Avalanche Forecast
Issued: Mar 13th, 2019 11:09AM
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
Cloudy skies and even a chance of a passing snow shower will limit the avalanche danger around the West-North region. If you travel to higher elevations, be leery of steep convex rollovers, unsupported slopes, and very steep terrain if you see any sign that the wind affected the snow surface.
Summary
Discussion
The only avalanches reported from the West-North Wednesday were small loose avalanches. Several natural loose wet avalanches occurred on steep sunny slopes, while human triggered loose dry avalanches occurred in very steep shaded terrain. The recent storm snow seems to be bonding well to the old snow interface. The strong March sun made quick work affecting the surface snow. Any slope that received sunshine Wednesday will show signs of a new surface crust.
Thursday’s avalanche forecast is a bit tricky. We expect overcast skies, however, it’s March, and the sun is strong. It won’t take many sun breaks, or thinner clouds to cause conditions to change quickly. Make frequent weather observations, be alert to changes in the weather that differ from the forecast, and be ready to alter your plans if observations don’t line-up with the forecast.
Snowpack Discussion
March 10, 2019
February started stormy and cold with very active avalanche conditions on persistent weak layers. The month finished with continued cold temperatures but drier weather allowed avalanche activity to taper off. Persistent weak layers that were buried early in the month (Feb 8th) are now unreactive, though you can still find the grains.
Aside from the series of storms in early to mid-February, most regions have measured relatively light snow accumulation in the past 3 weeks. Snow has stayed soft especially on shaded slopes and faceting and surface hoar have been plentiful. While there has been plenty of sunshine since mid-February, very cold temperatures have kept melt-freeze crust to a minimum on sunny slopes.
A natural loose wet avalanche (D1.5) on a southeast aspect of Lichtenberg Mtn at 5,400ft. 3/10/2019. Photo: Josh Hirshberg
Recent Avalanches
Our attention shifted to the upper snowpack. Recent avalanche problems have largely focussed on wind transported snow and weak snow surfaces on both dry, shaded slopes and sun-exposed aspects. In some locations, a weak layer of surface hoar and facets was buried on March 6th. We’ll keep an eye on this interface as we move into the future.
A skier-triggered storm slab avalanche (D1.5) on Shuksan Arm, north aspect, 5200ft. 03/09/19 Adam U Photo.
Moving Forward
As we move further into March, there are two points to consider:
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The strength of the March sun: As the sun creeps higher into the sky and the days grow longer, the sun can have a greater impact on the snow surface. When the sun comes out, expect things to change quickly. You may see avalanches conditions change with natural loose avalanches originating from sunny slopes, surface snow becoming thick and heavy, and slabs taking on a moist to wet snow character.
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Weak surfaces: facets and surface hoar have blanketed snow surfaces in many zones. Any significant snowfall will bury well-developed and widespread persistent weak layers. Recently, most storms have not delivered enough precipitation to cause a problem.
Problems
Wind Slabs
If you travel to higher elevations, you’ll need to look and feel for clues that the wind affected the snow. Does the snow feel firm under you? Are there sharp cornices overhanging nearby slopes? Does the snow appear textured? These all indicate that the wind affected the snow. Some of these clues may be hidden by a few inches of soft snow. Be aware of features in the terrain where you are more likely to trigger a lingering wind slab such as near convex rollovers, under cornices, and on unsupported slopes.
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
We don’t often list “unlikely” avalanches in our problem list, but this situation is a bit different. March brings with it a strong sun and freezing levels are expected to reach elevations we haven’t seen since January. It won’t take much to get a loose wet avalanche cycle going. If you see new rollerballs, find new fan-shaped avalanches, or experience moist surface snow, loose wet avalanches may occur. With all the new snow, if the slope is big enough, any loose wet may entrain significant new snow and become large. If you see signs of loose wet avalanches, steer away from steep slopes receiving any sunshine.
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: East, South East, South, South West, West.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Mar 14th, 2019 11:09AM
