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Avalanche Forecast

Archived

Feb 25th, 2018–Feb 26th, 2018

Alpine
Natural avalanches possible, human triggered probable.
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

Olympics.

New snow and wind created dangerous avalanche conditions. Avoid steep slopes were the wind deposited snow near and above treeline. It may now be easier to trigger a dangerous Persistent Slab avalanche. Stay away from large open slopes steeper than 35 degrees that are suspect for these avalanches.

Detailed Forecast

What a difference a day will make from Sunday to Monday! Following a strong storm passage Sunday, showers should end overnight Sunday with Monday expected to be very cool, light winds and partly cloudy with a few scattered light snow showers possible, but little if any accumulation expected. 

The strong storm Sunday produced widespread wind and storm slabs in the Hurricane Ridge area. These wind and storm slabs will gradually heal and strengthen Monday. 

You could still trigger an avalanche Monday, especially in areas that received significant wind loaded snow, such as steep slopes below ridges, mainly facing North to Southeast. 

Dense storm slabs that have built over weak lower density snow from Saturday will need more time to settle and stabilize. Back country travelers should continue to travel with extra caution and best to travel in shallower angled terrain well away from avalanche paths or runout zones from avalanches releasing on slopes above. 

The persistent slab problem remains a concern, though reports from the WA Cascades indicate it is very gradually healing, but still showing propagation potential in snow tests. Weak sugar-like facets can be found just above a firm crust layer formed in early February on all but south aspects. This layer is down about 2-3 feet, shallow enough to be human triggered. Observations on the distribution and sensitivity of this layer can be difficult to gather. When uncertainty increases, safe terrain choices decreases. Avoid steep open slopes where a persistent slab avalanche may occur.

Be cautious on slopes above cliffs, creeks, and gullies where even small avalanches may have large consequences.

Snowpack Discussion

An active storm cycle began Friday evening. By late Sunday afternoon about 2 ft of new snow had accumulated in the Hurricane Ridge area. The most recent snow Sunday fell with slightly warmer temperatures and was easily transported by winds depositing fresh wind and storm slabs on weak low density snow deposited by early Saturday. 

As of Sunday evening, about 3 or more ft of of settled snow sits on top of the weak sugary facets that formed on a strong crust earlier in February. Snowpack tests continue to suggest that these facets can fail and produce avalanches. This layer has not been reported on South aspects in the Hurricane Ridge area.

There are no other significant layers of concern in the mid and lower snowpack.

One person was killed in an avalanche south of Snoqualmie Pass on Sunday. Our condolences go to the friends and family of the victim. If conditions allow, NWAC staff will visit the accident site. A very large natural avalanche crossed Highway 20 east of Newhalem on Sunday.

Observations

Sunday, NPS rangers indicated the additional new 8 inches of snow was stiffer and more cohesive, being deposited over the previous days weak snow.

On Saturday morning, NPS rangers reported that 8 inches of new storm snow had low cohesion with little wind effects noted and no slab structure.

On Thursday Matt Schonwald and NPS rangers observed 20-30 inches of generally right-side-up surface snow over weaker snow (facets). Snowpack tests indicated the weak facet layer could still fail and produce avalanches. Northeast winds during the day Thursday redistributed snow onto SW-W-NW aspects.

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.

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.