Avalanche Forecast
Issued: Jan 17th, 2018 10:58AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Storm Slabs.
NWAC,
Northwest Avalanche Center
Human-triggered avalanches are likely on Thursday. New snow and moderate SW winds will combine to create sensitive wind slabs near and above treeline. The danger increase will be storm snow dependent. Both storm and wind slab will increase in size throughout the day.
Summary
Detailed Forecast
A frontal band of rain will progress across the Cascade West slopes on Wednesday night, with moderate to occasionally strong southerly winds during the evening hours. Precipitation has started as rain in most of our forecasting terrain. Light to moderate rain and snow showers on a moderate SW flow and a significant cooling trend follow the frontal passage during the late evening hours. Thursday will feature continued gradual cooling with the showery pattern initiated Wednesday night and light to moderate SW winds.
Note that a refreezing 1/17 rain crust will form Wednesday night and provide a bed surface for wind and storm slab avalanches on Thursday.
Cascade West-North above treeline terrain will have dangerous wind slab and storm slab on Thursday. The upper elevations of the above treeline terrain in this zone will not see all snow with the Wednesday night storm system, adding far more new snow relative other locations during the peak intensity of the storm. During the period of strong southerly winds snow will be available for transport, creating dangerous and touchy wind slabs. Storm slabs will be touchy and deep on Thursday due to the high precipitation rates Wednesday evening with additional loading on Thursday. Travel in above treeline terrain is not recommended in this zone and larger paths on higher North Cascade peaks may produce very large natural avalanches so avoid large slide paths at lower elevations on Thursday.
In all other zones and at lower elevations rain provides a delay in the development of storm and wind slab problems. These areas are described below.
Wind slab is the most significant avalanche problem on Thursday. In the early morning hours the snow line lowers and winds will be moderate and shifting to the SW, transporting storm snow as it falls. Wind slabs will continue to load during the day Thursday as lower density snow will be easier for light to moderate winds to move. Wind slabs are most likely on NW-N-NE-E-SE aspects, but remember that complex terrain can move snow onto aspects you don't always anticipate. Wind slabs will be sensitive to human trigger and increasingly large at higher elevations on Thursday. Avoid wind-loaded slopes, particularly near and above treeline.
Storm slab will form in all elevation bands as the snow line lowers, but is most likely to form and become reactive in locations receiving higher snowfall rates that are favored in SW flow, such as Mt. Baker and to a lesser extent Mt. Rainier. Adjust your terrain selection in accordance with the new snow accumulation, becoming more conservative where 8 or more inches of fresh snow has fallen.
Lower snowfall amounts anticipated in the Cascade Passes will make storm slab less massive/reactive and wind slabs generally smaller, reducing danger relative to other areas.
Even a small small avalanche can have significant consequences in a bad location such as avalanche terrain exposed terrain traps or rock/tree hazards.
Remember that new snow will falling on a snowpack that has yet to gain full winter coverage, particularly at lower elevations. Terrain travel and access will still be a challenge in many some areas on Thursday and submerged obstacles will exist.
Snowpack Discussion
A trace to 2" of high density snow fell Wednesday morning on a warming trend as warm front lifted across the Cascades and should be well bonded to 2-4" of higher elevation snow that fell on Tuesday. These small amounts of recent snow should be melting and rounding from rainfall currently saturating the snowpack will into the above treeline terrain on Wednesday afternoon.
Recent snowfall sits on well-drained, generally saturated snowpack with less saturation in the above treeline terrain, but multiple crusts extending into this terrain. There are no notable layers of concern at this time. Thus, focus your attention on new storm instabilities in these zones moving forward.
Observations
North
On Wednesday afternoon, Mt. Baker Pro Patrol observed evidence of several large, natural loose wet avalanches visible on Shuksan arm at approximately 3000 ft. The avalanches are suspected to be from earlier Wednesday.
Central
No recent observations
South
No recent observations
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: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
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: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Jan 18th, 2018 10:58AM
