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 22nd, 2017–Dec 23rd, 2017

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

Regions

.

Recent storm snow from the past week continues to settle and strengthen. Watch for areas of wind transported snow as winds increase and change direction Saturday. Due to the uncertainty that still exists around the distribution and sensitivity of buried persistent weak layers, choose terrain to avoid the lower likelihood/high consequence of a large persistent slab avalanche.  

Detailed Forecast

Cold and fair weather Saturday will continue to allow storm snow weakness to gain strength. Watch for areas of newly formed wind slabs in exposed terrain especially near and above treeline. Winds are expected to increase and shift ahead of the approaching system Sunday causing wind slabs to form on a variety of aspects. Take the time to identify areas of wind loaded snow near ridgelines and mid-slope cross loaded features. 

Lots of uncertainty exists around the distribution and sensitivity of the 12/15 layer. Limited observations on this layer make it difficult to forecast. When uncertainty goes up, terrain selection can go down to maintain wide margins of safety. Time and patience will allow this layer to gain strength. 

Snowpack Discussion

A few inches of fresh snow fell during  the day Friday adding to the snow totals for the past week. One to three feet of recent snow now sits atop the 12/15 crust/persistent weak layer. Weaknesses within the storm snow observed over the past week are gaining strength leading to an overall stabilizing trend.

Ridge top winds Wednesday and Thursday redistributed snow in exposed and wind prone areas forming wind slabs on a variety of aspects. Limited information about the location and sensitivity of these winds slabs has been received. 

A large amount of uncertainty exists around the 12/15 crust/persistent weak layer. This layer has generally been observed at elevations below 6000 feet. Snowpack test in a few locations continue to demonstrate results and the potential for an avalanche to propagate. The limited amount of incoming information makes it difficult to paint a clear picture of this potential avalanche problem. 

Snowdepth decreases substantially the further east from the Cascade crest. In many areas below treeline, there is not enough snow to present an avalanche hazard. 

Observations

North

Observations from NCMG throughout the week continue to support a stabilizing upper snowpack. Reports show the 12/15 layer/PWL down 16-24 inches and reactive to snowpack tests. They observed moderate wind transport along ridges on Friday. 

Central

On Thursday, professionals on Dirtyface Peak observed the 12/15 layer/PWL interface reactive in PST and ECT tests at 5000 ft on an east aspect. The interface was 2.5 ft down. Wind transport was occurring near and above treeline and actively loading lee slopes.  Recent storm instabilities were healing. 

South

No recent 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.

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.