Avalanche Forecast
Issued: Feb 21st, 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
The avalanche danger should gradually decrease on Monday. Watch mainly for new or previous wind slab on the lee slopes near ridges or mid-slope where cross loading may have occurred.
Summary
Detailed Forecast
West winds and orographic snow showers following the front should taper off on Monday with lower snow levels. The avalanche danger should gradually decrease on Monday.
The winds and snow on Sunday and Sunday night may have built new shallow wind slabs on lee slopes mainly near and above treeline by the end of the day. Older wind slab may linger on similar slopes. Watch for new or previous firmer wind transported snow mainly on the lee N to SE slopes near ridges or mid-slope where cross loading may have occurred.
New shallow storm slab is possible if you are in an area where more than several inches of snow rapidly accumulated Sunday to Monday morning.
Snowpack Discussion
Weather and Snowpack
Heavy rain and mild temperatures dominated last Sunday and Monday forming the latest crust along the west slopes. NWAC sites in the near and below treeline recorded 2-6 inches of water mostly as rain over the 2 days ending Tuesday morning. The Paradise station recorded over 5 inches of water mostly as rain in the 24 hours ending Monday morning!
An active and at times stormy, cooler pattern began to bury the crust on February 17th and brought about 1-2 feet of storm snow mainly near and above treeline to the west slopes Wednesday to Friday. Another front is crossing the area on Sunday followed by a short wave Sunday night that will generally cause shifting winds and a few inches of snow.
Storm layers in the snow from Wednesday to Friday should be strengthening. Bonding of storm snow to the February 17th crust has been good in some areas and not in others along the west slopes.
The mid and lower snow pack along the west slopes should be a stable mix of crusts and layers of wet rounded snow crystals.
Recent Observations
Widespread skier triggered avalanches near the Mt Baker ski area were reported Saturday by the Mt Baker pro-patrol and via the NWAC Observations page with some partial burials but no injuries. These were 12-18" deep x 100-200 yards wide storm slabs running fast on the February 17th crust on the northeast side of Shuksan Arm in the near treeline 4500-5000 feet.
NWAC pro-observers Dallas Glass and Ian Nicholson were on Mt Snoqualmie Saturday and reported 2 skier triggered wind slabs 20 cm deep x 30-50 feet wide with ridges limiting the widths of the slabs. This was at about 5500 feet on a cross loaded southwest slope. They reported a good bond of storm snow to the February 17th crust.
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: All elevations.
Likelihood
Expected Size
Valid until: Feb 22nd, 2016 10:00AM
