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Archived

Avalanche Forecast

Jan 23rd, 2014–Jan 24th, 2014
Alpine
2: Moderate
The avalanche danger rating in the alpine will be moderate
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
1: Low
The avalanche danger rating below treeline will be low
Alpine
2: Moderate
The avalanche danger rating in the alpine will be moderate
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
1: Low
The avalanche danger rating below treeline will be low

Regions: Stevens Pass.

The Bottom Line: The main concern should be isolated areas of wind slab on northerly or lee slopes near and above tree line that may become more sensitive to trigger during the ongoing dry spell.

Detailed Forecast

The main concern on Friday should remain to be small areas of slab layers on some northerly or lee slopes. This should be mainly northwest to southeast slopes near or above tree line, although noted facet/crust combos were recently found on west aspects in the Olympics. Such layers may be hard to trigger but require more caution and analysis before committing to a steep lee or northerly slope.

Freezing levels and temperatures should warm further Friday but light to moderate east winds up to crest level should locally keep conditions cooler and snow surfaces firm where exposed to wind.  Away from the Cascade crest there will be a better chance of small wet loose avalanches.  Avoid steep solar aspects or gullies in the sun if you see surface snow there deeper than a few inches or natural wet loose avalanches.

 

 

Snowpack Discussion

The last storm cycle to impact the Northwest lasted about a week, ending about 10 days ago by January 14th.  The storm produced about 2-4 feet of snow at most NWAC sites near and west of the crest with somewhat lower amounts near Hurricane Ridge. Warming and heavy precipitation towards the end of the cycle caused an upside down heavy dense snow and slab layers to high elevations and caused a widespread natural avalanche cycle.  Crowns of slab avalanches size D2 to D3 are still visible on many slopes throughout the Olympics and Cascades on lee slopes near and above tree-line.  

Over the past 10 days, clear weather and warm temperatures have helped to maintain a variety of surface snow conditions.  Varying amounts of low clouds and temperature inversions kept lower elevations cooler with many strong surface crusts noted. The recent warm weather allowed for additional small wet loose snow avalanches on solar aspects mainly near and above treeline. 

The warm days and cooler nights lately have been producing generally stable surface crusts on solar aspects at all elevations until mid-day softening occurs, and on most other aspects up to mid-mountain where a rain crust exists. A generally favorable mid and lower snowpack of crust layers and melt form crystals exists throughout the west slopes. The avalanche danger is low at lower elevations where there is little snow and and ample terrain and vegetation anchors. There is little snow cover on solar aspects even into the near tree-line zone near Hurricane Ridge. (see photo below). 

Recent layers of concern: On northerly or shaded aspects near and above treeline not affected by the warm temperatures or weak January sun small areas of surviving slab layers may still be found. Strong radiational cooling at night has increased temperature gradients and faceting in the upper snowpack.  NWAC observers at Hurricane and Nason Ridge (near Stevens Pass) the past couple days are reporting moderate or hard compression and extended column results with sudden collapse or sudden planar fracture characteristics. These results are found in the upper snow pack, as far down as 65 cm near Nason Ridge, releasing due to crusts and thin faceted snow layers. Due to the moderate and hard results, weak layers, would likely require larger triggers, perhaps a cornice or a snow mobile but are becoming more likely to propagate across the slope over time. The upper snowpack is becoming more spatially variable during this ongoing dry spell. If future loads buried these weak layers these may be classified as persistent slabs. Widespread reports of surface hoar growth this week will have to withstand additional sunshine, warm temperatures and easterly winds to become an issue when we finally receive more precipitation. 

ECTP29 SC Q1 at 67 cm due to crust and facet layers at Nason Ridge, NWAC observer Tom Curtis, 20 January.

Faceted crystals from the upper snow pack, Klahhane Ridge near Hurricane Ridge, NWAC observer Katy Reid, 20 January.

Quite a difference on north and south slopes in the vicinity of Hurricane Ridge,NWAC observer Katy Reid, 20 January.

Report of the cornice/avalanche fatality on Lewis Peak available at https://www.nwac.us/accidents/accident-reports/ 

Local non-avalanche hazards:  On 13 January a heavy local freezing rain event occurred in the Snoqualmie Pass area above about 4500 feet, covering the surface with about a 2-3 inch ice crust.  This crust should break down over time but a fall on a slope on this surface could result in a long and dangerous ride!  Additionally, guides in the North Cascades have noted some thin bridging over several crevasses on lower glaciers.  During this shallow snow season, any thin and potentially weak bridges over crevasses may present an additional danger for those travelling on glaciated terrain.     

Avalanche Problems

Wind Slabs

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: North, North East, East, South East, North West.

Elevations: Alpine, Treeline.

Likelihood: Possible

Expected Size: 1 - 1

Loose Wet

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: South East, South, South West.

Elevations: Alpine, Treeline.

Likelihood: Possible

Expected Size: 1 - 1