Avalanche Forecast
Regions: Olympics.
Increasing storm and wind slab layers should become sensitive to human triggers Sunday, especially where these are poorly bonded to a smooth underlying crust layer. Best to avoid most steeper avalanche terrain and terrain connected to large open avalanche terrain above. Continue to choose terrain conservatively and avoid large open avalanche terrain as the concern for a deep persistent slab remains, even though it is a low frequency, unlikely outcome, the consequences are high.
Detailed Forecast
Increasing snow Saturday night with gradual warming Sunday and further increasing moderate rain and snow is expected.  The initial cold low cohesion snow should form a poor bond to the surface crust in most areas.  Increasing strong wind, rising temperatures with heavier precipitation rates Sunday should all combine to cause a significant increase in the danger Sunday. Â
New unstable storm layers should form on most terrain near or below treeline with unstable slabs forming on open lee terrain above treeline. Sensitive triggered slab avalanches should become likely by later Sunday and potentially run far and fast on a smooth hard crust layer.  Therefore terrain choices should be very conservative as dangerous avalanche conditions are expected.  Avoid steep open slopes, slopes connected to steep open slopes above and lee slopes receiving wind deposited snow. The best choices should be lower angled terrain supported by denser trees.  Also watch for recently formed large cornices along ridges and give them a wide birth and safety margin.  Some of these cornices have grown large over the last 2 weeks and a cornice failure has the potential to trigger a large avalanche on the persistent weak layer from late January where present.Â
Snowpack Discussion
Recent Northwest Weather
An extended mild and dry period occurred the latter half of January generally forming a crust in all areas. Very cold weather followed in early February which produced light low density snowfall, near surface faceting, and faceting near and just above the crust.
Weaker frontal systems last weekend followed the frequent strong storms over the past 2 weeks.  The extended storm cycle wrapped up Monday night, capping off impressive amounts of some 6-7 feet of new snow with a rain event that reached to near 7000 feet in the Hurricane Ridge area and likely other Olympic areas.Â
Much of the week saw sunshine and some very mild temperatures, especially Tuesday through Thursday with Hurricane Ridge high temperatures reaching into the 40's all 3 days. Friday began some cooling with colder air arriving Saturday along with gradually increasing ENE winds.  Significant snowpack settlement of some 14-16 inches has occurred through the week, especially below treeline during the warm stretch.
Observations for Hurricane Ridge area
Following the storm cycle, the deep cold snow layers took a dramatic turn Monday in the near and below tree line zones as rain returned and fell through Monday night. The generally light to moderate rain that fell through late Monday night quickly changed the recent low density dry snow to wet snow conditions.  There were many field reports in  the Hurricane Ridge area Tuesday through Friday giving evidence of widespread wet snow avalanches running on most steeper slopes and a variety of aspects. The majority of these slides released Tuesday with less each day through Friday.
The latest observation on Friday, Feb 28th by NWAC pro observer Tyler Reid comes from Klahhane Ridge to the east of Hurricane where plenty of wet unconsolidated snow was found on solar aspects below tree line from 4400-5600 feet.  No strong layers were seen in this area with easy pole probing to the ground.  Above 5600 feet, 3-5 cm of new snow existed over a thin crust on solar aspects with a 5 cm supportable crust on shaded slopes.  Some small surface hoar growth was noted in places.  There was much evidence of the recent rain an warming with runnels found on all aspects with many small size 1 loose wet slides. For the latest conditions report from that area see this video from Friday.
By Saturday morning a park ranger reported very hard surface crust had formed as a result of the significantly colder overnight and early morning temperatures.  The hard new surface crust layer should not present itself to good bonding for new snow as it begins accumulating overnight and into Sunday.Â
The late January crust layer and associated weak layers of early February are now deeply buried by all the new storm snow and were producing areas of collapsing and large whumpfing last weekend prior to the warm up and rain.  These conditions were found in a NE meadow below treeline by a frequent back country traveler in the Hurricane Ridge area last weekend. The same skier had pictures of some very large cornice collapses.  While triggering a slide on this layer has become much less likely, we are still urging more conservative travel plans as the consequences of a slide on that layer could be devastating.
The mid and base pack around the Hurricane Ridge area may still consist of some stable crusts and melt form layers from periods of warm weather earlier this winter.  However the the report from Friday on solar aspects below treeline indicating full depth unconsolidated wet snow indicates not all areas are maintaining deeper crust layers.  However the current cold temperatures will begin to refreeze that deeper wet snow as well.Â
Avalanche Problems
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.
Aspects: North, North East, East, North West.
Elevations: All elevations.
Likelihood: Likely
Expected Size: 1 - 1
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: North, North East, East, South East, West, North West.
Elevations: Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 1
Deep Persistent Slabs
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 or near the ground. 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. They commonly develop when Persistent Slabs become more deeply buried over time.
Deep Persistent Slabs avalanches can be destructive and deadly events that can take months to stabilize. You can trigger them from well down in the avalanche path, and after dozens of tracks have crossed the slope.
A snowboarder triggered this Deep Persistent Slab near treeline, well down in the path.
Deep, persistent slabs are destructive and deadly events that can take months to stabilize. You can triggered them from well down in the avalanche path, and after dozens of tracks have crossed the slope. Give yourself a wide safety buffer to handle the uncertainty, potentially for the remainder of the season.
Aspects: All aspects.
Elevations: All elevations.
Likelihood: Unlikely
Expected Size: 1 - 2