Avalanche Forecast
Issued: Dec 11th, 2013 7:26AM
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
Summary
Detailed Forecast
A front and upper trough should cross the Northwest on Thursday. This will cause southwest winds and increasing rain or snow with the rain or snow reaching the Olympics Thursday early morning and the Cascades Thursday late morning. This should result in new storm slab on a variety of slope aspects and new wind slab on lee slopes. These new storm and wind slab layers will build on a variety of previous snow surfaces including the scoured or icy surfaces at higher elevations and hoar frost, near surface facets or powder at lower elevations. So I would expect some potential dicey conditions out there by later Thursday.
Snowpack Discussion
The last significant rain and snowfall occurred on 1 and 2 December when NWAC weather stations near and west of the crest recorded 3-5 inches of precipitation and the Mt Hood area received over 10 inches of water. Most of the precipitation fell as rain at mid and lower elevations with higher terrain receiving rain and then 1-2 feet of snow.
The storm at the start of December helped form a generally thick, strong crust with generally cohesionless new snow above the crust. There was little avalanche activity following the storm with colder low density snow allowing for some loose slides on some steep slopes but little slab avalanche activity was noted..
Over the past week the Pacific Northwest has been in the freezer under a cold and dry northerly flow aloft. During this dry period there were strong easterly ridge top winds, mainly last Friday and Saturday, that transported loose surface snow and built some small wind slabs on some lee slopes. A shallow wind slab was triggered on East Peak near Crystal Mountain.
Then the winds shifted to strong west to northwesterly on Sunday and Monday. This redistributed weak loose surface snow back to some easterly facing slopes below higher ridges building additional isolated wind slab.
The cold dry weather has also been active in the surface snow producing thick surface hoar growth in many areas and near surface faceted snow.
So there is quite a mix of snow surfaces out there on Wednesday. This varies from small areas of shallow wind slab and scoured or icy surfaces at higher elevations near ridges to hoar frost, near surface facets or powder on sheltered slopes and at lower elevations.
Also at lower elevations shallow snow cover is still limiting the avalanche potential especially in the Olympics, the south Cascades and along the Cascade east slopes due to terrain and vegetation
Photos of near surface facets and surface hoar, Monday 12/09 by WSDOT avalanche forecaster John Stimberis at Snoqualmie Pass:
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: All elevations.
Likelihood
Expected Size
Valid until: Dec 12th, 2013 7:26AM
