Avalanche Forecast
Issued: Jan 13th, 2016 10:01AM
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
The avalanche danger should improve through the afternoon as storm and wind slab instabilities further stabilize. Evaluate wind loaded slopes carefully and choose more moderately angled slopes to minimize storm related avalanche problems.
Summary
Detailed Forecast
The WA Cascades will be between storms Thursday and the next system should move inland to have the greatest effect on the southern zones and mainly Mt Hood and south later Thursday and Thursday night. Only light amounts of new snow are expected in the north and central areas with gradually more storm snow in the south later Thursday and Thursday night at cool temperatures.
The cool weather, expected light winds and precipitation holding off during the day, should allow for a further slow decrease in the avalanche danger through the day Thursday.
Remain cautious, however, in steeper terrain giving storm instabilities time to settle. Look for wind slab mainly on lee NW-SE aspects near and above treeline. Watch for signs of firmer wind transported snow that can indicate wind slab layers near and below ridges.
Snowpack Discussion
Deep storm snow that fell during mid-late December is well settled, homogeneous and has stabilized, so the focus on the current avalanche danger will be on the upper snowpack and the most recent storm snow.
Fair weather over the New Year caused extensive surface hoar and near surface faceted snow that was later buried by light snowfall Jan 3rd-4th. Several small avalanches occurred on the Jan 3rd layer last week. Snowpack observations and tests along the west slopes indicate that this layer is no longer present or generally unreactive in snowpack tests. This could still be a layer of concern in isolated and sheltered locations near the Cascade Passes where offshore flow (east winds) has kept temperatures cooler and skies cloudier versus elsewhere along the west slopes, though observer Ian Nicholson searched sheltered areas extensively around Skyline Ridge, Stevens Pass, up to about 5000 feet Wednesday and found no buried surface hoar. In general, Ian found minimal storm slab conditions in that area with mostly settling storm snow with only a thin denser layer formed during warming Wednesday morning, but this was non-reactive in tests or in ski tests.
Ian was on the S-SW slopes of Litchenburg Mt (Stevens Pass area) Tuesday and found a generally right-side up top portion of the snowpack with no layers of concern and a very thin (1cm) suncrust under Monday's light snowfall on solar aspects. He also observed light to moderate east winds building shallow wind slab on a SW aspect near tree-line Monday afternoon. Through 4 pm Monday, a few inches of new snow had accumulated with snow levels 4000-4500 ft, except near or below 3000 ft near the Cascade Passes.
Storm and wind slabs were more prevalent in the Mt Baker and Snoqualmie Pass area Wednesday, especially Mt Baker where heavier precipitation fell. A natural cycle was reported Wednesday morning with widespread 1-2 ft storm or wind slab releases on steeper north facing slopes near tree line. Some slides caused sympathetic releases running to the valley floor depositing large debris. Alpental pro patrol reported ski and explosive .5-1 ft slab releases with decent propagation, especially in the mid-mountain region around 4500 ft. These slides corresponded to warming following the frontal passage before cooling improved conditions late Wednesday.
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: All aspects.
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: All elevations.
Likelihood
Expected Size
Valid until: Jan 14th, 2016 10:01AM
