Avalanche Forecast
Issued: Apr 11th, 2018 11:28AM
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
Light snow and moderate winds will create dangerous avalanche danger on wind loaded slopes near and above treeline Thursday. Use visual clues such as fresh cornices, wind drifted pillows and cracks in the snow all indicating that you could trigger a Wind Slab avalanche. Shallow Storm Slabs may build over a newly forming crust. Watch for and avoid deeper drifted snow on steep slopes. Be alert to overhead hazards such as cornices by giving them a wide margin of safety.
Summary
Detailed Forecast
The avalanche danger will shift to new Wind and Storm Slabs building with and after a frontal passage Wednesday night. Further storm showers are expected Thursday. Moderate winds with light to moderate snowfall at cooling temperatures will make triggered slab avalanches likely by Thursday, especially on steep slopes receiving wind transported snow near and above treeline. Use visual clues such as fresh cornices, wind drifted pillows and cracks in the snow all indicating that you could trigger a Wind Slab avalanche.
New Storm Slabs may build over a newly forming crust. Watch for and avoid deeper drifted snow on steep slopes.
Cooling temperatures will allow for old wet snow to consolidate and refreeze, limiting any Wet Snow avalanches.
Watch for other spring hazards such as recent cornices along ridges, open glide cracks and opening creeks.
Snowpack Discussion
New wind and snow is arriving at cooling temperatures late Wednesday. This is allowing old wet snow to begin re-freezing while building new Wind and Storm Slab layers.
A front brought rain and wind to Mt. Hood Tuesday. Snow levels fell to around 5500 ft by Tuesday afternoon. Warm and dry conditions were seen on Monday.
The dry weather followed the strong weekend storms that caused rain Saturday to high elevations followed by cooling and new storm snow totaling 1.5-2 ft in the Mt Hood area above 5000 feet through Sunday afternoon. Wet snow avalanches were seen earlier in the storm and continued at lower elevations through the weekend. New Wind and Storm Slabs developed at higher elevations later in the weekend with fresh cornices developing along ridgelines.
There are currently no significant layers of concern in the mid or lower snowpack.
Observations
During morning mitigation work Monday, Mt. Hood Meadow Pro-patrol reported that the new snow was not very sensitive to ski triggering. Explosives produced generally shallow soft slabs, with a few that were up to 1-2 ft deep on lee slopes near 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.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Apr 12th, 2018 11:28AM
