Avalanche Forecast
Issued: Feb 17th, 2018 12:10PM
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
Travel in avalanche terrain is not recommended Sunday. Moderate to heavy snowfall paired with strong winds Saturday night into Sunday morning will create very dangerous avalanche conditions especially near and above treeline Sunday. Avoid slopes steeper than 30 degrees or terrain where avalanches could release from above.
Summary
Detailed Forecast
Travel in avalanche terrain is not recommended Sunday. Moderate to heavy snowfall paired with strong winds Saturday night into Sunday morning will create very dangerous avalanche conditions especially near and above treeline. Avoid slopes connected to large start zones higher in the terrain where wind slab avalanches are capable of producing very large and destructive slides.
A strong cooling trend should help mitigate the storm slab likelihood, but at the same time preserve storm slab instabilities formed overnight. In non-wind affected terrain, avoid open slopes greater than 35 degrees and allow new storm slab instabilities time to stabilize.
New snow will likely bond poorly to a slick crust that formed Saturday morning near and below treeline. This would be a likely bed surface for any avalanche that steps down to deeper layers.
Snowpack Discussion
A strong storm system brought heavy precipitation in the form of snow, ice and rain to Mt. Hood Saturday along with moderate winds increasing to strong and gusty winds in the afternoon (WSW gusts 40-70 mph were common at the lower NWAC wind sites!). Scoured surfaces surfaces are expected on windward slopes as well as wind effected snow well into the below treeline band. Despite heavy precipitation, only about 4 inches of new snow was recorded at the Meadows and Timberline base plots through 5 pm Saturday. The rain-line is estimated to have reached near 5500 feet Saturday mid-day before cooling ensued.
Prior to this system, 12-18 inches of storm snow fell late last week and sits on a strong firm crust layer formed earlier in the month. There are no significant layers of concern in the mid and lower snowpack.
Observations
On Saturday morning, a new rain or freezing rain crust was noted up to 6600 ft. Rime ice had covered most surfaces. Strong winds prevented observations later in the day.
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: Feb 18th, 2018 12:10PM
