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Avalanche Forecast

Archived

Jan 15th, 2016–Jan 16th, 2016

Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.

Regions

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Back country travel in avalanche terrain near and above tree line along the Cascade east slopes is not recommended on Saturday.

Detailed Forecast

Active weather and avalanches should be seen this weekend!

A front will approach the Northwest Saturday morning and should cross the Cascades Saturday midday. This will cause increasing winds and moderate to heavy rain or snow with warming on Saturday. A change to rain or snow showers should be seen following the front Saturday afternoon and night.

This weather will build generally upside down snow layers of increasing density near the surface and snow may also change to rain in some areas. About 6-12 inches of upside down snow should be seen est of the crest in especially in the higher terrain bands by late Saturday.

This snow will load the January 3rd and January 11th persistent slab layers were they exist along the Cascade east slopes. This should produce natural or triggered avalanches that could be remotely triggered, propagate around terrain features and bury, injure or kill.

New wind slab will also be very likely mainly on lee north to east slopes.

New storm slab due to the warming trend is also very likely on varied aspects where winds are lighter and snow rapidly accumulates to deeper than a few inches.

Avalanches releasing in near surface layers on Saturday may entrain previous snow producing large avalanches.

 

Snowpack Discussion

Fair weather over the New Year caused extensive surface hoar and near surface faceted snow. This was buried by about 4-13 inches of snowfall along the east slopes from about January 3-6th.

Another fair weather period was seen from about January 7-10th. Surface hoar and near surface faceted snow formed again in many areas during this period.

The latest period of snowfall has been from about January 11-15th with about 4-16 inches along the east slopes.

There has been unexpected dangerous avalanche activity along the west slopes Thursday and today. It looks like persistent layers buried about January 3rd and and January 11th are beginning to get active due to loading from the past couple storm cycles. There is less information for the east slopes but there are always persistent weak layers along the east slopes if they are present on the west slopes.

The latest report via the North Cascades Guides is from Delancey Ridge on Wednesday where easily triggered wind slab of about 20-25 cm was seen on north and south slopes at about 6000 feet which was releasing on a persistent crust layer buried on January 11th. A 20-25 cm storm slab was also remotely triggered on a south slope at about 5000 feet which released either a persistent crust buried on January 11th or on persistent buried surface hoar from January 3rd. Natural avalanches were also heard in the area.

A couple reports are available for Thursday via the NWAC Observations page. Skiers triggered 10 inch slabs on a crust which may be a persistent layer on north slopes at 6000 feet Silver Star Mountain. Snow pits near Leavenworth had a easy to spot persistent buried surface hoar layer at 35-40 cm from the surface.

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.