Avalanche Forecast
Issued: Jan 11th, 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
Shallow storm and wind slab avalanches are possible by Tuesday afternoon as snowfall rates and transport winds increase later in the day. If storm conditions develop faster than expected, change your travel plans accordingly for the increased hazard.
Summary
Detailed Forecast
An incoming frontal system on Tuesday should largely stall along the coast during the day with periods of rain and snow for the Cascades and steady snow levels along the east slopes. More moderate precipitation and stronger transport winds should hold off until Tuesday evening and night.
Shallow storm and wind slab avalanches are possible by Tuesday afternoon as snowfall rates and transport winds increase later in the day. Northwest through East aspects should continue to see additional light loading near and above treeline Tuesday, but look for wind slab on a variety of aspects. Watch for signs of firmer wind transported snow that can indicate wind slab layers near and below ridges. If storm conditions develop faster than expected, change your travel plans accordingly for the increased hazard.
Snowpack Discussion
Fair weather for about a week over the New Year caused extensive surface hoar and near surface faceted snow along the east slopes. Then about 4-13 inches of snowfall over the east slopes last week buried the surface hoar and near surface faceted snow.
Reports via the NWAC observations page for January 2nd at Blewett Pass, email received at the NWAC from Holden for January 3rd, reports via the NWAC observations page for January 4th for Stevens and Snoqualmie and for Red Mountain near Salmon La Sac on January 5th (which includes a video) help confirm the extent of this layer buried on January 3rd.
Tom Curtis was west of Blewett Pass on Iron Mountain on New Years Day and a December 13 buried surface hoar layer 60 cm down that showed sudden collapses and propagation in several column tests with a firm slab. In this area the layer was prevalent on northeast to east aspects around 5300 feet near treeline. This may still be a layer of concern to track going forward in this specific area but this layer is likely gaining significant strength due to the mild temperatures over the last week.
The North Cascade Mountain Guides were at Washington Pass on Friday found that the recent storm snow was surprisingly well bonded to the previous snow in spite of the recently buried surface hoar and faceted snow at the New Year interface. Tests and ski cuts gave no results due to little slab structure of the recent snow.
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
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 12th, 2016 10:00AM
