Avalanche Forecast
Issued: Nov 29th, 2017 10:06AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs and Wind Slabs.
NWAC,
Northwest Avalanche Center
Watch for recently and newly forming storm and wind slabs, especially on wind loaded slopes near ridges and if venturing to higher elevations. Watch for early season terrain hazards, such as poorly covered rocks, vegetation and creeks, particularly at lower elevations.
Summary
Detailed Forecast
On Wednesday night, high pressure should allow for further stabilizing of recent storm and wind slab layers.
A weak to moderate front should move across the Cascades near midday Thursday, causing a period of moderate to briefly heavy snow and moderate ridge level winds. This weather should build new areas of storm and wind slabs, especially in wind affected terrain, such as higher elevations and near ridges.
Watch for storm slabs in areas that received more than a few hours of moderate or heavy snowfall intensities.
Watch for firmer wind-transported snow on lee slopes near ridges, generally northwest through southeast aspects, but expect wind deposited snow on a variety of aspects in areas of more complex terrain.
Storm or wind slab avalanches may entrain recent snow down to the firmer old refreezing snow from last week.
Although not listed as a current avalanche problem, loose dry avalanches are possible Thursday, particularly on steep slopes.
Watch for the numerous travel hazards such as open creeks, barely buried rocks and trees, creating poor and challenging travel conditions, especially below treeline.
The avalanche danger should generally increase for all elevation bands on Thursday.
Remember that closed ski areas without avalanche mitigation are equivalent to backcountry terrain!
Snowpack Discussion
The great start to the 2017/18 PNW Winter through mid November, retreated last week. Warm, wet weather before Thanksgiving caused wet snow and glide avalanches and snowpack consolidation with total snow depths decreasing by 50% or more from their mid-November peak depths. The old wet snow continues to slowly refreeze and from recent observations, the crust layer remains thin at the interface with the recent snow and buried 1-2 feet on average along the west slope zones.
Recent weak to moderate fronts from Monday to Tuesday deposited 24 hour storm snow amounts ranging from 6-12" ending Wednesday morning and 48 hour storm snow of 6-18".
Several weak layers within the storm snow have mostly settled during a break in snowfall and some solar warming through the day Wednesday.
Observations
Wednesday in the Alpental backcountry, an NWAC observer found areas of unreactive wind slab on some east facing terrain features near ridges. Daytime solar warming had allowed surface snow to consolidate and form shallow storm slabs, but no avalanches were observed or triggered.
On Tuesday, the Mt. Baker ski patrol reported strong winds and snow transport Monday night, but full ski testing on Tuesday morning did not trigger avalanches. There were some upside-down layering in the snow from the past day's storms.
Also Tuesday, a Park Ranger at Paradise reported 5-8" of new snow that was bonding well to refreezing saturated snow. Tests were non-reactive.
On Monday, Professional Observers Dallas Glass and Jeremy Allyn traveled into the Crystal backcountry. They also found 3-4" of new snow moderately well bonded to the old moist to wet snow interface. The old wet snow is beginning to refreeze. It was supportive to skis, but not boots. They ski triggered a small loose wet avalanche on a directly sun-exposed south-facing slope. They also noted minimal wind effects.
Problems
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
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
Valid until: Nov 30th, 2017 10:06AM
