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Archived

Avalanche Forecast

Dec 12th, 2018–Dec 13th, 2018
Alpine
4: High
The avalanche danger rating in the alpine will be high
Treeline
4: High
The avalanche danger rating at treeline will be high
Below Treeline
3: Considerable
The avalanche danger rating below treeline will be considerable
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate

Strong winter storms are continuing to create very dangerous avalanche conditions. This is the time to stay out of areas avalanches can start, run, and stop. Avalanche danger will rise and peak during times of heavy snow and warming temperatures.

Discussion

Avalanche Summary

We have received reports of natural avalanches in the Mt Baker area for the past three days. There is no simpler sign of unstable snow than recent avalanches. While observations from backcountry travelers on Wednesday showed less reactivity than earlier in the week, reports of whumphing, shooting cracks, and sudden snowpack test results continue to remind us that the snowpack is weak. More snow Wednesday night and Thursday combined with a weak snowpack means avalanches.

Snowpack Discussion

Regional Synopsis 20181211

Welcome winter!!!

The NW snowpack is evolving quickly and a near constant pattern of winter storms with fluctuating freezing levels is making things interesting for our 1st week of forecasting.

Here’s what we know:

  • Tuesday saw a significant avalanche cycle.

  • We have a lot of new snow...2-3’ above 4500ft.

  • We’ve gotten a lot of wind.

  • We have weak layers near the ground.

  • We are going to get more snow before the weekend.

All in all that’s enough to indicate that dangerous avalanche conditions will persist over the next couple of days. That said, it is early season and the flavor of avalanche you may run into is likely a function of elevation, timing, and dumb luck.

Here are some basic emerging patterns:

  • Storm total (Monday morning to Wednesday evening):

    • Mt. Baker: 24”

    • Washington Pass: 22”

    • Snoqualmie Pass: 10”

    • Paradise: 17”

    • Mt. Hood Meadows: 10”

  • Upper versus Lower Elevations: The change in the snowpack is still pretty dramatic with elevation. Height of snow decreases rapidly below 4500’ at Baker and Washington Pass, 5500’ at Crystal/Rainier. The Passes have better low elevation coverage, but it's still pretty thin below 4000’. With additional warm storms in the forecast, this pattern is expected to continue for awhile.

  • East versus West: Loading along the East slope has been more incremental, and a  variety of buried facet and surface hoard layers may be found. This is most pronounced near WA Pass. Although significant snowfalls and precip totals have resulted in thicker, more homogeneous snowpack in the western zones, lingering weak layers near the ground will persist through the week.

Avalanche Problems

Storm Slabs

Significant new snow and a warming will create an up-side-down snowpack, with heavier snow over drier weaker snow. As this occurs it will be easier for you to trigger avalanches. Storm slabs may fail also naturally, especially during periods of high snowfall intensity or as warmer air moves into the area. You will find areas of wind affected snow where stiffer slabs may release above you. Due to the large amount of recent snow this week, some of these avalanches may be very large and travel down into flat terrain.

We have observations of buried facets and surface hoar at the bottom of the recent storm snow. In the Mt Baker area that weak snow was 3 feet or more below the snow surface on Wednesday afternoon. If this layer fails in an avalanche if could release in surprising ways or propagate long distances. In you experience shooting cracks, hear whumphs, or see recent avalanches stay out of avalanche terrain. 

 

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: Very Likely

Expected Size: 2 - 2

Loose Wet

You will see loose wet fan shaped avalanches Thursday as temperatures warm and precipitation turns to rain. The size of these avalanches will be limited by low coverage and rough terrain. Don’t let these little avalanches fool you. They can still have dangerous consequences if they push you over rocks, or into trees and gullies.

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: Likely

Expected Size: 1 - 1