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Avalanche Forecast

Archived

Feb 24th, 2018–Feb 25th, 2018

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural avalanches possible, human triggered probable.

Regions

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A variety of new and old avalanche problems will create dangerous avalanche conditions Saturday. With incoming storms, increasing avalanche danger, and a buried persistent weak layer this is a time to dial back terrain travel. Avoid all steep open slopes where avalanches may start. Use caution if traveling in areas where large or very large avalanches may run or stop.

Detailed Forecast

With several overlapping avalanche problems on Saturday this will be a challenging time to travel in the mountains. Dangerous avalanche conditions will develop Friday night into Saturday morning with newly forming wind and storm slabs. These new problems will develop on top of our buried persistent weak layer. Stay off of steep slopes where avalanches may start. Do not linger in the tracks and runouts of larger avalanche paths. Select travel routes and techniques to minimize your exposure to overhead hazards.

Winds will transport snow near and above treeline forming reactive wind slabs on lee aspects. Visual clues such as blowing snow and fresh cornices indicate wind slabs are nearby.

Storm slabs will develop with additional snowfall Saturday. These will be most reactive during periods of peak snowfall intensity. If you find strong new snow over softer weak snow expect storm slabs. Hand pits and small slope tests can help identify storm slab instabilities as you travel.

Our persistent slab problem has not gone away. Conditions over the past week have not allowed it to significantly heal. As additional snow buries this layer it will become more difficult to trigger. However, if you are involved in an avalanche on this persistent weak layer it could be very large and destructive. You are most likely to trigger a deep persistent slab from shallow areas in the snowpack or when a smaller avalanche steps down into this deeper layer. Continue to avoid large terrain where large avalanche may occur.

If you find loose unconsolidated surface snow, you may be able to trigger a loose dry avalanche.  Even small loose dry avalanche can have dangerous consequences if they push you off a cliff, into a creek, or bury you in a gully.

Excellent travel conditions can still be found on slopes less than 30 degrees that are not connected to steep overhead slopes.

Snowpack Discussion

In general 2-4 feet of soft settled surface snow exists along much of the region. In many areas this snow sits on top of weak sugary facets buried on 2/13. These facets are located just above the most recent robust crust (2/8). Several avalanches occurred on this weak layer Saturday through Monday. Snowpack tests from around the area have generally shown this layer can still fail and produce avalanches. Observations from Stevens Pass, Snoqualmie Pass, and Mt Rainier confirm this persistent slab problem.

Steep SE-SW slopes formed a thin unsupportable surface crust Wednesday and Thursday. On shaded aspects loose surface snow exists in most locations.

Below the 2/8 crust no significant layers of concern exist.

Observations

An avalanche professional at Stevens Pass reported snowpack tests Wednesday indicating the weak facet layer could fail and produce avalanches. These were the most recent of numerous snowpack tests conducted over the last several days. All observations supported the same conclusion.

Several large persistent slab avalanches were triggered last Saturday 2/17 through Monday 2/19 in both the Snoqualmie and Stevens Pass backcountry (Stevens Pass-Hollywood Bowl-2-19). These avalanches failed on facets just above the 2/5 crust.

South

On Friday NPS rangers in the Paradise area reported 3-4 feet of snow over weak facets. No wind transported snow was observed.

NWAC Forecaster Dallas Glass observed a large audible whumph caused by a collapsing weak layer Tuesday near Paradise. Snowpack tests in the area indicated the persistent weak layer could fail and produce avalanches up to 4 feet deep.

No recent observations have been received from the Crystal backcountry. A higher level of uncertainty exists in this area.

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.

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.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.