Avalanche Forecast
Issued: Mar 5th, 2016 10:33AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Storm Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
The avalanche danger will likely peak Saturday night but conservative terrain selection will be essential near and above treeline as human triggered avalanches remain likely. Choose moderately angled terrain and avoid wind loaded slopes. Watch for wet snow conditions below treeline. Be aware that isolated areas in the Washington Pass zone may harbor a reactive buried surface hoar layer on non-solar aspects with the potential to produce larger avalanches.
Summary
Detailed Forecast
Light to locally moderate rain and snow Saturday night will transition to lighter showers Sunday morning with a slow cooling trend during the day. Showers may occasionally be intense on Sunday forming shallow yet unstable storm layers.
The avalanche danger will rise Saturday night during periods of peak precipitation and wind loading. On Sunday, wet snow conditions will prevail below treeline with a transition to storm hazards near and above treeline. Avoid steeper slopes where even a small loose wet avalanche could entrain significant amounts of recent snow or where you could be flushed into a terrain trap. Be wary of unstable layers created during heavier showers that would make shallow wind or storm slabs found at higher elevations more likely to trigger.
Wind slab will be the most likely avalanche problem on lee slopes near and above treeline Sunday, but shallow storm slabs will also be possible.
Be aware that isolated areas in the Washington Pass zone may harbor a reactive buried surface hoar layer on non-solar aspects with the potential to produce larger avalanches.
Cornices formed over the last week should still be weakened by the mild temperatures so be aware of the overhead hazard.
Snowpack Discussion
Weather and Snowpack
During a period of fair weather, wide-spread surface hoar formed in the NE Cascades, mainly surviving outside of steeper solar aspects and wind affected terrain above treeline.
A weak front buried the surface hoar layer in the Washington Pass area by 2/27, and to a lesser extent the central-east zone.
A Pacific frontal system passed the Cascades midday Sunday. By Monday morning, 2-10 inches of new snow had accumulated along the east slopes accompanied by a cooling trend.
Another strong frontal system brought 8-20 inches of snow along the east slopes Tuesday through early Wednesday along with a slow warming trend. Rain may have pushed as high as 4500-5000 feet along the central and northeast Cascades Tuesday afternoon before cooling Wednesday morning. The active weather continued Wednesday depositing another 3-6 inches of snow by Thursday.
In the NE zone near treeline there are about 2 feet of snow now over the 2/27 interface as of Thursday afternoon, providing some fantastic ski and riding conditions in areas away from wind effects. Generally mild temperatures were seen Friday and Saturday with very light precipitation seen from a front early Saturday morning.
We are no longer tracking any layers of concern formed earlier this winter in the mid or lower snowpack, due to lack of recent activity and confirming field observations.
Recent Observations
NWAC pro-observer Jeff Ward was out near Washington Pass a week ago Thursday and found surface hoar up to 15 mm on non-solar slopes. Jeff was out again last Friday 2/26 and noted widespread 6-8 mm surface hoar on non-solar slopes. By Saturday, 2/27 the surface hoar was noted as buried intact in the Washington Pass area, specifically the Cedar Creek drainage. There has been daily travel in this zone this week with a few slides noted up until an observation via our NWAC observation page came in March 2nd from the Pine Creek drainage in the Washington Pass area. A skier triggered and was caught and buried in a persistent slab avalanche on a N-NE aspect at 6600 ft releasing on buried surface hoar about 70 cm down. The full observation with photos can be found here. No one was injured.
Observations from Thursday 3/3 in the NE zone indicated very strong winds near and above treeline were rapidly building wind slab with abundant cold snow available for transport. Several natural wind slab releases were observed Thursday on N-E aspects, producing impressive powder clouds, one being triggered by a cornice failure. It was not felt they stepped to a deeper layer, namely the 2/27 layer, as the largest crown was about 30 cm or 1 ft. The slides ran good distances and indicate how quickly conditions can change!
Jeff Ward, made observations near Wedge Mountain in the Central-East zone Friday, March 4th. Mild daytime temperatures and sunshine Friday, allowed for the crust to soften on solar aspects, even providing spring corn conditions in some areas. Shaded terrain was still holding the recent colder powder. The interface at 2/27, buried a little over 2 ft, did not support propagation in a test pit at 6100 ft on NNE slope. Much of the loose surface snow has now been stripped and redistributed along exposed ridges, forming wind slab deposits on many lee northerly facing slopes below ridges. These wind slabs have been settling quickly, however, remaining the primary concern for east slope backcountry travelers.
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: North, North East, East, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Storm Slabs
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
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.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
Valid until: Mar 6th, 2016 10:33AM
