Avalanche Forecast
Issued: Feb 24th, 2018 10:55AM
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
A very strong storm will cause dangerous avalanche conditions in steep terrain Sunday. Wind slabs will become very large on slopes receiving wind transported snow on a variety of aspects. Unstable storm slab avalanches will become very sensitive during the expected high snowfall rates. Travel in avalanche terrain is not recommended Sunday.
Summary
Detailed Forecast
Another strong storm arrives early Sunday morning! This system will bring very strong winds, heavy snowfall rates, and a relative warming trend. The storm Sunday will load the existing 2 ft of storm snow from Saturday, much that has already been re-distributed by strong winds.
These back to back storms will create very dangerous avalanche conditions Sunday. Wind slabs in many areas near or above treeline are likely to be 3-6 feet or more by Sunday! In addition to wind slabs, denser storm slabs should build rapidly during expected high snowfall rates on most steep terrain near and below treeline.
Natural avalanches will be increasingly likely Sunday and you will be likely to trigger large slab avalanches on most terrain steeper than about 35 degrees.
It will be difficult to chose safe terrain Sunday and travel in avalanche terrain is therefore, not recommended.
Snowpack Discussion
Previous low density surface snow that has provided the Mt Hood area with excellent backcountry conditions earlier this week all began to change Friday as avalanche conditions worsened.
Loose surface snow began being redistributed Friday afternoon, beginning to build new wind slabs. By Saturday afternoon an additional 2 ft of new storm snow was received. The newest snow has been deposited with very strong winds building increasingly large wind slabs.
By Saturday, new sensitive storm slabs were reported below treeline as denser storm snow was deposited on weak lower density snow.
Avalanches releasing in the most recent storm snow Sunday may step down to older storm layers creating large avalanches.
The deepening storm snow now sits over a strong crust layer formed and buried on Saturday 2/17. This crust has been reported up to 6600 feet by professionals in the region.
Observations
MHM professional patrol on Saturday reported white out conditions above treeline with significant wind transport occurring. Below treeline the surface snow was forming unstable storm slabs. Still relatively shallow storm slab avalanches were releasing with explosives Saturday morning.
NWAC pro-observer Laura Green traveled in the Mt Hood backcountry Friday. Laura reported stable conditions prior to Friday’s incoming storm. She found a right-side-up snowpack with loose unconsolidated surface snow.
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: All elevations.
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.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
Valid until: Feb 25th, 2018 10:55AM
