Avalanche Forecast
Issued: Dec 15th, 2015 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
Previous wind and storm slab layers from the weekend should be the main concerns on Wednesday. Greater caution would be necessary if the weak front Tuesday night and Wednesday morning brings more snow than expected.
Summary
Detailed Forecast
A weak cold front will cross the Northwest Tuesday night and Wednesday morning. This may bring some very light amounts of new snow but should not greatly change conditions. Some fair weather should be seen by Wednesday afternoon. Light or moderate northwest winds and cool temperatures should also continue on Wednesday.
Previous wind slab layers from the weekend will possibly linger on previous lee mainly north to east slopes near ridges and summits on Wednesday. Storm slab from the weekend should become unlikely by Wednesday but would be found in what were calmer areas if it still exists.
Use more caution and watch for new small wind slab or small storm slab if the weak front Tuesday night and Wednesday morning brings more snow than expected.
Continue to watch for signs of the PWL mainly in the central east zone. Snow pits could be helpful for this. You don't want to take big chance when this type of layer may be present.
Terrain anchors are still causing significant anchoring at the lowest elevations.
Snowpack Discussion
We have varied conditions along the Cascade east slopes. The forecast discussion is the same for the east slopes but the danger rating and problem varies for a potential persistent weak layer (PWL).
An atmospheric river caused warm very wet weather last week. This caused more snowfall in the northeast zone, rain in the southeast zone, with a transition in between in the central east zone.
A stormy pattern with cooling was seen late last week and over the weekend with about 1/2-2 feet of snowfall along the east slopes.
A concern is the extent along the east slopes of the PWL from the cold weather at the end of November. On Wednesday NWAC pro observer Tom Curtis was at Blewett Pass and found sudden collapse test results in faceted snow 15 cm from the ground. On Thursday Tom found similar sudden collapse results due to buried surface hoar at about 50 cm down at 5100 feet on the north side of Mt Cashmere. Tom also experienced whumpfing and heard a natural avalanche.
Further east in the central east zone, the Mission Ridge pro patrol did not find evidence of a lingering PWL during control on Friday. Tom Curtis and fellow NWAC pro observer Ian Nicholson visited Jove Peak east of Stevens Pass on Saturday and did not find a PWL up to 5000 feet on south to west aspects.
A report from Delancy Ridge in the northeast zone on Saturday to near treeline indicated storm snow was well bonded to the underlying crust from last week. Small shallow wind slab were forming above 6000 feet but no instability or avalanches were observed. No observations were made above treeline where more widesperad wind slbb conditions were suspected.
A report via the guides from Washington Pass on Monday found CTH RP results at 60-90 cm near the rain crust from last week and steep slope tests gave no results.
With a lot of loading and few signs of instability at Washington Pass the PWL will be removed as a problem in the northeast zone. But it will be retained in the central east zone due to the observations from last week.
The southeast zone should have a much shallower snowpack more affected by recent rain and warm temperatures. However we have no recent observations from the southeast zone.
Cool weather with little new snow has been seen Monday and Tuesday. This will have mostly caused some consolidation and further stabilizing.
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: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Dec 16th, 2015 10:00AM
