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 from about Mt Hood 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.
Storm slabs may be more sensitive to trigger in the Mission Ridge area on sheltered aspects above 5000 feet if recently formed surface hoar was buried intact.
Snowpack Discussion
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. Observations near Blewett Pass, Holden, and for Red Mountain near Salmon La Sac in early January helped confirm the extent and initial sensitively of this layer. However, these layers and a buried surface hoar layer 60 cm down in the Blewett Pass area from mid-Dec have likely gained significant strength due to the mild temperatures over the last week. No recent slides on these layers have been reported.
The North Cascade Mountain Guides were at Washington Pass last Friday and 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 Jan 3rd interface. Tests and ski cuts gave no results due to less slab structure of the recent snow.
Avalanche professionals in the Mission Ridge area Sunday and an observation via the NWAC page Monday reported widespread surface hoar growth on northerly aspects above 5000 feet. Light snowfall Tuesday may have buried this weak layer in sheltered non-solar aspects above 5000 feet in the Mission Ridge area. An additional 4-6 inches of snow were received by Wednesday. No further observations were reported Wednesday, other than some pockets of loose-wet slides during daytime warming in the Tumwater Canyon area.
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, 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 14th, 2016 10:01AM
