Avalanche Forecast
Issued: Jan 12th, 2016 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs, Wind Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
The avalanche danger should remain elevated in the morning with a slow decreasing trend through the afternoon as storm instabilities begin to stabilize. Cautious route-finding and terrain selection will be essential on Wednesday to stay safe and avoid storm related avalanche problems.
Summary
Detailed Forecast
Moderate to occasionally heavy precipitation Tuesday night should taper down Wednesday. Moderate precipitation should hang on longest for the central and south Washington Cascades through the morning hours. The Cascade Passes, particularly Snoqualmie and White Pass, should see a brief warm-up Wednesday morning as pass level winds switch to light westerlies, but an overall cooling trend will take place later in the day throughout the west slopes.
The avalanche danger should be elevated in the morning with a slow decreasing trend through the afternoon as storm instabilities begin to stabilize. Cautious route-finding and terrain selection will be essential on Wednesday to avoid storm related avalanche problems.
Stay off steeper slopes and allow storm instabilities time to settle. Look for wind slab on a variety of aspects near and above treeline especially near the Passes due to shifting winds. Watch for signs of firmer wind transported snow that can indicate wind slab layers near and below ridges.
Loose wet avalanches are likely Wednesday below 4000-4500 feet on steeper slopes near the Passes Wednesday that see a warming trend and/or rain in the morning.
Snowpack Discussion
Deep storm snow that fell during mid-late December is well settled, homogeneous and stabilized so our focus will be on the upper snowpack.
Fair weather that lasted about a week 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) have kept temperatures cooler and skies cloudier versus elsewhere along the west slopes.
Pro-observer Ian Nicholson 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.
Problems
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
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
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: Below Treeline.
Likelihood
Expected Size
Valid until: Jan 13th, 2016 10:00AM
