Avalanche Forecast
Issued: Mar 9th, 2016 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Storm Slabs.
NWAC,
Northwest Avalanche Center
Very strong winds and snow showers should combine to form new wind slabs on lee slopes, especially NW-E facing. Watch for earlier storm slabs that may be masked by shallow colder surface snow. Dangerous avalanche conditions are expected near and above treeline Thursday.
Summary
Detailed Forecast
Yet another atmospheric river is slamming the Cascades Wednesday, bringing stormy wet weather with rising snow levels through Wednesday night.
The heavy precipitation at rising freezing levels should produce one or more natural avalanche cycles during the warmest periods, mainly Wednesday night.
By Thursday morning, cooling and light to moderate showers with continued strong winds are expected.
The cooling should help wet snow to begin consolidating and cause a lowering danger during the day with shallow new snow accumulations expected.
However, near and above treeline, very strong winds should build new areas of wind slab on a variety of aspects, but mainly on NW-E facing slopes below ridges.
Be especially aware that new shallow lower density snow may mask earlier storm slab formations, especially on slopes that did not avalanche overnight Wednesday.
The persistent slab avalanche problem will only be listed in the northeast zone and is most likely to be 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 depths approaching the 1 m mark the extended column test becomes a less reliable indicator of propagation across a column. Deeper tests like the propagation saw test or deep tap test may help but layer identification and terrain selection are your best friends for managing a persistent slab danger.
The rain, where heavy enough mainly in the central east and southeast zones Wednesday night may activate previous layers and cause releases of previous wind slab layers or wet slab avalanches mainly in the near and below treeline. Any natural avalanches should help to locally remove the persistent layer. More time will tell.
Snowpack Discussion
Weather and Snowpack
During a period of fair weather in late February, widespread surface hoar formed in the northeast zone mainly surviving outside of steeper solar aspects and wind affected terrain.
A weak front buried the surface hoar layer in the Washington Pass area about 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.
A front Sunday and a cooler upper trough Monday brought some snow with 2 day storm totals of up to about 6 inches along the east slopes ending Tuesday morning.
A very strong storm arrived midday Wednesday bringing heavy snowfall/rain at rising temperatures and very strong south ridgetop winds.
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 feet 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 March 3rd 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 February 27th 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 from February 27th buried a little over 2 feet did not support propagation in a test pit at 6100 feet on NNE slope.
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, South 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
Valid until: Mar 10th, 2016 10:00AM
