Avalanche Forecast
Issued: Dec 12th, 2018 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
Avoid open slopes greater than 35 degrees where you will be able to trigger avalanches. You are most likely to encounter dangerous avalanche conditions at higher elevations, in locations affected by wind, and in areas that receive more than 8 inches of new snow.
Summary
Discussion
We have received reports of a few large avalanches in the Crystal area. An NWAC professional observer in the Crystal area experienced whumphing, shooting cracks, and found weak old snow (facets and/or buried surface hoar) in the snowpack. These are all easy to interpret signs of unstable snow. While a high degree of uncertainty exist around the distribution this weak layer, more snow Wednesday night and Thursday combined with a weak snowpack means you will be able to trigger 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.
Problems
Storm Slabs
You will be able to trigger new storm slab avalanche in areas receiving more than 8 inches of new snow Wednesday night into Thursday. This is most likely to occur around Mt Rainier. In areas where less snow falls, the avalanche danger may not be as high. In some locations wind will form stiffer surface slabs. These firm slabs may feel hollow and can break above you. Give them a wide berth.
In some locations weak buried facets and/or surface hoar may exist. If avalanches fail on these layers they may act in suprising ways. You are most likely to find old weak snow above 6000’ and in sheltered terrain. If you experience shooting cracks, hear whumphig, or see recent large 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
Expected Size
Loose Wet
You will see loose wet fan shaped avlanches Thursday as temperatures warm and precipitiaton 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
Expected Size
Valid until: Dec 13th, 2018 10:00AM
