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 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 possible, human triggered probable.

Regions

.

New wind and storm slab should be likely on Sunday. Great caution remains warranted in this area due to the persistent slab.

Detailed Forecast

Decreasing northwest winds, decreasing light snow showers and a cooling trend should be the main story on Sunday. New snow should run in the 3-8 inch range east of the crest.

New storm and wind slab are likely on Sunday. Storm slab will occur in areas that receive more than an inch of snow an hour for several or more hours. For wind slab watch for firmer wind transported snow on lee slopes

The surface hoar will increase storm and wind slab instability where it gets buried intact. You can check for this layer using hand shear tests or shovel tilt tests.

Great caution 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 warm storm moved over the Northwest Tuesday and Wednesday with strong west winds followed by lowering snow levels. It looks like up to about 7 inches was seen in this area such as at Harts Pass at the tail end of the storm.

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 are 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 today. 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 is moving over the Northwest on Saturday and Saturday night causing west winds and moderate snow and snow showers east of the crest and a cooling trend. This should build some new wind and storm slab by Sunday.

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.

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.

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.