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Archived

Avalanche Forecast

Feb 23rd, 2014–Feb 24th, 2014
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
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
3: Considerable
The avalanche danger rating below treeline will be considerable

Regions: Olympics.

If enough warming and rain is received on Monday....loose wet avalanches on all aspects into the treeline will be the primary concern. Also watch for sensitive shallow new storm slab formed Sunday afternoon and evening and give recently formed cornices a wide berth and avoid travel under slopes with large overhanging cornices.  

Detailed Forecast

A frontal system passing through Sunday afternoon and evening should produce generally light to moderate amounts of new precipitation at moderate snow levels with minor wind transport.  Another warm front should lift through the area Monday afternoon but with light rain and snow expected.

Snow levels should continue to creep up overnight and through Monday...extending the reach for wet loose avalanches into the near treeline zone on all aspects with light rain saturating the most recent storm snow potentially up to 5500 feet Monday. Watch for heavier surface snow that begins to naturally pinwheel on steeper slopes.  Wet-loose avalanches may start small but entrain large amounts of snow.  Avoid terrain traps and steep slopes and watch for signs of natural wet loose activity.   

Cornices have grown large over the last 2 weeks. A cornice failure could provide a large enough load to trigger a destructive avalanche. With the warming Monday...rising temperatures and rain should cause cornices to become even more sensitive and likely to fail.  

Continue your careful snow pack evaluation and cautious terrain selections during the work week.

Snowpack Discussion

Recent Northwest Weather

An extended mild and dry period occurred the latter half of January generally forming a crust in all areas. Very cold weather followed in early February which produced light low density snowfall, near surface faceting, and faceting near and just above the crust.

Frontal systems over the weekend were weaker than the frequent systems seen over the past 2 weeks.  Until we have an extended break in the storm cycle...the running tally for water equivalent over the past 13 days is about 8-10 inches with snowfall about 6 - 7 feet at the NWAC weather station at Hurricane Ridge and the Waterhole Snotel.

Observations for Hurricane Ridge area

Several avalanche cycles have occurred over the last 2 weeks. The last natural cycle likely occurred during the middle of last week when the road to Hurricane Ridge was closed. NPS employees found avalanche debris up to 8' deep just off the Hurricane Ridge road at 3600 ft while plowing Thursday. The start zone was likely in the near-treeline zone on a S-SE aspect. 

The late January crust layer and associated weak layers are now deeply buried by all the new storm snow, but still producing areas of collapsing and large whumpfing as reported in a NE meadow below treeline by a frequent TAY-er in the Hurricane Ridge area. The same skier had pictures of some very large cornice collapses.

On Friday NWAC observer Katy Reid Friday found stubborn but still possible to ski trigger wind slab on northerly aspects above treeline. She also observed quickly releasing wet loose avalanches on below treeline on solar aspects reacting to the strengthening February sunshine. Finally, new surface hoar around 4mm thick formed on all wind and sun sheltered aspects near and below treeline and may have been buried by the light new snow with little wind Saturday.  If buried intact...this layer could become an active interface with shallow new storm snow slab becoming denser Monday. 

Road side roller balls and small wet loose on solar slopes at Hurricane on Friday by Katy Reid.

The mid and base pack at Hurricane should still consist of mostly stable crusts and melt form layers from periods of warm weather earlier this winter.

Avalanche Problems

Cornices

Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind lips of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.

 

Cornices can never be trusted and avoiding them is necessary for safe backcountry travel. Stay well back from ridgeline areas with cornices. They often overhang the ridge edge can be triggered remotely. Avoid areas underneath cornices. Even small Cornice Fall can trigger a larger avalanche and large Cornice Fall can easily crush a human. Periods of significant temperature warm-up are times to be particularly aware.

 

A corniced ridgeline. A large cornice has formed at the top of the ridge. A smaller cornice has formed to the left of the trees from crossloading.

Cornices are easy to identify and are confined to lee and cross-loaded ridges, sub-ridges, and sharp convexities. They are easiest to trigger during periods of rapid growth (new snow and wind), rapid warming, and during rain-on-snow events. Cornices often catch people by surprise when they break farther back onto flatter areas than expected.

Aspects: North, North East, East, South East.

Elevations: Alpine, Treeline.

Likelihood: Likely

Expected Size: 1 - 2

Storm Slabs

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: Alpine, Treeline.

Likelihood: Possible

Expected Size: 1 - 1