Avalanche Forecast
Issued: Dec 23rd, 2019 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Persistent Slabs.
NWAC,
Northwest Avalanche Center
You can trigger large and dangerous avalanches on old weak layers of snow. Use caution near slopes 35 degrees and steeper and avoid lingering below steep terrain. The western half of the zone, near the Cascade Crest, holds the most dangerous conditions after a major winter storm hit the area last week.
Summary
Discussion
Observers at Holden Village and in the Icicle Creek drainage reported a cycle of large, natural avalanches on Friday and Saturday. These avalanches were easily big enough kill a person (up to size D2.5). Some broke widely across the terrain, ran on weak old facets, and may have been triggered remotely. The avalanche activity and snowfall were more significant in the western half of the zone, closer to the Cascade Crest. Snow totals ranged widely across the zone. Mission Ridge received less than 5” of snow while areas near to the Cascade Crest received up to 7” of water equivalent and now hold over 3 feet of settled storm snow on the ground. Rain fell at many locations Saturday and Sunday up to 5,000ft.
Snowpack Discussion
New Regional Synopsis coming soon. We update the Regional Synopsis every Thursday at 6 pm.
Problems
Persistent Slabs
You may be able to trigger dangerous and surprising avalanches even from below or on adjacent slopes. Triggered avalanches could be big enough to kill you and could break widely across the terrain. Use caution on or near slopes 35 degrees and steeper. Put a large buffer of space between you and any avalanche terrain. Watch out for rocky terrain, steep convexities, and unsupported slopes where you can trigger avalanches more easily.
Be observant by looking and listening for signs of avalanche danger. Some warning signs are: recent avalanches, whumpfing collapses, and cracks shooting through the snow. Take time to dig in the snow and look for older, weak layers. You can find a layer of facets, formed in November and capped by a thin crust, in the bottom foot of the snowpack. You can use snowpack tests to help identify this layer.
Release of a cohesive layer of soft to hard 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 Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Aspects: North, North East, East, West, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Dec 24th, 2019 10:00AM
