Avalanche Forecast
Issued: Apr 13th, 2018 12:07PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Storm Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
Dangerous avalanche conditions exist in the Mt. Hood area on Saturday and you may trigger a variety of reactive avalanche problems with strong winds forming large Wind Slabs, upside-down and intense snowfall Friday creating large Storm Slabs, and rainfall saturating the snow surface at lower elevations creating the possibility of Loose Wet avalanches. Avoid open slopes greater than 35 degrees, particularly near and above treeline. Also avoid exposure to large avalanche paths where a slide from the the upper slopes of Mt. Hood would be deadly.
Summary
Detailed Forecast
Very large avalanches may release from the upper elevations of Mt. Hood. Avoid large avalanche paths were avalanches may come down from the upper elevations of Mt. Hood. Check a map to ensure flat slopes are not connected to larger terrain before crossing open areas.
A relative break in the intense weather will not allow sufficient time for a variety of avalanche problems to heal. Dangerous avalanche conditions exist near and above treeline on Saturday.
Large Wind Slabs formed Wednesday afternoon through Friday during periods of heavy precipitation, strong winds, and warming temperatures. These slabs will continue to be reactive near and above treeline on Thursday. Avoid wind-loaded aspects steeper than 35 degrees. You are most likely to trigger a Wind Slab on NW through SE aspects below cornices, on convex roll overs, or on cross-loaded features.
You can trigger a storm slab as well from the top of the below treeline band upward on Saturday. These formed during periods of heavy precipitation Wednesday thorough Friday. A warming trend on Friday created unstable snow as denser snow sits on top of lower density layers within the 20-24" of recent snow that sits on a crust. You are most likely to trigger storm slabs on unsupported slopes, near convex rollovers, and in very steep terrain.
At lower elevations, rain on Friday created wet surface snow conditions. If you see new rollerballs, fan shaped avalanche debris, or experience periods of intense rain, stay off of slopes greater than 35 degrees. Loose wet avalanches can entrain additional snow allowing them to grow large. Use caution if you travel in areas where loose wet avalanches may carry you into or off of terrain with higher consequences.
Watch for other spring hazards such as recent cornices along ridges, open glide cracks and opening creeks.
Snowpack Discussion
In general 20-24 inches of new snow fell in the Mt Hood area stations Wednesday afternoon through Friday afternoon, but new snow distribution varied by a factor of 2 due to wind transport. Moderate to strong south to west winds increased to strong on Friday at upper elevations and easily transported the new snow, forming new wind slabs near and above treeline on a variety of aspects and these are likely to be large. On the upper mountain at Mt. Hood, Wind Slabs are likely to be very large. The heavy snowfall rates on Friday also created storm slabs with heavier, denser snow on top of lower density layers.. This most recent round of precipitation fell on a new melt-freeze crust (4/11) formed during warm weather earlier this week. Observations demonstrate the new crust is strong and supportable. The snowpack now consists of a slab (wind or storm) on top of lower density snow, sitting on an older crust. That is a prime recipe for dangerous avalanche conditions in the upper snowpack.
Reports indicate that the snowpack consist of various layers of melt –freeze crusts, refrozen snow, and strong rounds. This has resulted in a generally strong snowpack without any significant layers of concern.
Observations
On Friday, Pro Observer Laura Green traveled in the Newton/Clark drainage between 4500 and 5900 ft. She avoided higher elevation or larger avalanche terrain due to strong winds, heavy snowfall and poor visibility. With warming temperatures during the day, snow surfaces were moist to wet. On a NNE aspect at 5600' she observed up to 2' of upside down snow on top of lower resistance wet snow layers. Although she didn't find a particularly cohesive slab in the location tested, she suspected they exist higher up.
On Friday, Mt. Hood pro patrol reported reactive 6-18" explosive triggered slabs with limited visibility up high. Snow was falling at the base, but it was very heavy and wet.
On Thursday, Mt Hood Meadows ski patrol reported wind slabs ranging from 8 to 12 inches (20-30cm). Wind and storm slabs were reactive during avalanche control work Thursday morning. The most recent melt-freeze crust was reported as strong and supportable.
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
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: Below Treeline.
Likelihood
Expected Size
Valid until: Apr 14th, 2018 12:07PM
