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Archived

Avalanche Forecast

Mar 10th, 2019–Mar 11th, 2019
Alpine
2: Moderate
The avalanche danger rating in the alpine will be moderate
Treeline
1: Low
The avalanche danger rating at treeline will be low
Below Treeline
1: Low
The avalanche danger rating below treeline will be low
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

Expect increasing clouds, warming temperatures, and strong southwest winds on Monday ahead of an approaching frontal system. You may be able to trigger fresh wind slab avalanches in leeward terrain at upper elevations. Loose avalanches could still run far and fast - use caution near cliffs and gullies where even a small avalanche could be dangerous. 

Discussion

Snow and Avalanche Discussion

Several dry loose avalanches have been reported in steep shaded terrain over the past several days. The strong March sun caused small wet loose slides on solar aspects on Sunday. A natural wind slab (D1.5) was observed recently in steep terrain below a cliff band on a northeast aspect at 7600ft that entrained quite a bit of loose snow and ran 300ft downslope. Several days of cold weather with little snowfall and light winds have allowed the avalanche danger to ease, as the upper snowpack often lacks slab. The old persistent weak layers in the snowpack appear to be rounding and are unreactive in most areas throughout the East-North zone. Be cautious if you venture into areas further east and south of Mazama where you may still find a weaker snowpack structure.

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:

  • 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.

  • 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.

Avalanche Problems

Wind Slabs

Strong and gusty southwest winds are expected on Monday. This will be a change to relatively calm winds over the past several days. In upper elevation areas, winds may drift snow into shallow slabs. Anywhere cohesive slabs exist, they likely rest on weak old snow surfaces and could be sensitive to triggers. Evaluate how even a small slab may be dangerous on consequential slopes. Look for shooting cracks, blowing snow, and fresh cornice formations. Slopes just below ridgelines, exposed gullies, and convex rollovers are the most likely places to trigger a slide. On steep wind sheltered slopes, loose avalanches have been common, and are the most likely avalanche problem to encounter. Be especially careful of wet loose avalanches on solar slopes as sun and warming continue on Monday morning.

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: Possible

Expected Size: 1 - 1