Avalanche Forecast
Issued: Dec 1st, 2016 10:15AM
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
New shallow wind and storm slab layers seem likely on Friday mainly in the above treeline band along the east slopes due to winds, new snow and a warming trend.
Summary
Detailed Forecast
A frontal system will cross the Northwest on Friday. This will cause increasing southwest to west alpine winds, increasing rain or snow and a rise in snow levels.
Perhaps about 5 inches of snow seems likely along the east slopes mainly in the above treeline band on Friday with a warming trend.
This weather should build shallow new upside down wind slab on lee slopes which should be mainly southeast to northwest slopes above treeline. Watch for firm wind transported or hollow sounding snow.
New shallow storm slab is possible in areas that see rapid accumulations of new snow. The warming trend will also aid in the development of new upside down storm slab.
The danger level in this forecast has been nudged up a notch from the outlook issued the previous day. Small human triggered avalanches in many areas along the east slopes in the above treeline seems likely on Friday due to the alpine winds, new snow and the warming trend.
In the lower part of the below treeline band there is still insufficient snow for avalanches but watch for early season hazards such as terrain traps, rocks and creeks.
Snowpack Discussion
Weather Discussion for the Cascade East Slopes
A frontal system passed across the Cascades Sunday night and was followed bynorthwest flow aloft on Monday. Along the east slopes snow amounts were in the 3-5 inch range with lesser amounts further east. Moderate west northwest winds continued Monday as recorded at the NWAC Mission Ridge and Dirty Face weather stations.
Tuesday was a fair day with light winds along the east slopes.
Another front passed over the Northwest on Wednesday. NWAC stations along the crest had winds generally in the 20-30 mph range with gusts places to the 40-50 mph range. It looks like new snow on Thursday morning was light along the east slopes.
A bit of a break has been seen along the east slopes on Thursday.
Recent Reports for the Cascade East Slopes
Last Friday a skier triggered a deep wind slab on a 40 degree northwest slope at about 7500 feet on Slate Peak near Harts Pass. The crown varied from about 2 to 7 feet and ran about 2-300 vertical feet with car sized chunks of debris. Fortunately the skier was not caught. He noted that windward slopes across the valley were scoured indicating recent wind transport.
On Tuesday NWAC pro observer Jeff Ward was in the Washington Pass area. He found generally good ski conditions and a stable snowpack. Previous wind loading was obvious but varied by aspect throughout the terrain. Wind slabs were stubborn and unlikely to trigger in the drainages he traveled through.
Jeff relayed another report to the NWAC from the Washington Pass area for Wednesday. On east slopes near the pass no signs of instability were seen with compression tests giving inconsistent hard or negative results at a layer at about 45 cm. This coverage or open creeks were still an issue at the pass elevation. Stubborn wind slab was the only noted problem.
A layer of interest in the Washington Pass area was buried surface hoar at 65 cm down on a north aspect at 7000 feet seen about a week ago. From Jeff's report the layers is apparently not widespread and there have not been significant or alarming tests recently for this layer. It might still be worthwhile to check for it in snowpit tests.
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: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Dec 2nd, 2016 10:15AM
