Avalanche Forecast
Issued: Mar 8th, 2018 11:48AM
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
You can trigger wind slab or storm slab avalanches and the possibility of natural avalanches will remain into the day. Steer around wind drifts on lee slopes near and above treeline and avoid wind-stiffened snow, fresh cornices, or snow pillows. Storm slab will be reactive in wind-sheltered locations. Avoid open slopes steeper than 35 degrees. Large wind slabs may release from high on Mt. Hood, so stay out from under large avalanche paths.
Summary
Detailed Forecast
Large wind slabs may release from high on Mt. Hood, so stay out from under large avalanche paths.
Natural avalanches are possible on Thursday and human triggered avalanches are likely Thursday morning, particularly, as moderate precipitation through Thursday night combines with strong winds creating significant unstable wind slab which may still be quite reactive. Avalanche danger will decrease as temperatures gradually rebound and mostly cloudy skies prevail on Friday. In the NWAC rated near and above treeline terrain, watch for cracking, wind stiffened snow, and freshly formed drifts. Steer around fresh wind features, convex rolls, and slopes holding a foot or more of new, cohesive snow that are 35 degrees and steeper. Shallow Storm Slabs may develop by the afternoon in wind sheltered terrain. The avalanche danger will continue to increase Thursday night.
Heavy, dense snow from Thursday afternoon through Thursday night is creating storm slabs near and below treeline. These slabs are likely to be reactive, particularly in the morning hours. The snow will be wet and heavy, particularly down low, so expect these slabs to glide slightly slower than normal, but have a lot of power.
Snowpack Discussion
Snow levels dropped on Thursday, averaging about 4500-5000 feet and a storm brought .5" of snow water equivalent. Winds increased into the moderate to strong range and visibility was reported by Hood Meadows patrol as a whiteout during the afternoon, limiting snowpack observations. However, it can be assumed that large wind slabs were forming near and above treeline and storm slabs may have been beginning to develop in wind-sheltered locations given high precipitation rates Thursday afternoon. The cooler temperatures on Thursday likely created a crust near and below treeline from the moistened snow surfaces created by above-freezing temperatures and overcast skies on Wednesday.
Loose wet avalanches were reported Tuesday on sunny aspects.
The recent warm weather should have reduced the reactivity of the prior 8" shallow wind slab that formed on lee (easterly) slopes Sunday night and Monday morning.
In many locations more than 3’ of snow now sits on a firm buried crust layer (2/17). This crust has been reported up to 6600’ by professionals in the region. There are currently no significant layers of concern below the 2/17 crust.
Observations
NWAC pro-observer Laura Green was in Mitchell Creek Monday and observed active wind loading occurring near and above treeline. No new or recent avalanches were observed.
On Monday, Mt. Hood Meadows Pro Patrol reported west winds transporting snow, 8" of new and generally non-reactive wind slab on lee aspects limited to E facing slopes. Winds eased mid-day. Small loose wet avalanches occurred on solar, especially below treeline.
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.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
Valid until: Mar 9th, 2018 11:48AM
