Avalanche Forecast
Issued: Feb 23rd, 2016 10:33AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
Wind slab should continue to be touchy on recently loaded aspects Wednesday. Depending on how much sunshine is received, loose wet avalanches may also become a problem on steeper solar slopes in your area.
Summary
Detailed Forecast
Cool easterly flow should abate on Wednesday with warming temperatures forecast across the west slopes and Cascade Passes. However, a dissipating frontal band will push mid and high clouds over the area, potentially limiting sun effects on Wednesday.
Recent E-SE winds have redistributed recent snowfall to non-traditional westerly aspects forming generally shallow but touchy windslab primarily in the near and above treeline elevation bands, but may still be possible below treeline depending on local loading patterns.
Watch for wind transported snow on a variety of aspects Wednesday: Don't let the combination of Moderate avalanche danger with non-traditional lee aspects/cross-loaded slopes catch you by surprise!
Loose wet avalanches may be the primary problem in your area depending on the amount of sunshine Wednesday. Freezing levels will rise on Wednesday and combined with sunshine should activate loose wet avalanches on steeper solar slopes. Watch for pinwheeling and rollerballs as precursors to more significant loose wet activity and especially be wary of loose wet avalanches around terrain traps.
Snowpack Discussion
Weather and Snowpack
Last week, heavy rain and mild temperatures dominated along the west slopes forming the latest rain crust. NWAC sites in the near and below treeline recorded 2-6 inches of water mostly as rain over the 2 days ending Tuesday February 16th.
An active and cooler pattern began to bury the crust on February 17th, dropping about 1-3 feet of storm snow mainly near and above treeline along the west slopes from Wednesday through Saturday morning. A weak front on Sunday only brought a few inches of additional snowfall through Monday morning.
Storm layers in the snow received from Wednesday to Saturday have had many days to stabilize. Bonding of storm snow to the February 17th crust varied along the west slopes initially, but also should have improved over the last few days.
Moderate E-SE winds Monday night and Tuesday redistributed surface snow most effectively near the Cascade Passes and the central and southern Cascades including the Crystal and Paradise areas.
The mid and lower snow pack along the west slopes should be a stable mix of crusts and layers of moist and rounded snow crystals.
Recent Observations
Widespread skier triggered avalanches near the Mt Baker ski area were reported Saturday by the Mt Baker pro-patrol and via the NWAC Observations page with some partial burials but no injuries. These were 12-18" deep x 100-200 yards wide storm slabs running fast on the February 17th crust on the northeast side of Shuksan Arm in the near treeline band of 4500-5000 feet.
NWAC pro-observers Dallas Glass and Ian Nicholson were on Mt Snoqualmie Saturday and reported 2 skier triggered wind slabs 20 cm (8 inches) deep x 30-50 feet wide with ridges limiting the widths of the slabs. This was at about 5500 feet on a cross loaded southwest slope. They reported a good bond of storm snow to the February 17th crust.
Dallas was in Crystal backcountry on Monday and reported wind affected snow on a variety of aspects due to the redistribution of recent storm snow. He also observed a skier triggered soft slab avalanche on a north aspect draining into Union Creek @ 6300 feet Monday afternoon. Luckily there were no injuries.
Observations from backcountry ski tourers in the Snow Lake area of Snoqualmie Pass and NPS rangers near Paradise both reported shallow yet sensitive new wind slab formation Tuesday. New wind slabs were most likely formed from the Stevens Pass area and south along the west slopes.
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: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
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: South East, South, South West, West.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Feb 24th, 2016 10:33AM
