Avalanche Forecast
Issued: Mar 6th, 2016 10:12AM
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
Near the Cascade crest, be wary of unstable layers created during periods of rapid accumulation Monday that would make shallow wind or storm slabs more likely to trigger. Overall the potential for loose wet avalanches will decrease Monday and be confined to the lower elevations of the below treeline band and to steeper solar aspects during sunbreaks. The persistent slab avalanche problem will only be listed in the northeast zone.
Summary
Detailed Forecast
Light new snow received through Monday afternoon should accumulate at relatively cool and stable snow levels.
Be wary of unstable layers created during periods of rapid accumulation Monday that would make shallow wind or storm slabs more likely to trigger. Overall the potential for loose wet avalanches will decrease Monday and be confined to the lower elevations of the below treeline band and to steeper solar aspects during sunbreaks. Watch for new wind slabs on lee slopes mainly above treeline Monday.
Cornices formed over the last week may still be weak from the recent mild weather so be aware of the overhead hazard and give cornices a wide berth while traveling along ridgelines.
The persistent slab avalanche problem will only be listed in the northeast zone, most likely found on non-solar aspects in the above and near treeline band and stretching into the upper portion of the below treeline band. This interface is likely getting harder for a human to trigger, but if triggered is capable of producing large avalanches. Be aware that at the depths approaching 1 m the extended column test becomes less reliable for indicating propagation. You can attempt deeper column tests like the propagation saw test or deep tap test, but layer identification and terrain selection are your best friends.
Further east of the crest in the central and southeast zones, the avalanche danger will locally be lower due to a refreezing upper snowpack with minimal new snow concerns.
Snowpack Discussion
Weather and Snowpack
During a period of fair weather in late February, 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.
The recent active weather pattern continued over the weekend with a system on Saturday night producing 0.50 to 1 inch of rain along the east slopes, except in the Washington Pass zone where rain likely stayed below 5500 feet and a few inches of snow accumulated above 6000 feet.
The 2/27 buried surface hoar layer in the Washington Pass area should be down around 2-3 feet (60-90 cm) in the near treeline band.
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
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.
Strong winds on Thursday, 3/3 caused widespread natural wind slab avalanches in the NE zone. While many were contained to the recent storm snow, one larger slide on a N-NE aspect at Windy Pass likely released down to the 2/27 PWL.
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.
Observations from Mission Ridge pro-patrol Sunday indicated a firm upper snowpack after rainfall received Saturday night began to refreeze.
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
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: Mar 7th, 2016 10:12AM
