Avalanche Forecast
Issued: Dec 8th, 2017 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
Pay particular attention for lingering wind slab on west aspects in the central west and southwest Cascades. Be wary of small loose wet avalanches on solar slopes near terrain traps.
Summary
Detailed Forecast
Little change should be seen on Saturday with mostly sunny weather, warm temperatures at higher elevations, light alpine winds, with inversions and cool temperatures in the Cascade passes.
Especially in areas where the temperatures have become so mild, the snowpack should continue to gradually consolidate and strengthen, and the avalanche danger should continue to gradually decrease Friday to Saturday.
Along the Cascade west slopes recently formed wind slab will linger on non-solar slopes. Pay particular attention to west aspects in the southwest Cascades. Wind slabs are possible on more varied aspects in the passes. Wind slabs are expected to be stubborn or difficult to trigger. Watch for areas of firmer, wind-transported snow.
Snow on steep solar slopes should have mostly slid or consolidated in sunny warm weather late in the week. But continue to watch for loose wet snow on steep solar slopes especially near sun warmed terrain features.
Early season terrain hazards still exist, such as poorly covered rocks, vegetation and creeks, particularly at lower elevations.
Snowpack Discussion
A series of frontal systems from post-Thanksgiving through the first weekend in December produced about 1.5 to 4.5 ft of snow along the west slopes of the Cascades with the most falling at Mt. Baker. There are no snowpack concerns below the Thanksgiving rain crust.
An upper ridge this week has brought sunshine and greatly warming temperatures at higher elevations. Meanwhile an inversion and offshore surface flow is causing cool temperatures in the Cascade Passes and at low elevations along the Cascade east slopes.
The most recent active wind loading was by E-SE winds mid week in the Mt. Rainier area where there was more snow available for transport near and above treeline and to a lesser extent in the Cascade Passes and the Crystal area. These winds winds generally loaded westerly aspects.
Observations
North
After an active weekend, the Baker pro-patrol reported little avalanche activity Wednesday. Natural pinwheeling and small point releases were noted on steeper solar slopes. Wind loading reported above treeline on Tuesday was not observed on Wednesday.
On Thursday the Baker pro-patrol reported appliance sized chunks of snow and ice releasing from rocks in the sun, but no other activity.
Central
By Tuesday, observations in the Alpental valley indicated sun and warming were quickly creating moist surface snow with several small loose-wet slides releasing from rock or unsupported features exposed to direct sun.
A report via the NWAC Observations for Stevens for Tuesday indicates right side up snow on some non-solar slopes, moderate to hard tests in recent snow and a good bond to the T Day crust.
By Thursday and Friday the Alpental pro-patrol minor roller balls and point releases on steep solar slopes and cold snow on non-solar slopes. Good bonds were seen on the Thanksgiving crust down 67 inches near mountain top.
Several reports on Turns All Year the past few days through Friday indicate good snow conditions on non solar slopes.
South
NWAC observer, Ian Nicholson, traveled in the Crystal back country Tuesday, finding new shallow wind slabs that had formed overnight from increasing easterly crest level winds. These were small and isolated to features near ridges, causing terrain choices to be altered to avoid exposure to obvious wind loaded pockets. By afternoon, surface snow had become moist on any solar facing slopes, from E-SW, ending further wind transport.
Wind transported snow was a different story in the Paradise area on Tuesday. A snowboarder triggered a wind slab on a west aspect of Panorama Point that ran about 250 vertical feet and had some larger blocks in the debris pile (see picture below). A natural loose wet avalanche was also reported in the same area on a steep solar aspect and ran roughly 200 feet. By Wednesday NPS rangers had observed the path has already filled back in due to ongoing transport by east winds. Significant wind transport was also reported in the nearby Tatoosh range and wind effect was noted down to the Paradise parking lot.
Photo Seth Waterfall NPS, MRNP Panorama Point on 12-6-17. Avalanche occurred 12-5-17.
The Crystal pro-patrol on Wednesday reported east winds has stripped snow along windward ridges but little snow remained for significant transport. Settled powder remained on northerly aspects not wind buffed.
On Friday the Crystal pro-patrol reported only a few roller balls on steep solar slopes that were not entraining snow, with some previously scoured areas getting melted to the ground.
An NWAC avalanche class on a west aspect of Mazama Ridge near Paradise on Friday found a strong snowpack, with 65 cm of snow and good bonds on the Thanksgiving crust. Previous wind transport was obvious above but the above treeline was not visited.
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: North, North East, East, West, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Loose Wet
Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Aspects: South East, South, South West.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Dec 9th, 2017 10:00AM
