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Avalanche Forecast

Archived

Jan 22nd, 2016–Jan 23rd, 2016

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

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Improving trend, however, continued cautious routefinding and conservative decision-making remain the proper travel advice, until the persistent slab problems can be ruled out. 

Detailed Forecast

Cooling overnight Friday and Saturday with a weak system, depositing shallow new snow in showers Saturday with light winds, should all lead to an overall stabilizing snowpack. Any previously wet or moist surface snow may begin to form a thin surface crust, locally decreasing danger. Further continued snowpack settlement should also allow for an improving trend.

Continued cautious routefinding and conservative decision-making remain the proper travel advice, until the persistent slab problems can be ruled out. 

Snowpack Discussion

Weather

Two fair weather periods earlier this month allowed surface hoar and near surface faceting to occur. These persistent weak layers were buried intact on Jan 3rd and 11th and have been reported through Monday from the Methow, the Icicle Creek area, the Blewett Pass area, the Nason Ridge area and the Chiwaukum range east of Stevens Pass. It's safe to say this layer is extensive throughout the east slopes.

A progressive weather pattern this week added about .5-1 feet of snowfall to the east slopes from about Friday to Sunday. Another front deposited up to 10 to 14 inches of new snow recorded at the Hart's Pass snotel and Holden Village respectively, with 7 inches at Blewett Pass by Thursday morning. Temperatures warmed above freezing to near or above 6000 feet along the east slopes during the day Thursday. WSW ridgetop winds stayed strong at Mission all well. 

Snow and Avalanche Observations

NWAC pro observer Jeff Ward on Sunday found the reactive persistent weak layer on shaded slopes up to about 5000 feet in the Early Winters drainage. A video by Jeff shows a similar persistent weak layer and a remotely triggered avalanche in the Icicle Creek Drainage near Leavenworth on Monday.

On Monday NWAC pro observer Tom Curtis was on Diamond Head near Blewett Pass in the 4-6000 foot range and found a persistent weak layer at 30-35 cm on north to east aspects with propagation indicated via PST and ECT tests. Tom's video is here.

NCMG on Monday found several persistent weak layers near Goat Peak at 6400' on a north aspect, reactive to column test and sensitive to human triggering.  

Remotely triggered avalanches on persistent weak layers were reported in TAY in the Mission Ridge area Monday. 

NWAC pro observer Tom Curtis visited the Rainy Creek drainage of the central-east zone (near Jove Peak) Tuesday and traveled from 2000-5800 feet and did not find any signs of lingering persistent weak layers.

NCMG found touchy conditions below treeline near Delancey Ridge a week ago Friday, on moderate slopes, with sensitive slabs releasing on the Jan 11th weak layer about 60 cm down. They also heard natural avalanches in their general vicinity. 

With the recent precipitation and warming temperatures experienced late this week, Thursday and Friday, it remains an uncertainty about just how sensitive the persistent layer currently is. It seems likely there are warring factions working to in opposition with a new moist or denser slab adding stress to that layer, while overall snowpack settlement works to stabilize that layer. Best to treat this as a real threat in tour planning and err on the side of caution.   

Problems

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.

Wind Slabs

Wind Slab avalanches are the 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.

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). 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.