Avalanche Forecast

Archived

Feb 11th, 2019–Feb 12th, 2019

Alpine

Natural and human triggered avalanches likely.

Treeline

Natural and human triggered avalanches likely.

Below Treeline

Natural and human triggered avalanches likely.

Alpine

Natural avalanches possible, human triggered probable.

Treeline

Natural avalanches possible, human triggered probable.

Below Treeline

Natural avalanches possible, human triggered probable.

Regions

.

Heavy snowfall and winds, combined with a unique and scary snowpack structure will create very dangerous avalanche conditions on Monday night into Tuesday. The unusually cold and snowy storm pattern continues to stress a deeper weak layer, even at low elevations, and a natural avalanche cycle is likely to occur. Travel in, or underneath avalanche terrain is not recommended, as avalanches may run large from upper elevations and well into the runout zones. Very large avalanches may also be triggered from below.

Discussion

Snow and Avalanche Discussion:

Observers continue to report multiple collapses on a wide range of aspects and elevations. Monday's information leads us to believe we are headed for a natural avalanche cycle, and it may be a big one. A report of a huge, tree shaking collapse at Squilchuck State Park came in. Two from Blewett Pass near Tronsen Meadows that extended a long distance. Another on the top of Tumwater Mountain was said to have failed below the ice lens that is found in the Icicle and north of Leavenworth. This one was also reported to be very large, and snow crystals shot up, as the air space beneath the crust compressed. On Snoqualmie Pass, a skier remotely triggered an avalanche on Monday that failed on facets over a crust. A skier was able to trigger a small slab that ran on facets over a crust near Lanham Lakes on Stevens Pass. These signs are bullseye pieces of information. Ding, ding, ding...

Prior to the storm cycle that started on February 8, the very cold temperatures turned a layer of low density snow into facets. These facets rest over a mostly supportable crust, and are fairly widespread. These facets over a crust have been documented across the region, and we may be reaching critical load for this layer to wake up in a more widespread fashion. As the fluffy snow continues to pile up and the winds blow, it is becoming more and more cohesive. The size of the potential avalanches are increasing, and once the slab begins to communicate a fracture across the terrain, watch out. This is a fairly unusual situation for the Cascades, and one to be taken very seriously as the snowfall and winds picks up Monday night into Tuesday. Tomorrow is not a good day to venture out into avalanche country, whether it be the Salmon la Sac drainage, Blewett Pass, Icicle Creek, or the foothills of Wenatchee.

Facets are angular snow grains that bond poorly to other grains around it. Once formed, buried, and preserved, they tend to persist in the snowpack. They are often the culprit of deep and destructive avalanches that result in accidents.

Problems

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.