Avalanche Forecast
Regions: Mt Hood.
Dangerous avalanche conditions persist at higher elevations following strong storms over the past two days. Warming, precipitation, and strong winds mean you could trigger a large slab avalanche Thursday, particularly in high elevation start zones.
Discussion
On Tuesday morning, Mt. Hood Pro Patrol found high avalanche danger at all elevations. One notable slide occurred naturally on a path that normally only goes in east wind loading events. Given our predominant southerly and southwesterly winds during the recent storm, this event was surprising and suggests the possible reactivity of persistent grain types (although the bed-surface of this slide was not identified due to poor visibility).Â
Weak snow formed and buried earlier in the month can still be found in the upper snowpack. On Monday, Mt. Hood Meadows pro-patrol found buried surface hoar most intact in sheltered locations from 5500 to 6500 ft with the surface hoar layer below 5500 ft. While this layer was preserved intact 16-20" below the snow surface, it was neither reactive in snowpack tests nor involved in any recent avalanche activity. Several rain crusts were found above this layer. We will continue to track this layer and there is a small chance that with loading or rain it could produce a wet slab.
Snowpack Discussion
Regional Synopsis: December 19, 2018
Why has the avalanche danger been so high for so long? Two reasons: A very active and wet weather pattern and widespread persistent weak layer.
For perspective, the approximate snow totals from 12/9-12/19 are:
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Mt Baker: 102â€
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Washington Pass: 55â€
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Stevens Pass: 76†mid-mountain
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Snoqualmie Pass: 68†mid-mountain
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Crystal Mountain 70†Green Valley
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Paradise: 78â€
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Mt Hood Meadows: 44†mid-mountain
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Olympics: 48â€
In many areas, a layer of buried surface hoar and/or weak sugary facets was buried on December 9th. This layer has been the cause in numerous natural, explosive, and skier triggered avalanches. As this layer gets deeper it gets harder to assess … in short, the scenario is tricky and getting more dangerous by the day.
Higher snow totals along the Hwy 542 corridor /Mt Baker area have driven several avalanche cycles during this period. Loading from recent storms has been more incremental to the east and south and we have not yet experienced a widespread cycle …
When will we reach the breaking point? It’s hard to say. What we do know is we have a deep weak layer, reports of very large explosives triggered slides at Mission Ridge and Crystal Mountain, and more storms on the way.
Be patient and continue to stick to lower angle slopes with nothing above you. Â This is a good time to avoid areas where avalanches can start, run, and stop.
We’d like to thank all of you who have sent NWAC your observations. If you are out in the mountains, let us know what you see.
Avalanche Problems
Wind Slabs
Recent and expected strong winds, precipitation and warming will maintain dangerous and large wind slabs that will be reactive to human trigger. Natural avalanches are possible and may come down from the higher slopes on Mt. Hood. Watch for scoured ridge-lines, rounded snow pillows, and sastrugi as confirmation that wind slabs are present in the terrain.
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: Possible
Expected Size: 1 - 1
Loose Wet
Warming and rain to above treeline will cause small wet loose avalanches. If enough recent snow is available, some of these avalanches could be large enough to cause injury.
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: Likely
Expected Size: 1 - 1