Warm-weather avalanche hazards will persist on Friday. You can trigger a Loose Wet avalanche that can gouge deeply and gain mass rapidly in soft, poorly bonded snow. Give cornices a wide buffer while traveling on ridges and avoid spending time on slopes below them.
Summary
Detailed Forecast
Friday will be a transitional day out of our warm, dry pattern with low clouds spilling into the Mt. Hood area early and thickening, possibly bringing some light rain in the afternoon.
You can trigger Loose Wet avalanches, especially with the impact of morning solar radiation, at all elevations and on any aspect. Temperatures have not been below freezing since Monday morning at mid-elevation stations and with a lack of refreeze on the surface, Loose Wet avalanches may initiate at any time of the day. Use extra caution on large, steep, and unsupported slopes where you may be able to trigger a large avalanche. If you sink in wet snow up to the top of your boots, avoid slopes over 35 degrees.
Give cornices a wide buffer. They are sagging under the warm temperatures and will break naturally or you could easily trigger them. Cornices will often break surprisingly far back from the edge. Make sure you are well off and out from under cornices, especially while direct or filtered sunshine is reaching the snow surface. Cornice fall could be big enough to kill you and could also trigger Loose Wet avalanches.
Snowpack Discussion
A large ridge of high pressure has developed over the Mt. Hood region and temperatures have progressively warmed from Sunday through Thursday. Since late Monday morning, most mid-elevation sites have remained above freezing and by Thursday, daytime high temperatures were in the 50's and 60's at mid-elevation stations. A l lack of overnight refreeze for multiple consecutive nights, created unstable surface snow conditions with poor bonding of moist or wet, rounding grains in the upper snowpack.
Warm temperatures should have healed wind slabs formed 4/21 and prevented snow transport on 4/23 and 4/24 during moderate easterly flow.
About an inch of new snow fell 4/20 as a frontal system largely missed Mt. Hood and affected areas further north. Moderate west winds near and above treeline did not have any new snow to transport. No new avalanches were reported 4/21.
Warm and generally sunny weather affected the snow in the Mt Hood area 4/19 and 4/20.
Significant snowfall accumulated in the Mt Hood area earlier in the week. Around 16-20 inches (40-50 cm) of snow fell 4/15 through 4/17. Strong west-southwest winds during the storm transported the new snow forming wind slabs which have subsequently healed.
While numerous layers can be found within the snowpack, there are no significant layers of concern at this time.
Problems
Loose Wet
Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind lips of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
Cornices can never be trusted and avoiding them is necessary for safe backcountry travel. Stay well back from ridgeline areas with cornices. They often overhang the ridge edge can be triggered remotely. Avoid areas underneath cornices. Even small Cornice Fall can trigger a larger avalanche and large Cornice Fall can easily crush a human. Periods of significant temperature warm-up are times to be particularly aware.
A corniced ridgeline. A large cornice has formed at the top of the ridge. A smaller cornice has formed to the left of the trees from crossloading.
Cornices are easy to identify and are confined to lee and cross-loaded ridges, sub-ridges, and sharp convexities. They are easiest to trigger during periods of rapid growth (new snow and wind), rapid warming, and during rain-on-snow events. Cornices often catch people by surprise when they break farther back onto flatter areas than expected.
Aspects: North, North East, East, South East, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Apr 27th, 2018 11:00AM
