Avalanche Forecast
Issued: Feb 3rd, 2016 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
Wind slabs formed Wednesday, as well as expected continued wind transport Thursday, should maintain dangerous avalanche conditions, especially near and above treeline on a variety of aspects.
Summary
Detailed Forecast
Light to occasional moderate snow showers are expected Thursday with moderate WSW ridgetop winds. Generally light expected snowfall amounts and gradual warming should allow for settlement of storm related weaknesses. However, winds may still be strong enough to transport loose surface snow, continuing to deposit wind slabs on lee aspects, more NE-E-SE facing.
Watch for signs of cracking or firmer wind transported snow.
New snow may have initially bonded poorly to a variety of snow surfaces Wednesday, including possible surface hoar or weak lower density snow on sheltered and shaded slopes, or a possible thin sun crust on southerly facing slopes. Travel cautiously and make route choice decisions conservatively Thursday.
Snowpack Discussion
Weather and Snowpack
Two heavy rain events, one about January 21st and another January 27-28th have helped stabilize the mid and lower snowpack, having now formed a very strong rain crust that is being loaded by recent storms. This has shifted the current avalanche problems to upper storm snow layers, though they are now getting fairly deep.
A strong occluded front with strong winds crossed the Northwest on Friday. NWAC stations along the west slopes received about 1-1.5 feet of new snow by Saturday morning. Light amounts of snow followed at the tail end of the storm by Sunday morning. The most recent snowfall fell on top of the rain crust from late last week. Cool and benign weather followed Sunday through Tuesday. Varying amounts of sunshine Tuesday along the west slopes formed a thin sun crust on steeper solar aspects as reported in the most recent observations.
A front is depositing new snow with strong winds Wednesday afternoon, 2/3. Likely building new storm and wind slabs on a variety of aspects, with wind slab expected, mainly on NW-N-NE facing slopes below ridges, near and above treeline.
Recent Observations
Three NWAC pro-observers visited the Stevens Pass and Paradise areas on Sunday and generally reported possible wind slab on previous lee slopes and possible storm slab in the storm snow in the near and above treeline as the main avalanche problems.
NWAC pro-observer Lee Lazzara was in the Mt Baker area on Monday, reporting small wind slab varying in depth from 20-40 cm over slightly lower density snow but lacking clean shears. Skiers also triggered, loose dry avalanches on steep 45 degree slopes. Lee reported similar conditions in the Baker backcountry on Tuesday with the rain crust 50-80 cm down, no avalanches and an overall quite stable, right side up storm snow well bonded to the crust.
Pro-observers Ian and Dallas were on Snoqualmie Mt Tuesday and generally found low density snowfall above the most recent rain crust providing good skiing conditions with minimal wind effects. The most recent snowfall was not cohesive enough to form a storm slab in their specific area.
NWAC observer Jeff Ward was in the Stevens Pass area Tuesday. He generally found a stable upper snowpack, but observed new surface hoar growth most prominent in Highland Bowl near ridgecrest.
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: Feb 4th, 2016 10:00AM
