Avalanche Forecast
Issued: Apr 6th, 2018 1:05PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wet Slabs and Wind Slabs.
NWAC,
Northwest Avalanche Center
Dangerous wet snow conditions will develop Friday night and Saturday. Large and destructive Wet Slabs could run naturally. Avoid avalanche terrain if you see slab avalanches in areas of wet snow. Minimize your exposure to large avalanche terrain from above. Expect fresh Wind Slabs to build quickly near and above treeline in the afternoon as the snow level lowers.
Summary
Detailed Forecast
Rainfall received late Friday night and Saturday morning will be followed by a cooling trend with snow levels falling to 5500 feet in the afternoon. Very windy conditions will also develop Saturday in the Mt. Hood area. Rain, snow and wind will combine to create dangerous avalanche conditions Saturday.
You can trigger a dangerous Wet Slab avalanche at most elevations through midday. These avalanches may even run naturally. Watch for liquid water reaching buried crusts or softer layers of snow well-below the snow surface as a sign that Wet Slab avalanches could occur. Wet Slabs are dangerous because they are hard to predict, give little warning, and can be very destructive. If you see slab avalanches in areas of wet snowpack, avoid traveling in avalanche terrain. You may be able to trigger smaller Loose Wet avalanches more easily in the same types of terrain where the Wet Slab problem exists. Avoid slopes with terrain traps where even a small Loose Wet avalanche would have serious consequences.
Rain will change to snow Saturday afternoon near and above treeline. Wind and new snow will form Wind Slabs on lee slopes near and above treeline. Visual clues such as blowing snow, fresh cornices, and cracks in the snow all indicate that you could trigger a Wind Slab avalanche.
Large to very large Glide avalanches that naturally release to ground are possible in isolated areas. Use extra caution and avoid stopping near and below snow covered rock slabs where Glide avalanches could occur.
Recent rain and warm temperature have weakened Cornices. Give Cornices a wide berth if traveling along ridgelines as they often break back further than expected. Be aware of overhead hazard if traveling on slopes with Cornices above as they may fail naturally or be human-triggered.
Snowpack Discussion
Periods of rain and a rising snowline allowed wet snow conditions to extend to near and above treeline Thursday and Friday. A series of crusts and moist or wet snow exist in the upper snowpack.
There are currently no significant layers of concern in the mid or lower snowpack.
Observations
NWAC professional observer Laura Green was in the Timberline area on Friday. She observed some natural pinwheels in the wet surface snow and small broken cornice chunks, but no recent avalanche activity of note. Snow surfaces were wet on all aspects and all elevations that she traveled on up to 6500'.
Problems
Wet Slabs
Release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slab avalanches can be very destructive.
Avoid terrain where and when you suspect Wet Slab avalanche activity. Give yourself a wide safety buffer to handle the uncertainty
A Wet Slab avalanche. In this avalanche, the meltwater pooled above a dusty layer of snow. Note all the smaller wet loose avalanches to either side.
Wet slabs occur when there is liquid water in the snowpack, and can release during the first few days of a warming period. Travel early in the day and avoiding avalanche paths when you see pinwheels, roller balls, loose wet avalanches, and during rain-on-snow events.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
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
Valid until: Apr 7th, 2018 1:05PM
