Avalanche Forecast
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.Â
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.Â
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.Â
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.Â
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.Â
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.
Avalanche Problems
Wind Slabs
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.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood: Possible
Expected Size: 1 - 1
Storm Slabs
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. 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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Elevations: Treeline, Below Treeline.
Likelihood: Possible
Expected Size: 1 - 1
Persistent Slabs
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, South East, South West, West, North West.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 1 - 1