Avalanche Forecast
Issued: Apr 7th, 2018 11:39AM
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
Winter-like stormy weather will create very dangerous avalanche conditions by Sunday. Wind and Storm Slabs will grow on a variety of terrain features and lee slopes, possibly forming lower on slopes than typical, due to very strong winds. Natural or triggered avalanches are likely Sunday so stay well away from avalanche start zones or areas where avalanches could travel to. Use visual clues such as blowing snow, fresh cornices, and cracks in the snow all indicating that you could trigger a Wind Slab avalanche. Give fresh Cornices a wide margin of safety.
Summary
Detailed Forecast
Heavy rain Saturday morning transitioned to snow by late Saturday afternoon following a strong frontal passage. Very strong winds and further snowfall in moderate showers will continue Saturday night and Sunday.
Continued winter-like stormy weather will create very dangerous avalanche conditions at mid and higher elevations by Sunday. The greatest impact of the current storm is affecting the volcanic peaks, including the Mt Hood area where the greatest precipitation and hence danger increase is expected.
Wind Slabs will build on lee slopes and a variety of cross loaded terrain features, possibly well below ridges due to the strength of the winds where natural or triggered avalanches are likely to very likely. Use visual clues such as blowing snow, fresh Cornices, and cracks in the snow all indicating that you could trigger a Wind Slab avalanche.
While the bonding of new snow to the old wet snow surface should be favorable, unstable Storm Slabs will build on a variety of steep terrain, especially in those areas receiving the greatest new snow amounts. Watch for deeper stiffened slabs developing Sunday on a variety of aspects.
Fresh Cornices are likely to build along ridges, so 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
A vigorous storm Saturday caused periods of heavy rain and warm temperatures early Saturday that transitioned to snow or wet heavy snow by Saturday afternoon with shallow new snow beginning to accumulate by Saturday afternoon. The old rain soaked snowpack has begun to refreeze and form a crust under continued cooling. New snow has fallen on rain wet surfaces on most slopes.
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
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 8th, 2018 11:39AM
