Avalanche Forecast
Regions: Cascades - South West.
Pay particular attention for wind slab on west aspects in the southwest Cascades. Be especially wary of small loose wet avalanches near terrain traps.
Detailed Forecast
Expect some high clouds and slightly cooler temperatures on Friday with light winds at higher elevations. Some low clouds with slightly less east winds should be seen in the Cascade passes.
The avalanche danger should slightly decrease Thursday to Friday.
In most areas along the Cascade west slopes recently formed, mostly small wind slab are most likely to 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 Mt Baker area and 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 on Thursday. Any high clouds and slightly cooler temperatures should diminish the danger of loose wet avalanches on Friday. 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.Â
High pressure during this week has brought plenty of sunshine and increasing temperatures/freezing levels. Moderate cool offshore flow Tuesday to Thursday have locally suppressed temperatures in the Cascades Passes and the Crystal area while areas further west (Baker, Paradise) warmed significantly on Wednesday and Thursday. This has also allowed for gradual snowpack settlement.Â
Recent active wind loading by east-southeast winds was most pronounced in the Mt. Rainier area where there is more available snow for transport near and above treeline and to a lesser extent in the Cascade Passes and the Crystal area. These winds winds will have generally loaded westerly aspects. Â
A variety of snow surfaces now exist due to local wind, sun and temperature effects. Â
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
On Monday, several observations in the Alpental Valley noted recent storm layers were gaining strength and becoming less likely to initiate. Some recent wind effect was noted in the near and above treeline bands. Â
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 test in recent snow and a good bond to the T Day crust.
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 westerly 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. Â
Crystal 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.Â
Photo Seth Waterfall NPS, MRNP Panorama Point on 12-6-17. Avalanche occurred 12-5-17.Â
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: North, North East, South West, West, North West.
Elevations: Alpine, Treeline.
Likelihood: Possible
Expected Size: 1 - 1
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: Possible
Expected Size: 1 - 1