Avalanche Forecast
Issued: Apr 9th, 2018 12:09PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
Tuesday will be a transition day as we begin to enter a cooler snowier pattern. You may still encounter large and destructive Wet Snow avalanches at lower elevations, while fresh Wind Slabs develop at higher elevations. During this transition, continue to give fresh Cornices a large margin and minimize travel on slopes below.
Summary
Detailed Forecast
On Tuesday, a frontal system will bring a cool down with new snowfall accumulating above 5000-5500 ft by the afternoon. You may still trigger Loose Wet avalanches or they may be triggered by additional rainfall Tuesday morning. Watch for signs of wet snow conditions such as sluffing, roller-balling and any natural Loose Wet avalanches as signs of an increasing hazard. Loose Wet avalanches still have the ability to become large in areas where they gouge down to deeper layers of moist or wet snow.
Shallow new Wind Slabs may form on lee slopes above treeline by the afternoon. 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.
Large fresh cornices will still be fragile due to recent warm temperatures and initial rainfall Tuesday morning. Give cornices a large margin of safety and avoid travel on slopes below, as cornice failures may trigger large avalanches on slopes below.
Minimize or avoid exposure below unsupported slopes as the potential for Glide Avalanches will increase during prolonged rainfall and warm temperatures.
Snowpack Discussion
Monday was warm with filtered sunshine. The dry weather followed the strong weekend storm that caused rain Saturday to high elevations followed by cooling and deep new storm snow of 1.5-2 ft over the Mt Hood area by Sunday afternoon. There will be a strong difference on new snow amounts with elevation as temperatures were near freezing in the lower terrain below treeline. The new snow was transported by periods strong or very strong winds that built fresh Wind Slabs on a variety of aspects, especially below ridges. The recent storm built fresh cornices along exposed ridges.
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
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.
Mt Hood Meadows pro patrol reported 8 inch average soft slabs being easily released with ski cuts during avalanche control Sunday morning. In lower elevation terrain from 4500-5400 ft, the new storm snow of 10-14 inches was very reactive to ski trigger with avalanches running long distances and gouging into old wet snow.
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.
Likelihood
Expected Size
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.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
Valid until: Apr 10th, 2018 12:09PM
