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Avalanche Forecast

Archived

Jan 28th, 2016–Jan 29th, 2016

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

Regions

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The main problem Friday is expected to be wind slab in the above and near treeline zones as well as storm slabs of heavy, dense snow or wet surface snow stressing weaker underlying snow at all elevations. Storm slabs may me masked by the drier, colder surface snow. 

Detailed Forecast

Another front should arrive later Friday, bringing additional moderate snowfall at lower snow levels. Winds should be strong, especially above treeline. The main concern Friday will be newly forming wind slab on lee exposed slopes, especially NW-N-E facing near and above treeline. 

With cooling temperatures, the new snow should have a favorable profile, but may mask older unstable storm slabs.

 

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 were reported throughout the Cascade east slopes.

A warm front last Thursday caused light or moderate amounts of snow which changed to freezing rain or rain east of the crest.

This was followed by cooler weather and some snow. NWAC stations along the east slopes indicate up to about 4 inches for the 2 days ending Sunday morning.

Warm air arrived Wednesday with freezing levels climbing to near 7000 feet.

A strong front moved across the region Thursday, bringing significant warm air and likely causing snow levels to fluctuate greatly before cooling arrived late Thursday. 

This recent warm and wet weather should hopefully have cleaned out the earlier January persistent layers, however, until confirmation, we will still list this as an unlikely problem.  

Snow and Avalanche Observations

A report here on Turns All Year for the Clara Lake area near Mission Ridge on January 17th is dramatic.

See the NWAC YouTube page here for videos from Blewett Pass and Icicle Creek for January 18th.

NWAC pro-observer Tom Curtis was on Iron Mountain near Blewett Pass Sunday and found the January 11th layer 35-70 cm down on NW-SE aspects in the below and near treeline bands. He found that the buried surface hoar crystals are rounding and the tests gave low quality results and did not indicate propagation.

Tom was out again on Table Mountain near Blewett Pass on Monday in the 4-6000 feet elevation range and found the January 3rd and 11th layers well defined down 70 and 50-60 cm and still giving sudden planar results with ECT test indicating propagation. He also noted crowns from natural avalanches that released on the January 11th layer late last week.

Crown of persistent slab avalanche from about January 21-22 by Tom Curtis.

A report via the NWAC Observations page for Stemilt Basin near Mission Ridge strongly indicated that persistent weak layers at about 40 and 60 cm are still present east of the crest with test columns failing on isolation and evidence of previous widespread persistent slab avalanches.

Thanks to those taking the time to submit so many high quality, pertinent observations to the NWAC Obs page here: https://www.nwac.us/observations/ 

Problems

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.

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.