Avalanche Forecast
Issued: Feb 15th, 2018 10:35AM
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
Stay away from steep open slopes Friday as dangerous avalanche conditions develop with additional snow and wind. Wind and storm slab avalanches will become larger and easier to trigger throughout the day.
Summary
Detailed Forecast
Additional snow and wind will cause the avalanche danger to increase Friday creating dangerous avalanche conditions in most locations. Wind and storm slabs will grow larger and easier to trigger as the day passes.
You will be able to trigger wind slabs on exposed terrain near and above treeline. Moderate to strong westerly winds will transport snow forming new and sensitive wind slabs. Identify and avoid areas of wind deposited snow. Visual clues such as blowing snow, plumes, and fresh cornices all indicate wind slabs are present on slopes below.
Storm slab avalanches will be found on all aspects and elevations in steep sheltered terrain. Warming air temperatures, higher density snow, and additional precipitation will cause storm slabs to become easier to trigger. Avoid open slopes above 35 degrees where you will be able to trigger an avalanche.
At lower elevations, especially around Snoqualmie Pass, snow will transition to rain late in the afternoon. If you receive rain, expect loose wet avalanches on all steep slopes. Stay off of terrain where being caught in even a small avalanche could have large consequences.
Snowpack Discussion
Around the west slopes of the Cascades 8-12 inches of storm snow fell Valentine’s Day. In most locations reports showed soft weak surface snow conditions with small loose dry avalanches reported Wednesday and Thursday. Locally at Snoqualmie Pass the new snow formed shallow cohesive slabs resulting in one snowboarder triggered avalanche and numerous natural storm slab avalanches.
This recent snow fell on a variety of snow surfaces including firm melt-freeze crust, wind affected snow, and weak near surface facets. The bonding of the new snow to the old snow surface was improving Thursday.
Unstable snow has been isolated to the recent storm snow and its bonding to the most recent crust. Warm wet weather during early February has created a strong consolidated mid and lower snowpack. Although we are tracking older buried crust, there are currently no significant layers of concern.
Observations
NWAC staff in the field Wednesday and Thursday in the Stevens and Crystal Mt backcountry areas found loose surface snow conditions resulting in numerous loose dry skier triggered avalanches. Wind transported snow was seen to be isolated to exposed features near and above treeline.
At Snoqualmie Pass, NWAC staff and Alpental ski patrol reported natural storm slab avalanches from Wednesday. While this storm snow weakness was still evident, observations demonstrated it was gaining strength.
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: Feb 16th, 2018 10:35AM
