Avalanche Forecast
Issued: Mar 8th, 2016 10:00AM
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
Significantly changing weather and snow conditions should be seen along the east slopes Wednesday afternoon. You will need to be able to make careful snowpack evaluations Wednesday afternoon.
Summary
Detailed Forecast
A weakening front should move over the Northwest Tuesday afternoon and night. This may cause some light amounts of snow at higher elevations along the east slopes but probably not as much as earlier expected.
The bigger story is the incoming atmospheric river on Wednesday. A warm front should south to north over the Olympics and Cascades Wednesday morning to afternoon. This should bring stormy wet weather with rising snow levels initially to Mt Hood Wednesday morning and to the Olympics and Washington Cascades Wednesday afternoon.
Little change may be seen by Wednesday morning along the Cascade east slopes. But by Wednesday afternoon watch for increasing alpine winds and increasing moderate rain or snow and rising snow levels.
Mainly in the above treeline new wind slab of increasing density due to warming is likely to form on lee slopes. Watch for firmer wind transported snow as the storm develops Wednesday afternoon.
Also mainly in the above treeline new storm slab of increasing density is also likely due to warming. Watch for snowfall that begins to accumulate Wednesday afternoon at more than an inch an hour.
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 by Wednesday afternoon may activate previous layers and cause releases of previous wind slab layers or wet slab avalanches mainly in the near and below treeline. Rain in the same areas is also likely to create loose wet avalanche conditions. Watch for pinwheels and natural loose wet avalanches
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.
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.
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 9th, 2016 10:00AM
