Avalanche Forecast
Issued: Jan 8th, 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
New wind slab is most likely on Monday on northwest to southeast slopes mainly in the near and above treeline. Avoid slopes above terrain traps in the below treeline where recently formed weak layers may have been buried by new snow.
Summary
Detailed Forecast
New snow by Monday morning along the west slopes should be up to about the 5-10 inch range. Temperatures will be cooler by Monday morning.
Southwest winds aloft should decrease on Monday. Light snow showers along the west slopes should decrease on Monday with further cooling.
If you travel into the back country Monday watch for new wind slab mainly on northwest to southeast slopes mainly in the near and above treeline. Previous wind slab may linger on other slope aspects also mainly in the near and above treeline. Watch for firmer wind transported snow.
New storm slab is possible Monday on sheltered slopes where there was more than several inches of rapidly accumulating snowfall.
New wind and storm slab layers should be more stable by the end of the day on Monday.
Remember that recently formed surface hoar and surface faceted snow has been seen lately in the below treeline. It will be a good plan to avoid slopes above terrain traps in the below treeline where these weak layers may have been buried by new snow.
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, re-distributing it to lee slopes and building sastrugi in other areas.
Reports indicate the winds eliminated most of the faceted surface snow and surface hoar that formed in the near and above treeline during the cold weather. But these weak surface crystals are still reported below treeline.
A warm front is moving south to north over the Northwest on Sunday eroding and replacing the Arctic air mass and causing warmer temperatures. Southeast winds aloft on Sunday will shift to southwest by late Sunday and Monday. A change from east winds and cold temperatures to warmer west winds should be seen in the Cascade passes Sunday evening with a bump there in the snow levels.
Recent Observations
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 report will be posted soon on the NWAC web site.
A report via NWAC Observation page for Thursday for the Snoqualmie area indicated extremely wind affected snow in the near and above treeline.
NWAC pro-observer Lee Lazzara was out in the Glacier Creek drainage west of Mt Baker on Saturday and noted surface hoar there in the below tree line. This layer could be an issue on slopes where it gets buried above terrain traps.
NWAC pro-observer Ian Nicholson was out at Paradise on Saturday and found that stiff 10-15 cm thick wind slab on N-W-S facing slopes was not as reactive as 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 9th, 2017 10:00AM
