Avalanche Forecast
Issued: Jan 7th, 2017 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
Avoid backcountry travel on slopes with previous firm wind slab where you could trigger the slab from an area where the wind slab is shallower. Watch for new wind slab that is expected to form during the day on Sunday especially in the south Cascades such as at Paradise.
Summary
Detailed Forecast
A moderate warm front will move south to north over the Northwest on Sunday. S-SE winds should increase on Sunday and shift to S-SW in the south Cascades Sunday afternoon. East winds should be seen in Stevens and Snoqualmie passes until Sunday night. About 5-10 inches of new snow seems possible in the south part such as at Paradise by the end of the daylight hours Sunday with a warming trend so this is where the main danger increase is expected. Less new snow is expected by the end of the daylight hours in the north part such as at Baker and in Stevens and Snoqualmie Passes.
Watch for new wind slab mainly on northwest to southeast slopes in the near and above treeline on Sunday. Previous wind slab will linger on other slope aspects in all the terrain bands and will have been slow to heal due to the cold temperatures.Watch for firmer wind transported snow.
New storm slab is likely in the south Cascades such as at Paradise by later Sunday where there is more than several inches of rapidly accumulating snowfall. The slight warming trend will aid in developing new storm slab.
Snowpack Discussion
Weather and Snowpack
A cold dry Arctic air mass was over the Northwest last week. The main weather event last week was a day or so of nuclear east winds on Wednesday that went on a powder wrecking rampage, scouring windward slopes, and re-distributing it to lee slopes or other areas.
It sounds like the winds may have eliminated much of the faceted surface snow and surface hoar that may have formed out there in the near and above treeline during the cold weather. But these weak surface crystals may still be found in sheltered areas below treeline and will need to be avoided where buried by snowfall especially above terrain traps.
Recent Observations
A report via the NWAC Observation page for Wednesday reported extensive ongoing wind transport at Stevens Pass with plumes off all nearby peaks due to the east winds.
We received a report of a skier triggered wind slab avalanche in the Crystal backcountry around midday Wednesday. A 15 cm (6") wind slab avalanche of pencil hardness broke 40-50 ft across and carried the skier 200 ft on a NW aspect in the upper portion of the below treeline band. The skier was not seriously injured, but lost equipment. NWAC is compiling more information on this incident and will post a report soon.
Additional field observations ranging from the Mt Baker area to Mt Rainier Thursday highlighted the effects the recent strong east winds on the snowpack. In general surface conditions in exposed terrain were a mixture and a mess, ranging from shallow to deep wind slabs of mostly hard firmness, sastrugi, scoured and wind sculpted surfaces and some soft snow.
We received bad news late Thursday of an avalanche fatality in the Crystal Mountain south backcountry. The Crystal ski patrol came across the wind slab avalanche and found the shallowly buried victim late Thursday morning. The avalanche likely released Wednesday, had a crown of 1-3 feet with a start zone on a northwest facing slope at 6400 feet and ran about 400 vertical feet on a run with a local name of Joe's Shoulder. Members of the NWAC team were on-site Friday to assist with the investigation. A full report will be posted soon on the NWAC web site.
Another report via NWAC Observation page for Thursday for the Snoqualmie area indicated extremely wind affected snow in the near and above treeline.
In some sheltered areas below treeline some stable powder conditions and excellent skiing was still seen late last week.
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: All aspects.
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 8th, 2017 10:00AM
