Avalanche Forecast
Issued: Dec 28th, 2018 10:05AM
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
Dangerous avalanche conditions exist and conservative decision making will be essential. Thick hard slabs which produced avalanches two days in a row will grow in size and reactivity today above treeline while rain below 6000 ft will rapidly weaken the upper snowpack.
Summary
Discussion
The exact nature of the problems you encounter today depends on the elevation of the rain/snow line. Dry snow problems such as wind slabs will occur above the rain/snow line while wet avalanches will occur below. The travel advice remains similar with conservative decision-making being essential.
As of 12/26, Meadows pro-patrol is still finding weak snow (facets) buried 12/7 in backcountry snow pits between 5500 to 6600 ft on NW through SE aspects, in sheltered terrain roughly 2-3 ft down in the snowpack. The layer is trending toward unreactive. No natural or explosive avalanches have released down to this layer, but we'll continue to track this layer as it evolves over time.
Snowpack Discussion
In most parts of the state, a stout melt freeze crust was formed when it rained to high elevations around Thanksgiving. The one exception to this event was in the East North Zone, where the precipitation from the Thanksgiving storm was all snow. A quick storm at the end of November put a small amount of snow above the melt-freeze crust, and preserved the older basal facets in the northeastern areas.
Cold and clear weather dominated the first week in December, with valley fog and very cold temperatures east of the crest. The surface snow sat around and decomposed. Surface hoar grew large on top of this.
The jet stream took aim at the Pacific Northwest in the 2nd week of December. Most notably, light storms buried and preserved a widespread layer of surface hoar and/or near surface facets on December 9th. From December 9th to December 23rd, storms kept coming. Freezing levels fluctuated, but never moved much above 5000ft throughout the Cascades (although the southernmost volcanoes and Mt. Hood saw rain well above 6000).
Initially, the storm track favored the northern zones. The accompanying avalanche cycle began on December 11th. Most of these slides were soft slabs, but some propagated widely on the December 9th layer. Higher snowfall totals in the West North resulted in very large (D3+) avalanches in the mountains along Hwy 542.
A second, and larger avalanche cycle occurred during heavy snowfall and strong wind events between December 18th and 20th. Although these cycles were once again most prevalent in the northern and eastern zones, big storm totals around Mt. Rainier tipped the balance down south as well. This 2nd cycle was impressive, with very large and destructive avalanches (some D4) reported. The culprit was once again the December 9th surface hoar/facets (and/or the basal facets in the northern and eastern zones).
Today we have a large difference in snowpack depths between the Pacific Crest and the Eastern Slope. This is nothing unusual, as more often than not the west side of the Cascades and the passes get more snow than areas further east. Moving forward, places with a deep snowpack (say greater than 5ft) and warmer temperatures may continue to gain strength. Areas with a shallow snowpack (say less than 3.5ft) may take much longer. In a general and applied sense, this means the avalanche danger/conditions may begin to diverge between the western and eastern zones.
Happy Holidays
Problems
Wind Slabs
Wednesday and Wednesday night, moderate winds transported 10” of fresh low-density snow into stout (3-4 ft) hard slabs that were triggered by explosives at Mt. Hood Meadows on Thursday with a small skier triggered hard slab avalanche reported on Friday. You can trigger these slabs on wind-loaded slopes near ridge-lines above the rain/snow line. Expect hard wind slabs to break widely across terrain features and are capable of taking you with it. Give wind-loaded slopes greater than 35 degrees a buffer. During a storm, it is difficult to assess where wind loading has occurred previously, so give exposed terrain a wide buffer.
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.
Likelihood
Expected Size
Loose Wet
Expect loose wet avalanches on steep slopes where rain falls on dry snow. As these wet avalanches run downslope, they could entrain large amounts of snow. Water may pool on crust layers in the snowpack, creating larger avalanches. Look for roller balls and pinwheels as indicators of weakening snow surfaces. Avoid gullies and other confined terrain features where debris could pile up and make these powerful avalanches difficult to survive.
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.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
Valid until: Dec 29th, 2018 10:05AM
