Avalanche Forecast
Issued: Dec 6th, 2017 11:16AM
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
Expect fresh wind slab size and sensitivity to vary considerably near and above treeline Thursday but to generally be found on westerly aspects. The largest potential winds slabs will be in the Mt. Rainier area and steeper wind loaded slopes above treeline should be generally be avoided Thursday. Small loose wet avalanches are possible on steep solar slopes - be wary of them around terrain traps.
Summary
Detailed Forecast
Continued sunny and mild weather is expected Thursday with crest level easterly winds decreasing significantly Wednesday night and becoming light SW by Thursday afternoon. However moderate Pass level easterlies are expected to continue through the day Thursday, only easing later in the afternoon. In general, maximum temperatures will be several degrees warmer than Wednesday with a few NWAC stations outside the Cascade Passes likely pushing past 50 degrees on Thursday afternoon.
Fresh wind slab on westerly aspects will be the primary avalanche problem in the southwest zone (Crystal, Rainier) and the Cascade Passes. Expect fresh wind slab size and sensitivity to vary considerably near and above treeline in these zones but to generally be found on westerly aspects. The largest potential winds slabs will be in the Mt. Rainier area and steeper wind loaded slopes above treeline should generally be avoided Thursday. The Cascade Passes may harbor some unreactive wind slabs on more easterly aspects.
Loose Wet avalanches on steep solar aspects will be of higher concern in the Mt. Baker area. Stubborn wind slab may exist in specific locations near and above treeline in this zone.
In all areas, watch for generally small loose wet avalanches on steep sun exposed terrain releasing during the late morning or afternoon, especially on unsupported slopes or near rocks. Locally cooler temperatures in the Cascade Passes may mute the loose wet potential Thursday.
A variety of snow surfaces now exist due to local wind, sun and temperature effects. 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.
High pressure during this week has brought plenty of sunshine and increasing temperatures/freezing levels. Moderate cool offshore flow (easterlies) on Tuesday and Wednesday have locally suppressed temperatures in the Cascades Passes and the Crystal area while areas further west (Baker, Paradise) warmed significantly on Wednesday. This has also allowed for gradual snowpack settlement.
Active wind loading has been 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. Moderate east winds in the Paradise area have generally loaded westerly aspects.
A variety of snow surfaces now exist due to local wind, sun and temperature effects.
Observations
North
After an active weekend, 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.
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.
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.
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.
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.
Photo Seth Waterfall NPS, MRNP Panorama Point on 12-6-17. Avalanche occurred 12-5-17.
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
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: East, South East, South, South West, West.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Dec 7th, 2017 11:16AM
