weather continued last weekend through early this week. Average freezing levels through the middle of April are significantly colder than during any previous month this winter or late Fall. Even though below normal, many mid and high elevation snowdepth stations are peaking right now. Snowdepths are not the only thing peaking this April...  powder conditions have accompanied many happy reports received throughout the Cascades and Mt. Hood during this stretch.   

The frontal system last Friday night was followed by orographic showers through Saturday night, depositing 4-17 inches over Olympics and west slopes of the Cascades. During the same time period, 3-8 inches was measured along the east slopes.

Weekend avalanche conditions were touchiest over the North Cascades with several reports of skier triggered storm slabs noted both Saturday and Sunday. Multiple reports from the Chinook Pass area reported the new storm snow bonding and settling well, and only small point releases during sunbreaks. 

A report from the east slopes at Washington Pass Saturday indicated about 5-8 inches of storm snow over 2-3 inches of small faceted crystals and a crust with test columns failing on isolation in snow pit tests. This layering was felt to be wide spread around Washington Pass.

A frontal system Monday was followed by cool NW flow and another round of orographic showers that brought another 4-12 inches (except 14 at Timberline) to the west slopes and Mt. Hood, with much lighter amounts across the east slopes. On Tuesday, NWAC observer Jeff Hambelton in the Twin Lakes area of the North Cascades reported sensitive cross-loaded lee slopes and new wind transport occurring in the above treeline elevation band. Crystal mountain ski patrol reported the NW winds Tuesday had transported the light amounts of new snow to lee slopes above treeline.  

A slightly warmer, but still cool day with partly to mostly cloudy skies was seen on Wednesday. 

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.

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.

Wind Slabs

Wind Slab avalanches are the 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.