Avalanche Forecast
Issued: Apr 16th, 2015 11:50AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Loose Wet, Cornices and Wind Slabs.
NWAC,
Northwest Avalanche Center
Slight cooling, increased cloud cover and increasing winds will lower the avalanche danger Friday afternoon. The mostly likely window for natural and skier triggered loose wet avalanches and cornice failure will be late morning through mid-day on Friday.
Summary
Detailed Forecast
This special avalanche advisory is meant to highlight the increased danger particularly in the above treeline band through the first half of Friday. While recent storm snow instabilities are expected to have settled out, continued high freezing levels and mostly sunny skies Friday will keep the avalanche focus on loose wet avalanches and cornices failures. 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 should hold between 9000-10000 feet through late Friday morning and begin to fall in the afternoon as a weak upper level trough passes through the PNW. W-NW winds will also increase in the afternoon. Slight cooling, increased cloud cover and increasing winds will lower the avalanche danger in the afternoon. The mostly likely window for natural and skier triggered loose wet avalanches and cornice failure will be late morning through mid-day on Friday.
Isolated pockets of 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.
A full avalanche and mountain weather forecast will be issued for the weekend.
Snowpack Discussion
New observations from Thursday, 4/16: NWAC stations warmed rapidly into the 40s and 50s on a beautiful sunny Thursday. Avalanche professionals in Washington Pass area triggered small to large loose wet slides during control work that reached the highway by late morning. A natural cornice release Wed afternoon or night in Spire Gulch triggered either a loose wet avalanche that entrained impressive amounts of recent storm snow becoming very large. Less activity was seen in the Chinook Pass area through late morning, but fresh cornice growth along ridges presented an increasing hazard as temperatures warmed. Impressive pinwheels were seen in the Mt. Baker backcountry with the warm up.
Previous discussion from Wednesday, 4/15: 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. During a partly to mostly cloudy day Wednesday, observer J. Allyn in the Mt. Baker backcountry noted D1-2 loose wet slides involving the recent snow near and below treeline on nearly all aspects.
Problems
Loose Wet
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. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. 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.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
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 lips 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.
Cornices can never be trusted and avoiding them is necessary for safe backcountry travel. Stay well back from ridgeline areas with cornices. They often overhang the ridge edge can be triggered remotely. Avoid areas underneath cornices. Even small Cornice Fall can trigger a larger avalanche and large Cornice Fall can easily crush a human. Periods of significant temperature warm-up are times to be particularly aware.
A corniced ridgeline. A large cornice has formed at the top of the ridge. A smaller cornice has formed to the left of the trees from crossloading.
Cornices are easy to identify and are confined to lee and cross-loaded ridges, sub-ridges, and sharp convexities. They are easiest to trigger during periods of rapid growth (new snow and wind), rapid warming, and during rain-on-snow events. Cornices often catch people by surprise when they break farther back onto flatter areas than expected.
Aspects: North, North East, East, South East.
Elevations: Alpine.
Likelihood
Expected Size
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.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Apr 17th, 2015 11:50AM
