Avalanche Forecast
Issued: Feb 7th, 2016 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Loose Wet and Wind Slabs.
NWAC,
Northwest Avalanche Center
A warm and sunny day with freezing levels pushing above 12,000 feet Monday will point the avalanche danger toward loose wet avalanches. Steeper solar slopes near and above treeline should be the most likely places for natural or skier triggered loose wet avalanches. Solar slopes involve more than just due south aspects as we head further into February.
Summary
Detailed Forecast
A warm and sunny day with lighter alpine winds should be seen on Monday. As freezing levels push above 12,000 feet, the avalanche danger will focus on loose wet avalanches. Steeper solar slopes near and above treeline should be the most likely places for natural or skier triggered loose wet avalanches. Solar slopes involve more than just due south aspects as we head further into February.
The hazard should be locally lower below treeline in the Cascade Passes in areas affected by cool easterly flow and in areas that form a stout crust Sunday night.
Be aware of loose wet avalanche potential above terrain traps (like above cliffs or near gullies), where even small wet avalanches can become powerful and have unintended consequences.
If pushing higher on the Cascade volcanoes Monday, the loose wet potential extends well above the top NWAC elevation band with larger avalanches possible.
Lingering wind slab on lee slopes near and above treeline should be stubborn to trigger, but still possible in isolated locations on Monday.
Snowpack Discussion
Weather and Snowpack
The last heavy rain event January 27-28th further stabilized the mid and lower snowpack and formed a strong rain crust. Subsequent sun crusts and surface hoar formed early last week have not played a role in recent avalanches outside of the Crystal backcountry.
After a moderate snowfall Wednesday/Thursday of last week, a quick but powerful front moved through Friday night. About 4-10" of snow was recorded for NWAC west slope stations through mid-day Saturday, however strong SW to W winds caused significant transport even down into the below treeline band. Rain moved up to 4000-4500' in the south Washington Cascades on Friday evening before snow levels lowered.
Sunny or filtered sunshine was seen for the better part of Sunday from just south I-90 to Mt. Hood. Periods of very light rain occurred in the Mt. Baker area with a broken overcast inbetween. Temperatures pushed into the low to mid 40s at many NWAC stations not affected by temperature inversions or easterly flow.
Recent Observations
On Saturday, Alpental pro-patrol reported small yet extensive wind slab on the upper mountain averaging 6-10". Stevens, Crystal and Baker patrols saw less results despite significant wind transport. In general the new snow came in right side up and bonded well to the old snow surface. The exception was the slope involved in the Crystal accident. Surface hoar formed early last week was buried by lower density snow and then wind loaded. Other reports from the Crystal backcountry indicated sensitive wind slab with shooting cracks and triggered slides. Crystal patrol have not seen this weak layer reactive in the terrain they manage.
On Sunday, cloud cover and cool east flow at the Passes muted the warm up and loose wet activity from Snoqualmie Pass to Mt. Baker. Sunnier skies and warmer temps caused a small loose wet cycle in the Crystal area by late morning.
Accident details: There was a serious accident that occurred in the Crystal backcountry on Saturday. Two skiers descending East Peak crossed from a windward slope to a lee slope above Ted's Buttress a few hundred feet below the peak (5900 ft). One triggered a wind slab on a shallow portion of the slab (40 cm) that broke well upslope of his position. Both were caught, at least partially buried by the slide and sustained injuries. One was able to self-extricate and then eventually freed his partner after considerable digging. Initial estimates are the slab averaged 40-50 cm, max 130 cm in depth, and about 100 m in width. The slope in question would have been loaded by W-SW winds. A fracture line profile performed by Crystal Mt pro-patroller Peter Dale on Sunday identified the slab released on buried surface hoar from early last week on top of the 1/28 rain crust. The slide ran over 500 ft, with the skiers carried about less than 200 vertical feet.
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: South East, South, South West, West.
Elevations: All elevations.
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: Feb 8th, 2016 10:00AM
