Dashboard Regions Weather Stations Radar Alerts Glossary
Contact About
Log In

Register for an account and never miss a forecast again!

Register

Avalanche Forecast

Archived

Dec 28th, 2014–Dec 29th, 2014

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

Regions

.

The avalanche danger should decrease with lingering wind and storm slab should be the main avalanche problems on Monday. Great caution remains warranted in this area due to the persistent slab.

Detailed Forecast

Light northeast winds, clouds giving way to some sun and cool temperatures should be seen east of the crest on Monday.This should cause stabilizing of new layers and a decreasing avalanche danger on Monday.

Lingering wind slab will be rated as likely east of the crest on Monday. Watch for firmer wind transported snow on steep lee slopes.

Lingering storm slab may still be possible on Monday so watch for it in any areas that received heavier amounts of snow. Hand shears or shovel tilt tests can be fast ways to check for this type of layer.

Any hoar frost that survived the storm would increase the instability of wind or storm slab layers.

The sun should be out Monday but sun effects should be limited since we are barely past the solstice.

Note that northeast winds may increase Monday night and Tuesday so heads up for new wind slab potential on Tuesday.

Great caution also remains warranted near avalanche terrain in this area due to the persistent slab. Subsequent snowfalls may make this layer tougher to trigger but the increasing slab depth, especially on wind loaded slopes translates to larger avalanches. Slides beginning in new storm layers may possibly step down to this more deeply buried layer. Professional observers and mountain guides are treading carefully in this area with this snowpack structure and enjoying lower angled terrain not connected to large avalanche above.

Snowpack Discussion

A cooler period with little precipitation was seen about Thursday to Friday. Some skiers were reporting better conditions on lower angle slopes with skis scraping crusts on some steeper slopes. Surface hoar is possible from Thursday night.

Observations via the NWAC observers, North Cascade Mountain Guides and the NWAC Recent Observations page have been describing a persistent slab due to a facet/crust from 50-100 cm below the surface in the Washington Pass area. Remotely triggered avalanches were reported there Monday.

Remotely triggered slab avalanche near Washington Pass Monday by CB Thomas.

NWAC observer Tom Curtis was on Nason Ridge Saturday. He also a found a persistent slab with moderate but sudden collapse tests and likely propagation in snowpits due to a similar MF/facet layer at 55 cm on varied aspects.

PSTEnd SC results at Rainy Pass on Nason Ridge on Saturday by Tom Curtis.

A cold front moved over the Northwest on Saturday and Saturday night. West winds changed to northwest, with moderate snow east of the crest and a cooling trend. NWAC and Snotel sites east of the crest picked up about 3-10 inches of new snow by Sunday morning. 

Northwest winds are decreasing east of the crest on Sunday with little or no snow accumulation and cooler temperatures. New slab layers are likely Sunday morning but stability of new layers should have improved by Sunday afternoon.The cooling trend looks like it favored right side up snow and should help limit the extent and longevity of new slab layers.

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.